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Search Results: 1 - 10 of 462003 matches for " A. Ekert "
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Against Quantum Noise
A. Ekert,C. Macchiavello
Physics , 1999,
Abstract: This is a brief description of how to protect quantum states from dissipation and decoherence that arise due to uncontrolled interactions with the environment. We discuss recoherence and stabilisation of quantum states based on two techniques known as "symmetrisation" and "quantum error correction". We illustrate our considerations with the most popular quantum-optical model of the system-environment interaction, commonly used to describe spontaneous emission, and show the benefits of quantum error correction in this case.
Error Correction in Quantum Communication
A. Ekert,C. Macchiavello
Physics , 1996,
Abstract: We show how procedures which can correct phase and amplitude errors can be directly applied to correct errors due to quantum entanglement. We specify general criteria for quantum error correction, introduce quantum versions of the Hamming and the Gilbert-Varshamov bounds and comment on the practical implementation of quantum codes.
Universality in Quantum Computation
D. Deutsch,A. Barenco,A. Ekert
Physics , 1995, DOI: 10.1098/rspa.1995.0065
Abstract: We show that in quantum computation almost every gate that operates on two or more bits is a universal gate. We discuss various physical considerations bearing on the proper definition of universality for computational components such as logic gates.
Quantum Networks for Elementary Arithmetic Operations
V. Vedral,A. Barenco,A. Ekert
Physics , 1995, DOI: 10.1103/PhysRevA.54.147
Abstract: Quantum computers require quantum arithmetic. We provide an explicit construction of quantum networks effecting basic arithmetic operations: from addition to modular exponentiation. Quantum modular exponentiation seems to be the most difficult (time and space consuming) part of Shor's quantum factorising algorithm. We show that the auxiliary memory required to perform this operation in a reversible way grows linearly with the size of the number to be factorised.
Optimal universal quantum cloning and state estimation
D. Bruss,A. Ekert,C. Macchiavello
Physics , 1997, DOI: 10.1103/PhysRevLett.81.2598
Abstract: We derive a tight upper bound for the fidelity of a universal N to M qubit cloner, valid for any M \geq N, where the output of the cloner is required to be supported on the symmetric subspace. Our proof is based on the concatenation of two cloners and the connection between quantum cloning and quantum state estimation. We generalise the operation of a quantum cloner to mixed and/or entangled input qubits described by a density matrix supported on the symmetric subspace of the constituent qubits. We also extend the validity of optimal state estimation methods to inputs of this kind.
Conditional Quantum Dynamics and Logic Gates
A. Barenco,D. Deutsch,A. Ekert,R. Jozsa
Physics , 1995, DOI: 10.1103/PhysRevLett.74.4083
Abstract: Quantum logic gates provide fundamental examples of conditional quantum dynamics. They could form the building blocks of general quantum information processing systems which have recently been shown to have many interesting non--classical properties. We describe a simple quantum logic gate, the quantum controlled--NOT, and analyse some of its applications. We discuss two possible physical realisations of the gate; one based on Ramsey atomic interferometry and the other on the selective driving of optical resonances of two subsystems undergoing a dipole--dipole interaction.
Geometric quantum computation with NMR
J. A. Jones,V. Vedral,A. Ekert,G. Castagnoli
Physics , 1999, DOI: 10.1038/35002528
Abstract: The experimental realisation of the basic constituents of quantum information processing devices, namely fault-tolerant quantum logic gates, requires conditional quantum dynamics, in which one subsystem undergoes a coherent evolution that depends on the quantum state of another subsystem. In particular, the subsystem may acquire a conditional phase shift. Here we consider a novel scenario in which this phase is of geometric rather than dynamical origin. As the conditional geometric (Berry) phase depends only on the geometry of the path executed it is resilient to certain types of errors, and offers the potential of an intrinsically fault-tolerant way of performing quantum gates. Nuclear Magnetic Resonance (NMR) has already been used to demonstrate both simple quantum information processing and Berry's phase. Here we report an NMR experiment which implements a conditional Berry phase, and thus a controlled phase shift gate. This constitutes the first elementary geometric quantum computation.
Optimal State Discrimination Using Particle Statistics
S. Bose,A. Ekert,Y. Omar,N. Paunkovic,V. Vedral
Physics , 2003, DOI: 10.1103/PhysRevA.68.052309
Abstract: We present an application of particle statistics to the problem of optimal ambiguous discrimination of quantum states. The states to be discriminated are encoded in the internal degrees of freedom of identical particles, and we use the bunching and antibunching of the external degrees of freedom to discriminate between various internal states. We show that we can achieve the optimal single-shot discrimination probability using only the effects of particle statistics. We discuss interesting applications of our method to detecting entanglement and purifying mixed states. Our scheme can easily be implemented with the current technology.
Distributed Quantum Computation over Noisy Channels
J. I. Cirac,A. Ekert,S. F. Huelga,C. Macchiavello
Physics , 1998, DOI: 10.1103/PhysRevA.59.4249
Abstract: We analyse the use of entangled states to perform quantum computations non locally among distant nodes in a quantum network. The complexity associated with the generation of multiparticle entangled states is quantified in terms of the concept of global cost. This parameter allows us to compare the use of physical resources in different schemes. We show that for ideal channels and for a sufficiently large number of nodes, the use of maximally entangled states is advantageous over uncorrelated ones. For noisy channels, one has to use entanglement purification procedures in order to create entangled states of high fidelity. We show that under certain circumstances a quantum network supplied with a maximally entangled input still yields a smaller global cost, provided that $n$ belongs to a given interval $n\in [n_{min},n_{max}]$. The values of $n_{min}$ and $n_{max}$ crucially depend on the purification protocols used to establish the $n$-- processor entangled states, as well as on the presence of decoherence processes during the computation. The phase estimation problem has been used to illustrate this fact.
Optimal purification of single qubits
J. I. Cirac,A. K. Ekert,C. Macchiavello
Physics , 1998, DOI: 10.1103/PhysRevLett.82.4344
Abstract: We introduce a new decomposition of the multiqubit states of the form $\rho^{\otimes N}$ and employ it to construct the optimal single qubit purification procedure. The same decomposition allows us to study optimal quantum cloning and state estimation of mixed states.
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