Quantum state tomography, the ability to deduce the density matrix of a quantum system from measu... more Quantum state tomography, the ability to deduce the density matrix of a quantum system from measured data, is of fundamental importance for the verification of present and future quantum devices. It has been realized in systems with few components but for larger systems it becomes rapidly infeasible because the number of quantum measurements and computational resources required to process them
We propose a driven optical cavity quantum electrodynamics (QED) set up aimed at directly probing... more We propose a driven optical cavity quantum electrodynamics (QED) set up aimed at directly probing energy transport dynamics in photosynthetic biomolecules. We show that detailed information concerning energy transfer paths and delocalization of exciton states can be inferred (and exciton energies estimated) from the statistical properties of the emitted photons. This approach provides us with a novel spectroscopic tool for
Natural and artificial light-harvesting processes have recently gained new interest. Signatures o... more Natural and artificial light-harvesting processes have recently gained new interest. Signatures of long-lasting coherence in spectroscopic signals of biological systems have been repeatedly observed, albeit their origin is a matter of ongoing debate, as it is unclear how the loss of coherence due to interaction with the noisy environments in such systems is averted. Here we report experimental and theoretical verification of coherent exciton-vibrational (vibronic) coupling as the origin of long-lasting coherence in an artificial light harvester, a molecular J-aggregate. In this macroscopically aligned tubular system, polarization-controlled 2D spectroscopy delivers an uncongested and specific optical response as an ideal foundation for an in-depth theoretical description. We derive analytical expressions that show under which general conditions vibronic coupling leads to prolonged excited-state coherence.
The stabiliser formalism allows the efficient description of a sizeable class of pure as well as ... more The stabiliser formalism allows the efficient description of a sizeable class of pure as well as mixed quantum states of n-qubit systems. That same formalism has important applications in the field of quantum error correcting codes, where mixed stabiliser states correspond to projectors on subspaces associated with stabiliser codes.
Entanglement manipulation, and especially Entanglement Swapping is at the heart of current work o... more Entanglement manipulation, and especially Entanglement Swapping is at the heart of current work on quantum information processing, purification and quantum teleportation. We will discuss how it may be generalized to multiparticle systems and how this enables multi-user quantum cryptographic protocols to be developed. Our scheme allows us to establish multiparticle entanglement between particles which belong to distant users in a
ABSTRACT We apply quantum control techniques to a long spin chain by acting only on two qubits at... more ABSTRACT We apply quantum control techniques to a long spin chain by acting only on two qubits at one of its ends, thereby implementing universal quantum computation by a combination of quantum gates on these qubits and indirect SWAP operations across the chain. It is shown that the control sequences can be computed and implemented efficiently. We discuss the application of these ideas to physical systems such as superconducting qubits in which full control of long chains is challenging.
We present an analytical formula for the asymptotic relative entropy of entanglement with respect... more We present an analytical formula for the asymptotic relative entropy of entanglement with respect to positive partial transpose states for Werner states of arbitrary dimension. We then demonstrate its validity using methods from convex optimization. This is the first case in which the asymptotic value of a subadditive entanglement measure has been calculated.
We present a detailed study on the possibility of manipulating quantum information encoded in the... more We present a detailed study on the possibility of manipulating quantum information encoded in the "radial" modes of arrays of trapped ions (i.e., in the ions' oscillations orthogonal to the trap's main axis). In such systems, because of the tightness of transverse confinement, the radial modes pertaining to different ions can be addressed individually. In the first part of the paper we show that, if local control of the radial trapping frequencies is available, any linear optical and squeezing operation on the locally defined modes -on single as well as on many modes -can be reproduced by manipulating the frequencies. Then, we proceed to describe schemes apt to generate unprecedented degrees of bipartite and multipartite continuous variable entanglement under realistic noisy working conditions, and even restricting only to a global control of the trapping frequencies. Furthermore, we consider the transmission of the quantum information encoded in the radial modes along the array of ions, and show it to be possible to a remarkable degree of accuracy, for both finite-dimensional and continuous variable quantum states. Finally, as an application, we show that the states which can be generated in this setting allow for the violation of multipartite non-locality tests, by feasible displaced parity measurements. Such a demonstration would be a first test of quantum non-locality for "massive" degrees of freedom (i.e., for degrees of freedom describing the motion of massive particles).
... The goal will be to use these energy shifts to create a conditional dynamics of the two qubit... more ... The goal will be to use these energy shifts to create a conditional dynamics of the two qubits. ... In summary, we have demonstrated that a system con-sisting of ions trapped inside of a far detuned optical cavity can be used to implement coherent quantum in-formation ...
This article discusses the concept of information and its intimate relationship with physics. Aft... more This article discusses the concept of information and its intimate relationship with physics. After an introduction of all the necessary quantum mechanical and information theoretical concepts we analyse Landauer's principle which states that the erasure of information is inevitably accompanied by the generation of heat. We employ this principle to rederive a number of results in classical and quantum information theory whose rigorous mathematical derivations are difficult. This demonstrates the usefulness of Landauer's principle and provides an introduction to the physical theory of information.
We investigate the entanglement between two separated segments in the vacuum state of a free 1D K... more We investigate the entanglement between two separated segments in the vacuum state of a free 1D Klein-Gordon field, where explicit computations are performed in the continuum limit of the linear harmonic chain. We show that the entanglement, which we measure by the logarithmic negativity, is finite with no further need for renormalization. We find that the quantum correlations decay much
In this paper, we present new progress on the study of the symmetric extension criterion for sepa... more In this paper, we present new progress on the study of the symmetric extension criterion for separability. First, we show that a perturbation of order O(1/N ) is sufficient and, in general, necessary to destroy the entanglement of any state admitting an N Bose symmetric extension. On the other hand, the minimum amount of local noise necessary to induce separability on states arising from N Bose symmetric extensions with Positive Partial Transpose (PPT) decreases at least as fast as O(1/N 2 ). From these results, we derive upper bounds on the time and space complexity of the weak membership problem of separability when attacked via algorithms that search for PPT symmetric extensions. Finally, we show how to estimate the error we incur when we approximate the set of separable states by the set of (PPT) N -extendable quantum states in order to compute the maximum average fidelity in pure state estimation problems, the maximal output purity of quantum channels, and the geometric measure of entanglement.
Demonstrating the subtraction of a single photon from one mode of a two-mode squeezed state repre... more Demonstrating the subtraction of a single photon from one mode of a two-mode squeezed state represents a significant step in the realization of continuous-variable quantum repeater protocols [1, 2]. Photon subtraction is a non-Gaussian operation, taking Gaussian phase-space quasi-probability distributions (such as Wigner functions) to non-Gaussian phase-space distributions, and can increase entanglement [3]. Non-Gaussian operations are neccesary for continuousvariable entanglement distillation [2, 4]. Entanglement increase has been demonstrated by using a ...
We show how entanglement may be quantified in spin and cold atom many-body systems using standard... more We show how entanglement may be quantified in spin and cold atom many-body systems using standard experimental techniques only. The scheme requires no assumptions on the state in the laboratory and a lower bound to the entanglement can be read off directly from the scattering cross section of Neutrons deflected from solid state samples or the time-of-flight distribution of cold atoms in optical lattices, respectively. This removes a major obstacle which so far has prevented the direct and quantitative experimental study of genuine quantum correlations in many-body systems: The need for a full characterization of the state to quantify the entanglement contained in it. Instead, the scheme presented here relies solely on global measurements that are routinely performed and is versatile enough to accommodate systems and measurements different from the ones we exemplify in this work.
2008 Quantum Electronics and Laser Science Conference (QELS), 2008
A quantum protocol or experiment can be divided into three stages: preparation, processing, and m... more A quantum protocol or experiment can be divided into three stages: preparation, processing, and measurement. Quantum state [1, 2, 3] and process tomography [4, 5] respectively prescribe a procedure to completely characterise the first two stages and have been successfully shown experimentally. We complete this triad by implementing a method to determine the set of POVMs for an arbitrary quantum detector [6]. Thus without any knowledge of the inner workings of the detector we can predict its response to any input. Precise knowledge of the detector ...
2009 Conference on Lasers & Electro Optics & The Pacific Rim Conference on Lasers and Electro-Optics (CLEO/PACIFIC RIM), 2009
HB Coldenstrodt-Ronge1, AP Worsley1, JS Lundeen1,2, A. Feito3,4, KL Pregnell3,4, Ch. Silberhorn5,... more HB Coldenstrodt-Ronge1, AP Worsley1, JS Lundeen1,2, A. Feito3,4, KL Pregnell3,4, Ch. Silberhorn5, PJ Mosley1,5, BJ Smith1, G. Puentes1, N. Thomas-Peter1, TC Ralph6, J. Eisert3,4, MB Plenio3,4 and IA Walmsley1 ... 1Clarendon Laboratory, University of Oxford, Oxford, 0X1 3PU, UK 2Institute for National Measurement Standards, Ottawa, ON, Canada K1A 0R6 3Institute for Mathematical Sciences, Imperial College London, SW7 2PE, UK 4QOLS, The Blackett Laboratory, Imperial College London, Prince Consort Rd., SW7 2BW, UK 5Max- ...
Quantum computers require quantum logic, something fundamentally different to classical Boolean l... more Quantum computers require quantum logic, something fundamentally different to classical Boolean logic. This difference leads to a greater efficiency of quantum computation over its classical counter-part. In this review we explain the basic principles of quantum computation, including the construction of basic gates, and networks. We illustrate the power of quantum algorithms using the simple problem of Deutsch, and explain, again in very simple terms, the well known algorithm of Shor for factorisation of large numbers into primes. We then describe physical implementations of quantum computers, focusing on one in particular, the linear ion-trap realization. We explain that the main obstacle to building an actual quantum computer is the problem of decoherence, which we show may be circumvented using the methods of quantum error correction.
Quantum state tomography, the ability to deduce the density matrix of a quantum system from measu... more Quantum state tomography, the ability to deduce the density matrix of a quantum system from measured data, is of fundamental importance for the verification of present and future quantum devices. It has been realized in systems with few components but for larger systems it becomes rapidly infeasible because the number of quantum measurements and computational resources required to process them
We propose a driven optical cavity quantum electrodynamics (QED) set up aimed at directly probing... more We propose a driven optical cavity quantum electrodynamics (QED) set up aimed at directly probing energy transport dynamics in photosynthetic biomolecules. We show that detailed information concerning energy transfer paths and delocalization of exciton states can be inferred (and exciton energies estimated) from the statistical properties of the emitted photons. This approach provides us with a novel spectroscopic tool for
Natural and artificial light-harvesting processes have recently gained new interest. Signatures o... more Natural and artificial light-harvesting processes have recently gained new interest. Signatures of long-lasting coherence in spectroscopic signals of biological systems have been repeatedly observed, albeit their origin is a matter of ongoing debate, as it is unclear how the loss of coherence due to interaction with the noisy environments in such systems is averted. Here we report experimental and theoretical verification of coherent exciton-vibrational (vibronic) coupling as the origin of long-lasting coherence in an artificial light harvester, a molecular J-aggregate. In this macroscopically aligned tubular system, polarization-controlled 2D spectroscopy delivers an uncongested and specific optical response as an ideal foundation for an in-depth theoretical description. We derive analytical expressions that show under which general conditions vibronic coupling leads to prolonged excited-state coherence.
The stabiliser formalism allows the efficient description of a sizeable class of pure as well as ... more The stabiliser formalism allows the efficient description of a sizeable class of pure as well as mixed quantum states of n-qubit systems. That same formalism has important applications in the field of quantum error correcting codes, where mixed stabiliser states correspond to projectors on subspaces associated with stabiliser codes.
Entanglement manipulation, and especially Entanglement Swapping is at the heart of current work o... more Entanglement manipulation, and especially Entanglement Swapping is at the heart of current work on quantum information processing, purification and quantum teleportation. We will discuss how it may be generalized to multiparticle systems and how this enables multi-user quantum cryptographic protocols to be developed. Our scheme allows us to establish multiparticle entanglement between particles which belong to distant users in a
ABSTRACT We apply quantum control techniques to a long spin chain by acting only on two qubits at... more ABSTRACT We apply quantum control techniques to a long spin chain by acting only on two qubits at one of its ends, thereby implementing universal quantum computation by a combination of quantum gates on these qubits and indirect SWAP operations across the chain. It is shown that the control sequences can be computed and implemented efficiently. We discuss the application of these ideas to physical systems such as superconducting qubits in which full control of long chains is challenging.
We present an analytical formula for the asymptotic relative entropy of entanglement with respect... more We present an analytical formula for the asymptotic relative entropy of entanglement with respect to positive partial transpose states for Werner states of arbitrary dimension. We then demonstrate its validity using methods from convex optimization. This is the first case in which the asymptotic value of a subadditive entanglement measure has been calculated.
We present a detailed study on the possibility of manipulating quantum information encoded in the... more We present a detailed study on the possibility of manipulating quantum information encoded in the "radial" modes of arrays of trapped ions (i.e., in the ions' oscillations orthogonal to the trap's main axis). In such systems, because of the tightness of transverse confinement, the radial modes pertaining to different ions can be addressed individually. In the first part of the paper we show that, if local control of the radial trapping frequencies is available, any linear optical and squeezing operation on the locally defined modes -on single as well as on many modes -can be reproduced by manipulating the frequencies. Then, we proceed to describe schemes apt to generate unprecedented degrees of bipartite and multipartite continuous variable entanglement under realistic noisy working conditions, and even restricting only to a global control of the trapping frequencies. Furthermore, we consider the transmission of the quantum information encoded in the radial modes along the array of ions, and show it to be possible to a remarkable degree of accuracy, for both finite-dimensional and continuous variable quantum states. Finally, as an application, we show that the states which can be generated in this setting allow for the violation of multipartite non-locality tests, by feasible displaced parity measurements. Such a demonstration would be a first test of quantum non-locality for "massive" degrees of freedom (i.e., for degrees of freedom describing the motion of massive particles).
... The goal will be to use these energy shifts to create a conditional dynamics of the two qubit... more ... The goal will be to use these energy shifts to create a conditional dynamics of the two qubits. ... In summary, we have demonstrated that a system con-sisting of ions trapped inside of a far detuned optical cavity can be used to implement coherent quantum in-formation ...
This article discusses the concept of information and its intimate relationship with physics. Aft... more This article discusses the concept of information and its intimate relationship with physics. After an introduction of all the necessary quantum mechanical and information theoretical concepts we analyse Landauer's principle which states that the erasure of information is inevitably accompanied by the generation of heat. We employ this principle to rederive a number of results in classical and quantum information theory whose rigorous mathematical derivations are difficult. This demonstrates the usefulness of Landauer's principle and provides an introduction to the physical theory of information.
We investigate the entanglement between two separated segments in the vacuum state of a free 1D K... more We investigate the entanglement between two separated segments in the vacuum state of a free 1D Klein-Gordon field, where explicit computations are performed in the continuum limit of the linear harmonic chain. We show that the entanglement, which we measure by the logarithmic negativity, is finite with no further need for renormalization. We find that the quantum correlations decay much
In this paper, we present new progress on the study of the symmetric extension criterion for sepa... more In this paper, we present new progress on the study of the symmetric extension criterion for separability. First, we show that a perturbation of order O(1/N ) is sufficient and, in general, necessary to destroy the entanglement of any state admitting an N Bose symmetric extension. On the other hand, the minimum amount of local noise necessary to induce separability on states arising from N Bose symmetric extensions with Positive Partial Transpose (PPT) decreases at least as fast as O(1/N 2 ). From these results, we derive upper bounds on the time and space complexity of the weak membership problem of separability when attacked via algorithms that search for PPT symmetric extensions. Finally, we show how to estimate the error we incur when we approximate the set of separable states by the set of (PPT) N -extendable quantum states in order to compute the maximum average fidelity in pure state estimation problems, the maximal output purity of quantum channels, and the geometric measure of entanglement.
Demonstrating the subtraction of a single photon from one mode of a two-mode squeezed state repre... more Demonstrating the subtraction of a single photon from one mode of a two-mode squeezed state represents a significant step in the realization of continuous-variable quantum repeater protocols [1, 2]. Photon subtraction is a non-Gaussian operation, taking Gaussian phase-space quasi-probability distributions (such as Wigner functions) to non-Gaussian phase-space distributions, and can increase entanglement [3]. Non-Gaussian operations are neccesary for continuousvariable entanglement distillation [2, 4]. Entanglement increase has been demonstrated by using a ...
We show how entanglement may be quantified in spin and cold atom many-body systems using standard... more We show how entanglement may be quantified in spin and cold atom many-body systems using standard experimental techniques only. The scheme requires no assumptions on the state in the laboratory and a lower bound to the entanglement can be read off directly from the scattering cross section of Neutrons deflected from solid state samples or the time-of-flight distribution of cold atoms in optical lattices, respectively. This removes a major obstacle which so far has prevented the direct and quantitative experimental study of genuine quantum correlations in many-body systems: The need for a full characterization of the state to quantify the entanglement contained in it. Instead, the scheme presented here relies solely on global measurements that are routinely performed and is versatile enough to accommodate systems and measurements different from the ones we exemplify in this work.
2008 Quantum Electronics and Laser Science Conference (QELS), 2008
A quantum protocol or experiment can be divided into three stages: preparation, processing, and m... more A quantum protocol or experiment can be divided into three stages: preparation, processing, and measurement. Quantum state [1, 2, 3] and process tomography [4, 5] respectively prescribe a procedure to completely characterise the first two stages and have been successfully shown experimentally. We complete this triad by implementing a method to determine the set of POVMs for an arbitrary quantum detector [6]. Thus without any knowledge of the inner workings of the detector we can predict its response to any input. Precise knowledge of the detector ...
2009 Conference on Lasers & Electro Optics & The Pacific Rim Conference on Lasers and Electro-Optics (CLEO/PACIFIC RIM), 2009
HB Coldenstrodt-Ronge1, AP Worsley1, JS Lundeen1,2, A. Feito3,4, KL Pregnell3,4, Ch. Silberhorn5,... more HB Coldenstrodt-Ronge1, AP Worsley1, JS Lundeen1,2, A. Feito3,4, KL Pregnell3,4, Ch. Silberhorn5, PJ Mosley1,5, BJ Smith1, G. Puentes1, N. Thomas-Peter1, TC Ralph6, J. Eisert3,4, MB Plenio3,4 and IA Walmsley1 ... 1Clarendon Laboratory, University of Oxford, Oxford, 0X1 3PU, UK 2Institute for National Measurement Standards, Ottawa, ON, Canada K1A 0R6 3Institute for Mathematical Sciences, Imperial College London, SW7 2PE, UK 4QOLS, The Blackett Laboratory, Imperial College London, Prince Consort Rd., SW7 2BW, UK 5Max- ...
Quantum computers require quantum logic, something fundamentally different to classical Boolean l... more Quantum computers require quantum logic, something fundamentally different to classical Boolean logic. This difference leads to a greater efficiency of quantum computation over its classical counter-part. In this review we explain the basic principles of quantum computation, including the construction of basic gates, and networks. We illustrate the power of quantum algorithms using the simple problem of Deutsch, and explain, again in very simple terms, the well known algorithm of Shor for factorisation of large numbers into primes. We then describe physical implementations of quantum computers, focusing on one in particular, the linear ion-trap realization. We explain that the main obstacle to building an actual quantum computer is the problem of decoherence, which we show may be circumvented using the methods of quantum error correction.
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Papers by Martin Plenio