WEEKLY QUANTUM INFORMATION THEORY SEMINARS

TODAY  IMS Seminar Room 14:00:

Hector Bombin

Topological Subsystem Codes

We introduce a family of 2D topological subsystem quantum error-correcting codes. The gauge group is generated by 2-local Pauli operators, so that 2-local measurements are enough to recover the error syndrome. We study the computational power of code deformation in these codes, and show that boundaries cannot be introduced in the usual way.

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16/12/2009  IMS Seminar Room 14:00:

Lev Vaidman

Where is the Quantum Particle between two Measurements?

Wheeler Delayed Choice experiment, Elitzur-Vaidman Interaction-free measurement, and Hosten-Kwiat Counterfactual Computation will be discussed to answer Bohr\'s forbidden question: \"Where is a quantum particle while it is inside a Mach-Zehnder Interferometer?\" The analysis reveals a paradoxical feature of a pre- and post-selected quantum particle: it can reach a certain location without being on the path that leads to and from this location.

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03/11/2009  IMS Seminar Room 14:00:

Alex Lvovsky

Peeking into quantum black boxes

Assembling a quantum information processing circuit requires precise knowledge of the properties of its each component, i.e. the ability to predict the effect of the component on an arbitrary input quantum state. This knowledge is acquired through quantum process tomography - a procedure in which certain \"probe\" states are sent into the quantum \"black box\", and the corresponding outputs are measured. Surprisingly, black boxes that process quantum optical information can be completely characterized using coherent states, i.e. simple laser pulses, as the probe states. We shall discuss the theory and implementation of this procedure as well as a few examples of its practical application.

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07/10/2009  IMS Seminar Room 14:00:

Stephanie Wehner

Unconditional security from noisy quantum storage

We consider the implementation of two-party cryptographic primitives based on the sole assumption that no large-scale reliable quantum storage is available to the cheating party. We construct novel protocols for oblivious transfer and bit commitment, and prove that realistic noise levels provide security even against the most general attack. Such unconditional results were previously only known in the so-called bounded-storage model which is a special case of our setting. Our protocols can be implemented with present-day hardware used for quantum key distribution. In particular, no quantum storage is required for the honest parties.

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06/10/2009  IMS 14:00:

Austin Fowler

Fast, fault-tolerant quantum communication

Existing quantum communication protocols make use of entanglement purification, the combination of two low fidelity Bell pairs to create a single Bell pair with higher fidelity. This entails interactive classical communication between quantum repeaters, which can take significant time due to the finite speed of light. When this is the only technique used, the communication rate is severely limited. By making use of surface code quantum error correction, we show that it is possible to avoid entanglement purification once the Bell pair error rate p is less than 15/16 and furthermore show that p ~ 10% is sufficiently low for practical operation. In addition to making the timescale of quantum communication commensurate with the timescale of the quantum gates within each repeater node, which can be orders of magnitude faster than the light propagation time, we demonstrate an exponential saving in resources versus communication distance and the ability to achieve arbitrarily high fidelity quantum communication with finite fidelity quantum gates within each repeater node.

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08/09/2009  IMS seminar room 14:00:

Dr. Earl T. Campbell

Bound States for Magic State Distillation

Magic state distillation is an important primitive in fault-tolerant quantum computation. The magic states are pure non-stabilizer states which can be distilled from certain mixed non-stabilizer states via Clifford group operations alone. Hence, any distillable non-stabilizer state is a resource for fault-tolerant quantum computation. Due to the Gottesman-Knill theorem, convex mixtures of Pauli eigenstates are not expected to be magic state distillable, but it has been an open question whether all mixed states outside this octahedral set may be distilled. I will outline our recent result showing that, when protocols are of finite size, non-distillable states exist outside the stabilizer octahedron. In analogy with the bound entangled states, which arise in entanglement theory, we call such states bound states for magic state distillation.

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25/08/2009  IMS seminar room 15:00:

Akihito Ishizaki

Theoretical Examination of Quantum Coherence in Photosynthetic Systems - Please note, due to a technical error, this seminar was not broadcast.

The observation of long-lived electronic coherence in photosynthetic excitation energy transfer (EET) by Engel et al. [Nature (London) 446, 782 (2007)] raises questions about the role of the protein environment in protecting this coherence and the significance of quantum coherence in light harvesting efficiency. In order to elucidate the origin of the long-lived quantum coherence and its interplay with the protein environment in EET processes, it is crucial to develop a theoretical framework beyond conventional perturbative approaches such as Redfield theory and Förster theory. Recently, we developed a new quantum dynamic equation for EET by taking into account effects of the environmental reorganization dynamics on the pigments in a nonperturbative fashion. The developed equation can describe EET processes irrespective of the electron-environment coupling strength, and reduces to the conventional Redfield and Förster theories in their respective limits of validity. In the regime of coherent wave-like motion, moreover, the equation predicts several times longer lifetimes of the electronic coherence than does the Redfield equation. Additionally, although the experimental observations of long-lived quantum coherence provide valuable insights into the inner working of photosynthetic complexes, the measurements were performed outside the physiological range of temperature. Hence, the robustness of quantum coherence under physiological conditions is to large extent unknown. In order to address the issue, we apply the above equation to the calculation of EET in the Fenna-Mathews-Olson (FMO) photosynthetic pigment-protein complex under physiological conditions. The numerical results reveal that quantum coherence persists for several hundred femtoseconds even at physiological temperature. In this talk, we discuss the newly developed equation for EET and its application to the FMO complex under physiological conditions. Based on the numerical results, a potential role of quantum coherence in photosynthetic systems is also discussed.

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31/07/2009  IMS Seminar Room 12:00:

Michael Lubasch

Aspects of quantum chaos and entanglement in the Bose-Hubbard model

\"We address the question if the tools of entanglement theory can be used to detect quantum chaos. Additionally, we develop a novel way of characterizing the quantum chaotic regime and present an experimentally realizable technique for the generation of large amounts of entanglement.\"

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28/07/2009  IMS Seminar Room 14:00:

James D Whitfield

Quantum Generalizations of Random Walks: Quantum Stochastic Walks

We introduce the quantum stochastic walk (QSW), which determines the evolution of generalized quantum mechanical walk on a graph that obeys a quantum stochastic equation of motion. Using an axiomatic approach, we specify the rules for all possible quantum, classical and quantum-stochastic transitions from a vertex as defined by its connectivity. We show how the family of possible QSWs encompasses both the classical random walk (CRW) and the quantum walk (QW) as special cases, but also includes more general probability distributions. As an example, we study the QSW on a line, the QW to CRW transition and transitions to generalized QSWs that go beyond the CRW and QW. QSWs provide a new framework to the study of quantum algorithms as well as of quantum walks with environmental effects.

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24/07/2009  IMS Seminar Room 14:00:

Ramón Aguado

Shot noise and entanglement in Solid State charge Qubits

Non-equilibrium quantum noise in electronic transport (shot noise), provides us with information beyond that contained in usual dc transport experiments. Important examples include frequency-dependent shot noise and its relation to the dissipative dynamics of open qubits [1,2] as well as the intriguing connections between shot noise cross-correlations and bipartite entanglement. I will illustrate these ideas with various examples including the frequency-dependent shot noise of charge qubits coupled to reservoirs [2], entanglement and shot noise in capacitively-coupled double quantum dots [3], bath-mediated entanglement in charge qubits [4] and circuit QED systems based on Cooper pair Boxes [5]. [1] R. Aguado and Leo P. Kouwenhoven, Phys. Rev. Lett.,84, 1986 (2000). [2] R. Aguado and T. Brandes, Phys. Rev. Lett.,92, 206601 (2004). [3] N. Lambert, R. Aguado and T. Brandes, Phys. Rev B, 75, 045340 (2007). [4] D. Contreras and R. Aguado, Phys. Rev B, 77, 155420 (2008). [5] D. Contreras, C. Emary, T. Brandes and R. Aguado, work in progress.

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14/07/2009  IMS Seminar Room 14:00:

Jonathan Home

Scalable quantum information processing with trapped ions

I will describe recent work with trapped ions which combines all of the basic building blocks for a large-scale quantum information processor. We use scalable methods, allowing all operations to be performed at the same fidelity over long timescales, and before and after qubit transport. Decoherence of qubits stored in the ions\' electronic states is heavily suppressed by the use of a magnetic field insensitive transition. We have used this system to implement arbitrary unitary operations on two qubits. A crucial component of this work is simultaneous trapping of two species of ion (Mg+ and Be+), which allows the motional degree of freedom to be cooled while preserving qubit coherence. This has also allowed us to create a novel entangled state of two separated mechanical oscillators. This work was funded by NIST, IARPA, DARPA. J.P.H would like to acknowledge a Lindemann Trust fellowship from the English Speaking Union.

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07/07/2009  IMS Seminar Room 14:00:

Yeong-Cherng Liang

How to violate a Bell inequality in the absence of a complete classical reference frame?

Bell inequality violation is one of the most bewildering features allowed by quantum mechanics. However, even when equipped with an entangled state, the demonstration of a Bell inequality violation is not at all a trivial task. Specifically, the various spatially separated experimenters generally need to know what measurements to make in order to demonstrate a Bell inequality violation. What happens when the experimenters do not have complete knowledge of how their reference frames are related to their distant experimenters\'? Are they still able to demonstrate a Bell inequality violation in the absence of a shared classical reference frame? In this talk, I will discuss an approach to this problem that is based on random local measurements.

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03/07/2009  IMS Seminar Room 14:00:

Nicolas Brunner

Why is quantum non-locality limited?

Quantum mechanics is a non-local theory, however not a maximally non-local one according to relativity. More precisely, there exist theories containing more non-locality than quantum mechanics that still respect the no-signaling principle. Why these theories are unlikely to exist in nature, and what physical principle limits quantum non-locality is still not known today. Recently, an intensive research effort has been devoted to these issues. Here I will present recent progress related to information theoretic tasks (such as communication complexity and information causality) in general non-signaling theories. Then I will focus on the dynamical process of non-locality swapping, the analogue of quantum entanglement swapping, and show how quantum correlations partly emerge from this study.

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02/07/2009  IMS Seminar Room 14:00:

Martin Kiffner

Dissipation-induced Tonks-Girardeau Gas of photons

A one-dimensional system of Bosons that experience a repulsive two-particle interaction is an exactly integrable system that has been discussed by E. Lieb and W. Liniger in the early 1960\'s. This model caught a lot of attention since it predicts that the bosonic system may exhibit various features of a noninteracting spinless Fermi gas. In particular, the Lieb-Liniger model comprehends the limit of a so-called Tonks-Girardeau gas, where the particles behave like impenetrable hard-core bosons that can never be at the same position. The recent progress in cooling and trapping of atoms allowed the experimental observation of a Tonks-Girardeau gas. Moreover, an intriguing experiment carried out last year shows that the &#8220Fermionization&#8221 of atoms can also be achieved with inelastic two-particle collisions. In the first part of my talk, I will give an introduction to the Lieb-Liniger model and the relevant experiments. The second part gives a summary on recent theoretical work [D. E. Chang et al., nature physics 4, 884 (2008)] suggesting that a Tonks-Girardeau gas could be realized with photons. Finally, in the third part I discuss our theoretical efforts to create a strongly correlated system of hard-core photons without elastic two-photon interactions, but with two-photon losses only.

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01/07/2009  IMS Seminar Room 14:00:

Sophie Schirmer

Quantum Engineering: Hamiltonian and Markovian Reservoir Engineering

Note: This talk was not broadcast live.

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12/06/2009  IMS Seminar Room 12:00:

Gilad Gour

Quantum Resource Theories

Quantum information theory can be viewed as a theory of inter-conversions among different resources. These resources are diversely classified as quantum or classical, static or dynamic, noisy or noiseless, and therefore enable plethora of quantum information processing tasks. In this talk I will discuss the characterization, manipulation and quantification of quantum resources. I will focus mostly on the resource theories that follow from the presence of super-selection rules or the absence of shared reference frames. In particular, I will compare and discuss measures of quantum resources, such as the relative entropy of a resource, and show that these measures get different operational interpretations in different resource theories. Such comparisons provide a much broader perspective on all of these resource theories and allow us to use the insights gained from one theory to solve the problems that arise in the context of another resource theory.

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09/06/2009  IMS Seminar Room 14:00:

Gustavsson Anna

Constrained quantum dynamics

I will discuss a new framework that has been introduced to investigate the dynamics of constrained quantum systems, and the procedure for its implementation. The approach makes use of the fact that the space of pure states in quantum theory has both a Riemannian structure, the unitary-invariant Fubini-Study metric, and a symplectic structure. As a consequence, the idea of constrained quantum motion can be realised by requiring that the dynamics should lie on a suitably specified submanifold of the state space. I will show that for certain classes of systems the framework can be viewed as an extension of Dirac\'s theory for constrained systems applied to quantum mechanics. I will illustrate some of the results through examples and I will also discuss how the framework could be extended to include mixed quantum states.

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08/06/2009  IMS Seminar Room 16:00:

Gabriele De Chiara

Mott-insulator states of ultracold atoms in optical resonators

We investigate a paradigmatic example of cavity quantum electrodynamics with many body systems: an ultracold atomic gas in the potential formed inside an optical resonator. As in open space, when the optical potential is deep enough, the atomic gas is in the Mott-insulator state. Inside the cavity, the potential is critically affected by the photon-mediated interaction between the atoms. The presence of the atoms, however, determines the intracavity field amplitude, which then determines atomic localization and the quantum state of the gas. We derive an effective Bose-Hubbard model describing the physics of the system in one dimension, where its coefficients depend non-linearly on the number of atoms, and have to be evaluated self-consistently. We study the transition between the superfluid-Mott insulator quantum states and determine the regions of parameters where the atomic insulator states are stable.

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03/06/2009  IMS Seminar Room 12:00:

Cecilia Cormick

Studying entanglement dynamics in an ion trap

The evolution of the entanglement between two oscillators coupled with a common environment is non-trivial and has been extensively studied (Phys. Rev. Lett. 100, 220401). The long time limit exhibits qualitatively different behaviors (phases) depending on parameters such as the initial state and the temperature of the bath. The possible phases include cases where entanglement suffers from sudden death, others with infinite sequence of events of sudden death and revivals, or non-vanishing long-term entanglement. In this talk I will describe a simple way of realizing and observing these different scenarios in an ion trap experiment. The proposal involves three ions in a linear trap. The longitudinal degrees of freedom of two of them correspond to the two oscillators while the central ion, subject to continuous laser cooling, is the gateway to a decohering reservoir. The scheme proposed is another instance of a quantum simulator that seems to be realizable with current technology. The simulation is an example of an interesting class which involves the observation and control of the evolution of quantum open systems in the non-Markovian regime.

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20/04/2009  IMS 14:00:

Murray Batchelor

Corner transfer matrix variational approach to many-body problems

In this talk I will review Baxter\'s variational corner transfer matrix approach to the calculation of many-body problems. As an example, this approach is applied to calculate the universal scaling function of the square lattice Ising model in a magnetic field. The high precision numerical data is in perfect agreement with the remarkable field theory results obtained by Fonseca and Zamolodchikov, as well as with many previously known exact and numerical results for the 2D Ising model. This includes excellent agreement with analytic results for the magnetic susceptibility obtained by Orrick, Nickel, Guttmann and Perk. In general the high precision of the numerical results underlines the potential and full power of the variational corner transfer matrix approach.

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16/04/2009  IMS 14:00:

Alan Aspuru-Guzik

Quantum computers for chemistry: Correlated quantum systems, chemical reaction dynamics, and exploring classical potential energy surfaces

In this talk, I review the recent progress of our group in the development of efficient quantum computer algorithms for electronic structure and chemical dynamics. The algorithmic complexity and resource requirements will be discussed. I will also briefly discuss the potential of quantum computers to solve classical optimization problems such as the HP model for protein folding. We will also describe the first quantum computer experiment to obtain the electronic structure of the hydrogen molecule, carried out in collaboration with the group of Andrew White at the University of Queensland, Australia.

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07/04/2009  IMS Seminar Room 14:00:

Marco Piani

All entangled states are useful for channel discrimination

We prove that every entangled state is useful as a resource for the problem of minimum-error channel discrimination. More specifically, given a single copy of an arbitrary bipartite entangled state, it holds that there is an instance of a quantum channel discrimination task for which this state allows for a correct discrimination with strictly higher probability than every separable state.

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03/04/2009  IMS seminar room 12:30:

Laura Cattaneo

A rigorous representation of the Feynman-Vernon influence functional via infinite dimensional oscillatory integrals

A rigorous representation of the Feynman-Vernon influence functional used to describe open quantum systems is given, based on the theory of infinite dimensional oscillatory integrals. In particular, the case of the Caldeira-Leggett model of two quantum systems with a quadratic interaction is considered.

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17/03/2009  IMS 14:00:

Eduardo Martín Martínez

Entanglement decoherence settings in Relativistic Quantum Information

The talk will be centred in two different settings where the study of entanglement degradation requires a relativistic approach: Soft-photons effect on the entanglement of charged qubits Soft-photons appear when we build charged qubits from the asymptotic states of QED. The nonexistence of free charged particles (due to the long range of QED interactions) leads us to question the sense of the very concept of free charged qubit. On the contrary, one charged qubit is always undergoing the EM interaction even at asymptotic times, being inescapably surrounded by a cloud of soft-photons whose effect on entanglement is not known. We will analyse the problems introduced by the long range of the EM interaction (as infrared divergences or the loss of the particle interpretation). We will show that, using the ‘dressing’ formalism, we can build physical charged qubits from dressed fields which have the correct asymptotic behaviour, are gauge invariant, their propagators have a particle pole structure and are free from infrared divergences. Finally we will discuss the impact of the soft corrections on the spin entanglement. Spin and occupation number entanglement of Dirac fields for non-inertial observers Unruh effect on occupation number entanglement has been studied for scalar and Dirac fields. However, previous works did not take into account the spin degree of freedom of the Dirac field. We will investigate the consequences of including spin degree of freedom in these settings. This analysis enables us to study bipartite spin Bell states, obtaining their entanglement dependence on the acceleration of the observers. Then, we will consider simple equivalents to the occupation number entangled state but with spin quantum numbers for the one particle states. We show that, despite their apparent similitude, while the spinless case is always qubit x qubit, for the spin case acceleration produces a qubit x qu4it state. We will also introduce a procedure to consistently erase the spin information from our setting preserving occupation numbers. We will show how the maximally entangled state for occupation number emerges from our setting. We as well analyze its entanglement dependence on acceleration, obtaining greater entanglement degradation than in the spinless case.

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11/03/2009  IMS Seminar Room 12:00:

Tomaz Prosen

General solution of master equations for quadratic open Fermi systems and quantum phase transition far from equilibrium

Using a concept of quantization in the Fock space of operators, the master equation (e.g. of Lindblad, or Redfield type) for an arbitrary quadratic system of n fermions is solved explicitly in terms of diagonalization of a 4n × 4n matrix, provided that all the bath coupling operators are linear in the fermionic variables. As a nontrivial example of the method, we report a discovery of a novel type of far from equilibrium quantum phase transition in an open XY chain. The transition is characterized with a spontaneous emergence of long-range order in spin-spin correlation functions, and a dramatic change in the macroscopic entanglement properties of the nonequilibrium steady state.

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10/03/2009  IMS seminar room 14:00:

Ashley Montanaro

Quantum boolean functions

In recent years, the analysis of boolean functions has arisen as an important theme in theoretical computer science. In this talk I will discuss an extension of the concept of a boolean function to quantum computation. This extension allows many classical results to be translated to the quantum regime. Examples of the new results we obtain are efficient testing for whether a unitary is a stabiliser operator, efficient approximate learning of some types of quantum dynamics, and a \\\"hypercontractive\\\" inequality for the qubit depolarising channel. This talk is based on joint work with Tobias Osborne.

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24/02/2009  IMS Seminar Room 14:00:

Francesco Ciccarello

Scattering: a viable resource for control-limited entanglement distribution

The setup composed of a mobile particle, such as an electron or a photon, scattered by static quantum centers such as magnetic impurities or multi-level atoms, enjoys many intriguing physical features. I shall illustrate how these can be exploited in order to set maximum entanglement between the spin degrees of freedom of remote particles in control-limited situations [1-4]. In particular, I will try to shed light onto the issue: \"How much time is needed for two scattering spins to reach their entangled steady state?\". The answer to this question [4] reveals that the mechanism behind entanglement formation via dynamical scattering is characterised by features that are deeply different from the case of non-mobile coupled spins. Counter-intuitively, in the scattering case the time required for setting maximum entanglement is independent of the spin-spin coupling strength. [1] A. T. Costa, S. Bose, and Y. Omar, Phys. Rev. Lett. 96, 230501 (2006). [2] F. Ciccarello, G. M.Palma, M. Zarcone, Y Omar & V. R. Vieira, New J. Phys 8,214 (2006); J. Phys. A 40, 7993 (2007), Las. Phys. 17, 889 (2007); F. Ciccarello, G. M.Palma & M. Zarcone, Phys. Rev. B 75, 205415 (2007) F.Ciccarello, G. M.Palma, M.Paternostro, M. Zarcone and Y Omar, to appear on Sol. Stat. Sci. (2008). [3] F. Ciccarello, M.Paternostro, M. S. Kim, and G. M. Palma, Phys. Rev. Lett. 100, 150501 (2008); Int. J. Quant. Inf. 6,759 (2008). [4] F. Ciccarello, M.Paternostro, G. M. Palma, M. Zarcone, arXiv:0812.0755v1 [quant-ph].

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19/02/2009  IMS Seminar Room 14:00:

Matthew Woolley

Quantum States of Nanomechanical Systems

Recent technological progress has allowed the fabrication of mechanical systems approaching the quantum limit. Nanomechanical systems, composed of mechanical resonators fabricated on the nano-scale, offer the advantages of low mass (\"large\" quantum fluctuations), high quality factors (\"long\" coherence times), and high frequencies (prohibiting thermal occupation of the state at achievable temperatures). Such progress offers the possibility of studying of quantum mechanics in a new regime, including the generation of exotic quantum states and novel quantum dissipative physics. I will briefly discuss the experimental and theoretical progress in this field, and then describe one system that we have studied theoretically, being a nanomechanical resonator capacitively coupled to a superconducting microwave coplanar waveguide. I will discuss how it is possible to cool the mechanical resonator using such a system, and also how to generate and detect a squeezed state of the mechanical resonator.

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17/02/2009  IMS seminar room 14:00:

Madalin Guta

Local asymptotic normality in quantum statistics

Quantum statistics deals with the processing of statistical information carried by quantum systems. One of the central problems in quantum statistics is that of estimating an unknown quantum state of dimension d by performing general measurements on a (large) number of identically prepared quantum systems. We will show how to optimally estimate a completely unknown quantum state by developing the quantum analogue of the classical statistical concept of \\\"local asymptotic normality\\\" (LAN). LAN means that the statistical model described by the collective state of n identically prepared quantum systems converges to a model consisting of a (d-1)-dimensional classical Gaussian and a product of thermal equilibrium states of d(d-1)/2 harmonic oscillators. Both Gaussian models have fixed variance and unknown mean which can be easily estimated by means of standard (heterodyne/homodyne) measurements. This optimal measurement can be pulled back to an optimal strategy for estimating the quantum state.

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05/02/2009  IMS seminar room 14:00:

Koji Maruyama

CNOT-free entanglement purification by spin chain dynamics

We present a simple protocol to purify bipartite entanglement in spin-1/2 particles by utilizing only natural spin-spin interactions and Sz measurements on single spins. Even the standard isotropic Heisenberg interaction is shown to be sufficient to purify mixed state entanglement. Our protocol does not need controlled-NOT (CNOT) gates that are very hard to implement experimentally. This approach could be useful for quantum-information processing in solid-state-based systems.

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03/02/2009  IMS seminar room 14:00:

Tony Downes

Quantum Connectivity of Space-Time and Gravitationally Induced Decorrelation of Entanglement

We discuss an alternative formulation of the problem of quantum optical fields in a curved space-time using localized operators. We contrast the new formulation with the standard approach and find observable differences for entangled states. We propose an experiment in which an entangled pair of optical pulses are propagated through non-uniform gravitational fields and find that the new formulation predicts de-correlation of the optical entanglement under experimentally realistic conditions.

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14/10/2008  IMS Seminar Room 14:00:

Oscar C.O. Dahlsten

Nonclassicality without entanglement enables bit commitment

We investigate the existence of secure bit commitment protocols in the convex framework for probabilistic theories. The framework makes only minimal assumptions, and can be used to formalize quantum theory, classical probability theory, and a host of other possibilities. We prove that in all such theories that are locally non-classical but do not have entanglement, there exists a bit commitment protocol that is exponentially secure in the number of systems used.

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07/10/2008  IMS seminar room 14:00:

Leandro Alita

Scaling laws for the decay of multiqubit entanglement decay

We investigate the decay of entanglement of (pure) generalized N-particle Greenberger-Horne-Zeilinger (GHZ) states interacting with independent reservoirs. Scaling laws for the decay of entanglement and for its finite-time extinction (sudden death) are derived for different types of reservoirs. The latter is found to increase with the number of particles. However, entanglement becomes arbitrarily small, and therefore useless as a resource, much before it completely disappears, around a time which is inversely proportional to the number of particles. We also show that the decay of multi-particle GHZ states can naturally generate bound entangled states. Finally, the mixed-state case of generalized GHZ diagonal states is also studied and scaling laws for the decay of their entanglement are also provided. Such laws are universal for this family of states in the sense that they hold for any entanglement quantifier. For other more general families of states a numerical exploration is performed

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30/09/2008  IMS Seminar Room 14:00:

Andrzej Dragan

Why devil plays dice?

It is known that extending the Principle of Relativity (and Special Relativity) to superluminal observers leads to the disturbance of causality. It turns out, however, that such an extension does not result in any paradoxes, only leads to basic postulates of Quantum Theory. It will be shown that quantum indeterminacy based on complex probability amplitudes undergoing linear superposition principle is an inevitable consequence of Special Relativity involving all inertial observers.

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02/09/2008  IMS Seminar Room 14:00:

Masahito Hayashi

Quantum universal coding protocols and universal approximation of multi-copy states

We have constructed universal codes for quantum lossless source coding and classical-quantum channel coding. In this construction, we essentially employ group representation theory. In order to treat quantum lossless source coding, universal approximation of multi-copy states is discussed in terms of the quantum relative entropy.

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28/08/2008  IMS Seminar Room 14:00:

Christopher Hadley

Single-copy entanglement in a gapped quantum spin chain

The single-copy entanglement of a given many-body system is defined [J. Eisert and M. Cramer, Phys. Rev. A. 72, 042112 (2005)] as the maximal entanglement deterministically distillable from a bipartition of a single specimen of that system. For critical (gapless) spin chains, it was recently shown that this is exactly half the von Neumann entropy [R. Orus, J. I. Latorre, J. Eisert, and M. Cramer, Phys. Rev. A 73, 060303(R) (2006)], itself defined as the entanglement distillable in the asymptotic limit: i.e. given an infinite number of copies of the system. It is an open question as to what the equivalent behaviour for gapped systems is. In this paper, I show that for the paradigmatic spin-S Affleck-Kennedy-Lieb-Tasaki chain (the archetypal gapped chain), the single-copy entanglement is equal to the von Neumann entropy: i.e. all the entanglement present may be distilled from a single specimen. Journal reference: Phys. Rev. Lett. 100, 170001 (2008)

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13/08/2008  IMS seminar room 14:00:

Ignacio Wilson-Rae

Intrinsic dissipation in nanomechanical resonators due to phonon tunneling

Mechanical damping, which plays an ubiquitous role in determining the temperature limits in mesoscopic \"laser cooling\" schemes, is usually described in a phenomenological manner by introducing an Ohmic damping force. We analyze the dissipation mechanism that arises in mesoscopic mechanical beam-structures due to the tunneling of phonons between the beam and its supports (known as clamping losses), and provide a microscopic derivation of the environmental force spectral density that determines the quantum Brownian motion associated to a given resonance. Our treatment is valid for low frequencies and determines the leading contribution in the aspect ratio. This yields fundamental geometric upper bounds for the Q-values which are described by simple scaling laws and are relevant for state of the art structures. In this context, for resonant frequencies in the 0.1-1GHz range, while this dissipation mechanism can limit flexural resonators it is found to be negligible for torsional ones. In the case of structureless 3D supports the corresponding environmental spectral densities are Ohmic for flexural resonators and super-Ohmic for torsional ones, while for 2D slab supports they yield 1/f noise. Furthermore, analogous results are established for the case of suspended semiconducting single-walled carbon nanotubes which are found to be more resilient to this dissipation mechanism. Finally, we provide a general expression for the spectral density that allows to extend our treatment to other geometries and illustrate its use by applying it to a microtoroid. Our analysis is relevant for applications in ultra-high precision measurements and for the prospects of probing quantum effects in a macroscopic mechanical degree of freedom.

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31/07/2008  IMS Seminar Room 14:00:

Mile Gu

More is Different

In 1972, P.W.Anderson suggested that `More is Different\', meaning that complex physical systems may exhibit behaviors that cannot be understood only in terms of the laws governing their microscopic constituents. We prove this by showing that macroscopic observables of a simple physical system (the infinite periodic Ising lattice) cannot generally be derived from fundamental principles. This presents compelling evidence that emergent behavior must occur in such systems, and indicates that even if a `theory of everything\' governing all microscopic interactions were to be discovered, the understanding of macroscopic order will still require additional insights.

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28/07/2008  IMS Seminar Room 14:00:

Andrew White

Not your parents\' quantum computer

The realisation of a quantum computer presents one of the most challenging problems in modern science. The formidable computational power of such a device has implications across the domain of physics, and is now known to extend to fields such as chemistry, cryptography and nanotechnology. Recently, universal quantum-logic gate sets—the fundamental building blocks for a quantum computer—have been demonstrated in a number of physical architectures. However, a serious obstacle to a full-scale implementation is the sheer number of these gates required to implement even small quantum algorithms. Here we present an innovative technique that harnesses higher dimensions of quantum systems to significantly reduce the number of gates required to construct complex quantum circuits. This allows us to implement for the first time two key quantum circuits: the three-qubit Toffoli and the two-qubit controlled-unitary. We use a photonic architecture, and demonstrate the classical and quantum actions of our Toffoli by reconstruction the logical truth-table and testing its ability to control an entangling process. We also fully characterise, via quantum process tomography, the operation of four distinct controlled-unitary gates, achieving the highest process fidelity ever reported in any architecture. There are many candidate systems for encoding quantum information. All of these have an abundance of readily available higher dimensions, and will benefit from our technique. We also discuss a pair of deeply surprising results: it is possible to build a useful quantum computer without using entanglement; and there exist non-classical correlations *beyond* entanglement. We demonstrate this with a simple quantum circuit, clean measurements, and an accessible discussion. It is not well known that there are non-classical correlations other than entanglement: we not only show that these correlations exist in experiment, but demonstrate their usefulness in the context of quantum computation. Our results are in contrast to the widely held belief that entanglement lies at the heart of the power of a quantum computer.

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10/07/2008  IMS Seminar Room 14:00:

Shai Machnes

QLib - A Matlab Package for Quantum Information Theory Calculations with Applications

Developing intuition about quantum information theory problems is difficult, as is verifying or ruling-out of hypothesis. We present a Matlab package intended to provide the QIT community with a new and powerful tool-set for quantum information theory calculations. The package covers most of the \"QI textbook\" and includes novel parametrization of quantum objects and a robust optimization mechanism. New ways of re-examining well-known results is demonstrated. QLib is designed to be further developed and enhanced by the community and is available for download at www.qlib.info

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08/07/2008  IMS Seminar Room 14:00:

Adan Cabello

Proposed Bell experiment with energy-time (or time-bin) entanglement - PLEASE NOTE: This Seminar will NOT be broadcast Live.

Aerts et al. [Phys. Rev. Lett. 83, 2872 (1999)] showed that Franson\'s Bell experiment with energy-time entanglement [Phys. Rev. Lett. 62, 2205 (1989)] does not and cannot violate local realism. We show that Franson\'s postselection procedure applied to a local hidden variable model can produce any violation of the Bell-CHSH inequality, even those forbidden by quantum mechanics. We then introduce a novel two-photon energy-time (and time-bin) Bell experiment without this defect.

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29/04/2008  IMS seminar room 14:00:

Martina Hentschel

Fermi-edge singularities in mesoscopic systems: From quantum dots to graphene

Fermi-edge singularities are among the simplest many-body effects and have been a key interest in condensed matter physics for many years. They have been extensively studied, and are understood, for bulk systems such as metals. Here we show that Fermi-edge singularities of mesoscopic samples like quantum dots and graphene hold surprises related to their finite size, the intrinsic mesoscopic fluctuations, and the modifications of the electron dynamics in confined ballistic systems. A particular interesting behaviour is seen in graphene where the vanishing density of states at the Dirac point significantly modifies the system\\\'s many-body response. One focus of our study is the response of to a sudden, localized perturbation such as the excitation of a core electron by an x-ray. This gives rise to Anderson orthogonality catstrophe (the vanishing of the overlap between the unperturbed and perturbed many-body states in the limit of large systems) and the Fermi-edge singularities in the so-called x-ray edge problem which we find to be fundamentally different form the metallic case. We also discuss condensed matter realizations of the parity anomaly that originate in graphene\\\'s honeycomb lattice. In particular we investigate the parity anomaly induced by a termination of the graphene lattice in form of a zig-zag edge and show that it gives rise to a spin quantum Hall effect at zero magnetic field.

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25/04/2008  IMS seminar room 14:00:

Nilanjana Datta

Beyond Independence in Quantum Information

Optimal rates of quantum information protocols, such as compression and transmission of information,and manipulation of entanglement, were initially obtained under the assumption that information sources, channels or entanglement resources were memoryless. A memoryless quantum information source is one that emits successive signals which are independent of each other. A quantum channel is said to be memoryless if the noise acting on successive inputs to the channel is uncorrelated and hence the action of each use of the channel is independent of previous uses. Similarly, an entanglement resource is said to be memoryless if it consists of states which are multiple copies, i.e., tensor products, of a given entangled state. Hence, the assumption of sources, channels and entanglement resources being memoryless corresponds to an underlying assumption of independence. In real world communication systems, this assumption is not always justied. Hence, it is necessary to go beyond independence, and to take into account memory eects in evaluating optimal rates of protocols. In this seminar, we do this for entanglement dilution. More precisely, we evaluate the entanglement cost of an arbitrary sequence of bipartite states, which are not necessarily of the tensor product form. The technique that we use for this purpose is the so-called Quantum Information Spectrum approach. This approach provides a unifying mathematical framework for studying dierent quantum information protocols, without making specic assumptions about the nature of the sources, channels or entanglement resources used. The optimal rates in this framework are given by spectral entropy rates. An alternative approach to overcome the limitation provided by the memoryless criterion, is the so-called Smooth Renyi Entropy framework, developed by R.Renner. This approach introduces new entropy measures, called smooth Renyi entropies or smooth min- and max- entropies. The relationship between the two approaches is discussed. Further, we introduce two new relative entropy quantities, which act as parent quantities for the min- and max- entropies of Renner, and which satisfy interesting properties. We also dene a new entanglement measure, which we refer to as the max- relative entropy of entanglement and which is an upper bound to the relative entropy of entanglement.

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17/04/2008  IMS seminar room 14:00:

Anthony Laing

Experimental Quantum Process Discrimination - Please note: Due to a technical fault, there was no recording of this talk.

Perfect identification of an unknown quantum process that acts on the state of a quantum system can be achieved via quantum process tomography, but requires infinite uses of the unknown process. However the situation is different for quantum process discrimination (QPD) where we wish to correctly identify a quantum process chosen secretly from a set of two. Contrary to process tomography and quantum state discrimination, two non-orthogonal quantum processes can be deterministically and unambiguously discriminated with finite uses of the unknown process. In this talk I will give details of recent photonic experiments that realise some two-photon examples of QPD.

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02/04/2008  IMS seminar room 14:00:

Todd Brun

Improving single photon sources with continuous measurement

One method for designing a single-photon source involves an emitter (such as a quantum dot) in an optical microcavity, which subsequently leaks the photon into an outgoing waveguide. A problem such systems tend to have is low indistinguishability of their output photons. This is at least partially due to timing uncertainties in the emission, and reduces their ability to do two-photon interferometry. We study an engineering technique using continuous quantum measurement in conjunction with feed-forward to improve the indistinguishability. The technique involves continuously monitoring the state of the emitter, processing the noisy output signal with a simple linear estimation algorithm, and feeding this information forward to control a variable delay at the output. In the weak coupling regime, the information gained by monitoring the state of the emitter can reduce the time uncertainty in photon emission from the source, improving the indistinguishability of the emitted photons.

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12/03/2008  IMS seminar room 14:00:

Filippo Caruso

Quantum Information transfer over Quantum Channels

A complete analysis of Bosonic Gaussian channels is proposed. We clarify the structure of unitary dilations showing their normal form. It allows us, for instance, to simplify their classification in terms of weak-degradability features, that pave the way to the solution of their quantum capacity issues. The most relevant one- and two-mode cases are analyzed in details. Finally, by exploiting the composition rules of these maps and the fact that anti-degradable channels cannot be used to transfer quantum information, we identify a new set of one- and two-mode Bosonic Gaussian channels with zero quantum capacity.

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28/02/2008  IMS, seminar room 14:00:

Ines de Vega

Measuring the state of atoms in a lattice with light

Ultracold atoms in optical lattices open exciting prospects for the investigation of quantum many--body phases in a highly controllable setup. They can be used to simulate a rich variety of models from quantum magnetism. Furthermore, this system is ideally suited to implement a quantum register for quantum computation, and to realize multiparticle entangled states with cold controlled collisions. An important issue for all these applications is to measure the state of the atoms within the lattice. Such a state can often be characterized by correlations of the atomic spin operators. In this talk, I will present a method to measure these spin correlations, by using off resonant two-photon Raman scattering. It will be shown that scattered photons carry information of atomic spin operators, in such a way that correlations between photons emitted in different directions are proportional to correlations of spin operators in the momentum space. Hence, different photon detection schemes (i.e. photon counting or homodyne detection), will give information of different types of correlations.

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21/02/2008  IMS seminar room 14:00:

Berry Groisman

Special Relativity meets Quantum Information Theory: Instantaneous measurements of non-local variables

Non-local variable is a property of a quantum system related to more than one small region of space. Instantaneous measurability of a non-local variable, i.e. its measurability at a given time, is essential in order to grant it the status of an observable. Relativistic causality produces rigid restrictions on the measurements of non-local variables. In 1931 L. Landau and R. Peierls claimed that the instantaneous measurability of any non-local variable would contradict relativistic causality. Nevertheless, in 1980 Y. Aharonov and D. Albert found that some non-local variables can be measured instantaneously. Later some explicit methods for performing instantaneous non-local measurements were proposed. On the other hand, it was proved that the measurability of some other operators contradicts relativistic causality. The above mentioned causal restrictions were derived under certain assumptions that the measurement has to satisfy. Recently, we have shown that if some of the requirements from the measurement are relaxed then the majority of non-local operators can be measured instantaneously. Thus, in principle there are no causal restrictions on these measurements. In this talk I will present a brief historical review of the development of this subject, discuss our recent results and present interesting open questions. In particular, I will discuss the bounds on resources of entanglement required for instantaneous non-local measurements.

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06/02/2008  IMS seminar room 14:30:

Seth Lloyd

Non-quantum gravity

The application of concepts of quantum information to the problem of measuring spacetime geometry suggests that the spacetime metric should not be quantized. Instead, what is quantum are the clocks and signals that determine the metric. This talk presents a non-quantum theory of gravity. In this theory, intervals of space and time are derived from the states of quantum fields: the metric itself is not quantized. In non-quantum gravity, gravitational fields inherit the quantum fluctuations of the underlying quantum fields, leading to a systematic treatment of the back-reaction problem. The theory makes concrete predictions: in particular, the non-quantum gravitational fields do not exhibit independent zero-point fluctuations, predicting a suppression of tensor modes of the cosmic microwave background. Although the talk deals with quantum gravity, it will be accessible to anyone with a background in quantum information, too.

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31/01/2008  IMS seminar room 14:00:

Adolfo del Campo, University del Pais Vasco, Bilbao

Dynamics of ultracold atoms in Tonks-Girardeau regime

When an ultracold Bose gas is confined in a tight-waveguide at low enough density, the effective repulsive interactions are so large, that many properties of the system become fermion-like. This is the so-called Tonks-Girardeau regime, in which we shall describe the dynamics following a switch of the trapping potential. In particular, we shall show that the free expansion from a box lacks the self-similarity characteristic of harmonic potential case. Moreover, whenever the gas is allowed to leak out by tunnelling, its short-time dynamics exhibit a new regime of deviations from the exponential decay law. We shall make use of similar changes in the trap to propose a robust operational scheme to prepare high-fidelity atom Fock states.

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30/01/2008  IMS seminar room 14:00:

Ralf Schuetzhold, University of Dresden

Dynamical quantum phase transitions

A sweep through a quantum phase transition by means of a time-dependent external parameter (e.g., pressure) entails non-equilibrium phenomena associated with a break-down of adiabaticity: At the critical point, the energy gap vanishes and the response time diverges. Consequently, the external time-dependence inevitably drives the system out of equilibrium, i.e., away from the ground state, if we assume zero temperature initially. In this way, the initial quantum fluctuations are amplified and may become observable. By means of several examples, mostly based on ultra-cold atoms, possible effects of these amplified quantum fluctuations are studied and universal features (such as freezing) are discussed.

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15/01/2008  IMS seminar room 4 pm:

Gerard Milburn

Measurement and decoherence in mesoscopic quantum dots

Recent experimental work on mesocopic quantum dots enable the real time monitoring of a single quantum dot and coupled quantum dots. In this talk I will discuss these experiments focusing on measurement induced decoherence and conditional quantum dynamics.

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22/11/2007  IMS seminar room 2pm:

Mark Williamson (Leeds)

Parallel transport and entanglement

When the phase of the wavefunction in quantum mechanics is parallel transported i.e. moved without twisting or turning, the wavefunction can obtain a phase factor that depends only on the curvature of the surface it is parallel transported over. This phase is known as the geometric phase and in this study we use it as a tool to investigate the difference classical and quantum correlations a.k.a. entanglement make to the curvature of the state space. In the second part of the talk, instead of using the phase of the wavefunction as the rule for parallel transport we use the correlations between qubits themselves to define the rule. We show how entanglement and the Kempe invariant, a previously mysterious quantity, can be understood using this approach.

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12/12/2007  IMS seminar room all day:

Alex Retzker (Imperial), Joe Fitzsimons (Oxford) and Rob Spekkens (Cambridge) and more TBC.

QUOXIC

QUOXIC (QUantum OXford Imperial College) meetings are a series of one day quantum seminars at Oxford University and Imperial College London supported by the QIPIRC. The seminars cover different aspects of quantum computation and quantum information science and in particular create a common ground for collaboration between different branches of this diverse field. The seminars vary from tutorial-level presentations to reports on the latest research developments. Of course, participation is open to anyone, and not restricted to those from Oxford or Imperial College. More details can be found on the website which will soon be updated with titles, abstracts and the schedule. http://www.qunat.org/quoxic/

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12/12/2007  IMS Seminar Room 12pm:

Alex Retzker (Imperial), Libby Heaney (Leeds)

QUOXIC Session 1

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15/11/2007  IMS seminar room 2pm:

Stephen Bartlett (Sydney)

Quantum-computational universality and quantum phase transitions in the ground states of spin lattices

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01/11/2007  IMS seminar room 2pm:

Tony Dorlas (Dublin)

Quantum channels with memory

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25/09/2007  IMS seminar room 2pm:

Tokishiro Karasawa (NII Japan)

Dynamical Noise on Single Qubit Gates Induced by Conservation Laws

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18/09/2007  IMS seminar room 2pm:

Samuel Marcovitch (Tel-Aviv)

Is Communication Complexity Physical?

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07/06/2007  IMS seminar room 2pm:

Mathias Michel (Surrey)

Transport in Quantum Systems: An Approch Using Projection Operator Techniques

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18/05/2007  :

Hugo Cable (Louisiana)

N00N states, Metrology and Imaging

QI Live note: Only one stream is provided as this was a whiteboard talk.

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17/05/2007  :

Vladimir Korepin (SUNY, Stony Brook)

Entanglement measured by the entropy of a subsystem

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10/05/2007  :

QI@IC day II speakers

Andrew Fisher-Quantum Information Research in the London Centre for Nanotechnology Michael Hartmann-Effective Many-Body Systems in Coupled Arrays of Micro-Cavities [.PDF] Chris Phillips-Quantum Optics and strong coupling experiments with Semiconductor nanostructures. Michael Duff-Black holes, qubits and the Fano Plane

Talks that formed part of a day to connect quantum information scientists at Imperial. See also http://www3.imperial.ac.uk/quantuminformation/qiic/qiicday

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04/05/2007  :

Charles Adams

To appear

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04/05/2007  :

Axel Kuhn

To appear

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04/05/2007  :

Rosario Fazio

To appear

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04/05/2007  :

Topias Kippenberg

To appear

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04/04/2007  :

Atac Imamoglu

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04/05/2007  :

Dimitris Angelakis

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04/05/2007  IMS seminar room :

Fernando Brandao

Workshop talk: Strong atom-light interactions: theory, experiment and applications

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28/04/2007  IMS seminar room 2pm:

Pablo Barberis-Blostein (UNAM, Mexico)

Opacity of Electromagnetically Induced Transparency for Quantum Fluctuations

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19/04/2007  IMS seminar room 2pm:

Peter Ivanov (Sofia and Imperial)

Engineering of arbitrary U(N) transformation by quantum Householder reflections.

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29/03/2007  IMS seminar room 2pm:

Niel Debeaudrap (Calgary)

Efficient construction of Flows for the One-Way measurement model

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22/03/2007  IMS seminar room 2pm:

Koji Maruyama (Riken)

Observing the quantum nonlocality in the state |01>+|10> of a massive particle

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08/03/2007  IMS seminar room 2pm:

Karl Surmacz (Oxford)

Optimal Unitary Quantum Memory in Atomic Vapour Systems

In this talk I will outline the recent theoretical progress in the Oxford quantum memory group. After a brief introduction to quantum memories, I will introduce a figure of merit for a quantum memory which measures the preservation of a two qubit entangled state, with one qubit stored in and retrieved from the memory. An analytical expression for this entanglement fidelity can be calculated in terms of Gaussian fluctuations in the Hamiltonian parameters. We do this first for a general quantum memory model, and then consider a specific implementation based on stimulated Raman scattering, which theoretically allows storage of broadband photons. We show that storage of a quantum signal field with high probability can be achieved with reasonable coupling parameters. The retrieval of the field is hampered by modematching and phasematching issues, which we overcome with modifications to our scheme. To conclude we show that these modifications allow the possibility of storing a photonic qubit in a single atomic ensemble.

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03/03/2007  IMS seminar room 2pm:

Koenraad Audenaert (Imperial)

Quantum hypothesis testing

In statistics, the Chernoff information, a.k.a. the Chernoff bound, is a quantity describing how well one can distinguish between two given distributions. In a classic 1952 paper, H. Chernoff gave a closed-form formula for the asymptotic efficiency of the corresponding optimal hypothesis test, that is, for the rate at which the total error probability goes to zero when the number of draws in the test goes to infinity. In the quantum setting, one can similarly ask for the asymptotic efficiency of the optimal Hellstrom-Holevo POVM for distinguishing between to given quantum states. Finding the correct quantum generalisation of the Chernoff bound has been a long-standing problem in quantum information theory. In this talk I will discuss the two breakthrough results, the first by Nussbaum and Szkola, and the second by a group of researchers including the speaker, that taken together completely solve this important problem. The latter result is based on a remarkable new trace inequality for positive operators. I will also briefly discuss the solution of another open problem, which is the quantum generalisation of the Hoeffding bound, as obtained by Nagaoka and Hayashi, exploiting the techniques used for solving the Chernoff bound problem.

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22/02/2007  IMS seminar room 2pm:

Marcus Cramer (Imperial)

Entanglement area laws

We summarize the recent developments in entanglement scaling in many-body systems, focusing on the proven instances of area laws and violations thereof. In particular, we concentrate on Hamiltonians that are quadratic forms of either bosonic or fermionic operators. For the special case of $D$-dimensional half-spaces we find a difference in the scaling properties depending on whether the system is bosonic--- where an area-law is first proven to hold---or fermionic, extending previous findings for cubic regions. For bosonic systems with nearest neighbor interaction we prove the conjectured area-law by computing the logarithmic negativity analytically. For fermions we determine the multiplicative logarithmic correction to the area-law, which depends on the topology of the Fermi surface. QI LIVE note: we only provide a video+audio stream here (as well as downloadable slides) due to human error.

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15/02/2007  IMS seminar room :

Shashank Virmani (Imperial)

Spin Chains and channels with memory (QUOXIC talk)

In many studies of channel capacities an independent error model is assumed. However, in reality the environment will usually have a `memory\' that leads to correlations in the errors affecting successive transmissions. Despite their physical importance, `memory channels\' are exceedingly hard to analyze, and their communication capacities are rarely known. On the other hand correlations are a key concern in the seemingly unrelated area of statistical physics, and its key branch of interacting many-body systems. Here we provide a natural connection between the study of quantum memory channels and the study of the statistical mechanics of interacting many body systems. This connection enables us to develop non-trivial `exactly solvable\' models of memory channels that can display similar behaviour to many-body systems, including manifestations of `phase transitions\'. QI LIVE note: This was a whiteboard talk, hence there is only one stream.

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15/02/2007  IMS seminar room :

Yousef Ghazi-Tabatabai (Imperial )

Quantum measure theory (QUOXIC talk)

In the histories approach to quantum mechanics, the basic object is a sample space of histories, typically in configuration space, leading In the histories approach to quantum mechanics, the basic object is a sample space of histories, typically in configuration space, leading from an initial to a final state. In classical stochastic mechanics the dynamics of this system can be embodied in a probability measure mapping the power set of the sample space to R, and the classical interpretation can be viewed as a map from the sample space to {0,1} sending the `real\' history to 1 and all other histories to 0 - which can be extended to a map from the power set of the sample space to {0,1}, with a set mapped to one iff it contains the `real\' history. Quantum measure theory attempts to replicate this approach for quantum mechanics, encoding the dynamics in a more generalized `quantum measure\' that does not obey the Kolmogorov sum rule. This talk will focus on recent developments in the interpretation of quantum measure theory, generalizing the classical interpretation to a wider class of functions from the power set of the sample space to {0,1}from an initial to a final state. In classical stochastic mechanics the dynamics of this system can be embodied in a probability measure mapping the power set of the sample space to R, and the classical interpretation can be viewed as a map from the sample space to {0,1} sending the `real\' history to 1 and all other histories to 0 - which can be extended to a map from the power set of the sample space to {0,1}, with a set mapped to one iff it contains the `real\' history. Quantum measure theory attempts to replicate this approach for quantum mechanics, encoding the dynamics in a more generalized `quantum measure\' that does not obey the Kolmogorov sum rule. This talk will focus on recent developments in the interpretation of quantum measure theory, generalizing the classical interpretation to a wider class of functions from the power set of the sample space to {0,1}. QI Live note: we regret that we only provide the video+audio stream for this talk because of a technical problem. Please use the provided Powerpoint slides instead.

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15/02/2007  IMS seminar room :

Ho-Chi Lin(UCL)

A local view of continuous variable entanglement (QUOXIC talk)

We present a new approach to the analysis of entanglement in smooth bipartite continuous-variable states.  One or both parties perform projective filterings via preliminary measurements to determine whether the system is located in some region of space; we study the entanglement remaining after filtering. For small regions, a two-mode system can be approximated by a pair of qubits and its entanglement fully characterized, even for mixed states.  Our approach may be extended to any smooth bipartite pure state or two-mode mixed state, leading to natural definitions of concurrence and negativity densities: we will explicitly discuss the generalization to higher-dimensional (i.e. multimode) systems.

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15/02/2007  IMS seminar room :

David Gross (Imperial)

Computational potency of quantum many-body systems (QUOXIC talk)

We have recently described a framework which allows one to systematically construct novel schemes for measurement-based quantum computation [quant-ph/0609149]. The technique utilizes tools from many-body physics -- based on finitely correlated or projected entangled pair states -- to go beyond the cluster-state based one-way computer. A zoo of universal resource states with interesting properties has been constructed this way.

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30/01/2007  IMS seminar room 2pm:

Sandu Popescu (Bristol)

Entanglement and the Foundations of Statistical Mechanics

Statistical mechanics is one of the most successful areas of physics. Yet, almost 150 years since its inception, its foundations and basic postulates are still the subject of debate. Here we suggest that the main postulate of statistical mechanics, the equal a priori probability postulate, should be abandoned as misleading and unnecessary. We argue that it should be replaced by a general canonical principle, whose physical content is fundamentally different from the postulate it replaces: it refers to individual states, rather than to ensemble or time averages. Furthermore, whereas the original postulate is an unprovable assumption, the principle we propose is mathematically proven. The key element in this proof is the quantum entanglement between the system and its environment. Our approach separates the issue of finding the canonical state from finding out how close a system is to it, allowing us to go even beyond the usual boltzmannian situation.

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23/01/2007  IMS seminar room 2pm:

Jens Siewert (Regensburg, Germany)

Macroscopic quantum dynamics in superconducting nanocircuits

The detection of macroscopic quantum coherence in systems of small superconductors is one of the important achievements in solid-state physics during the past decade. In this talk, I will give an introduction to the basic ideas how to generate quantum dynamics in superconducting nanocircuits and then discuss various applications ranging from the detection of geometric phases to STIRAP, circuit quantum electrodynamics and quantum information processing.

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18/01/2007  IMS seminar room 2pm:

Vladimir Buzek (Slovak Academy of Sciences Bratislava, Slovak Republic)

Quantum infodynamics (An attempt to describe dynamics of open quantum systems using quantum information theory)

In my talk I will analyze how information encoded in a quantum system is “diluted” in reservoirs. In particular, I will describe a universal quantum homogenizer [1–3], which is a quantum machine that takes as an input a system qubit initially in the state  and a set of N reservoir qubits initially prepared in the same state . In the homogenizer the system qubit sequentially interacts with the reservoir qubits via the partial swap transformation. The homogenizer realizes, in the limit sense, the transformation such that at the output each qubit is in an arbitrarily small neighborhood of the state  irrespective of the initial states of the system and the reservoir qubits. This means that the system qubit undergoes an evolution that has a fixed point, which is the reservoir state . The model of the homogenizer allows us to understand various aspects of the dynamics of open systems interacting with non-equilibrium environments. In particular, the reversibility vs or irreversibility of the dynamics of the open system is directly linked to specific (classical) information about the order in which the reservoir qubits interacted with the system qubit. I will analyze possible physical realization of quantum homogenization [4]. In addition I will discuss how entanglement is established between particles involved in homogenization process [5]. Finally I will show how a master equation governing dynamics of the system qubit during the homogenization process can be derived [6]. [1] V. Scarani, M. Ziman, P. ¡Stelmachovi¡c, N. Gisin, and V. Bu¡zek: Thermalizing quantum machines: Dissipation and entanglement. Phys. Rev. Lett., 88, 097905 (2002). [2] M. Ziman, P. ¡Stelmachovi¡c, V. Bu¡zek, M. Hillery, V. Scarani, and N. Gisin: Diluting quantum information: An analysis of information transfer in system-reservoir interactions. Phys. Rev. A, 65, 042105 (2002). [3] P. ¡Stelmachovi¡c, M. Ziman, and V. Bu¡zek: Microscopic description of information transfer from a qudit to reservoir. Fortschritte der Physik, 51, 280 (2003). [4] D. Nagaj, P. ¡Stelmachovi¡c, V. Bu¡zek, and M. S. Kim: Quantum homogenization for continuous variables: Realization with linear optical elements. Phys. Rev. A, 66: 062307, (2002). [5] M. Ziman, P. ¡Stelmachovi¡c, and V. Bu¡zek: Saturation of Coffman-Kundu-Wootters inequality via quantum homogenization. J. Opt. B: Quantum Semiclassical Optics, 5, S439 (2003). [6] P. ¡Stelmachovi¡c, M. Ziman, V. Bu¡zek, P. Rap¡can, and M. Koniorczyk: Quantum infodynamics: Describtion of dynamics of open quantum systems using the tools of quantum information theory To be submitted (2007).

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08/01/2007  IMS seminar room 2pm:

Gerard Milburn (Queensland, Australia)

Measurement and Control of Quantum Electromechanical Systems

Quantum electromechanical systems (QEMS) are nano-fabricated mechanical resonators with vibrational frequencies approaching GHz, operated at low temperatures (mK). Under these conditions the mechanical vibration must be described quantum mechanically. Typically transducers for these systems are based on single electronics, but in this talk I will describe how superconducting microwave waveguides may be used to monitor the motion of the resonator. There is sufficient control over these systems to develop schemes for feedback mediated entanglement engineering as well as phonon counting measurements. I will briefly discuss a nano-photonic scheme for QEMS measurement based on NV centres in a diamond nanomechanical resonantor. QI Live Comment: This was the first QI Live talk. There was an unforeseen data-flow bottleneck (which was fixed for later broadcasts). Because of this we do not provide the .rv version of the slides as for later broadcasts.

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