摘要:

The essential operations of a quantum computer can be accomplished using solely optical elements, with different polarization or spatial modes representing the individual qubits. We present a simple all-optical implementation of Grover's algorithm for efficient searching, in which a database of four elements is searched with a single query. By ‘compiling’ the actual set-up, we have reduced the required number of optical elements from 24 to only 12. We discuss the extension to large databases, and the limitations of these techniques.

ISSN:0950-0340    

DOI:10.1080/09500340008244040

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We derive an experimentally testable criterion for the teleportation of quantum states of continuous variables. This criterion is especially relevant to the recent experiment of Furusawaet al.where an input—output fidelity of 0.58 ± 0.02 was achieved for optical coherent stages. Our derivation demonstrates that fidelities greater than 1/2 could not have been achieved through the use of a classical channel alone; quantum entanglement was a crucial ingredient in the experiment.

ISSN:0950-0340    

DOI:10.1080/09500340008244041

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Quantum teleportation is one of the essential primitives of quantum communication. We suggest that any quantum teleportation scheme can be characterized by its efficiency, i.e. how often it succeeds to teleport, its fidelity, i.e. how well the input state is reproduced at the output, and by its insensitivity to cross talk, i.e. how well it rejects an input state that is not intended to teleport. We discuss these criteria for the two teleportation experiments of independent qubits which have been performed thus far. In the first experiment (1997,Nature,390, 575) where the qubit states were various different polarization states of photons, the fidelity of teleportation was as high as 0.80 ± 0.05 thus clearly surpassing the limit of 2/3 which can, in principle, be obtained by a direct measurement on the qubit and classical communication. This high fidelity is confirmed in our second experiment (1998,Phys. Rev. Lett.,80, 3891), demonstrating entanglement swapping, that is, realizing the teleportation of a qubit which itself is still entangled to another one. This experiment is the only one to date that demonstrates the teleportation of a genuine unknown quantum state.

ISSN:0950-0340    

DOI:10.1080/09500340008244042

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Ideal dense coding protocols allow one to use prior maximal entanglement to send two bits of classical information by the physical transfer of a single encoded qubit. We investigate the case when the prior entanglement is not maximal and the initial state of the entangled pair of qubits being used for the dense coding is a mixed state. We find upper and lower bounds on the capability to do dense coding in terms of the various measures of entanglement. Our results can also be reinterpreted as giving bounds on purification procedures in terms of dense coding capacities.

ISSN:0950-0340    

DOI:10.1080/09500340008244043

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The concept of quantum information in a state is analysed. For a component of an entangled state, the potential measurements are claimed to define a context, against which the information is to be interpreted. Projective measurements are shown to allow the processing of information contained in the incoming quantum states. Finally the process of a quantum measurement is considered as a mapping between different parts of the system, a quantum information transfer.

ISSN:0950-0340    

DOI:10.1080/09500340008244044

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We analyse the role of entanglement for transmission of classical information through a memoryless depolarizing channel. Using the isotropic character of this channel we prove analytically that the mutual information cannot be increased by encoding classical bits into entangled states of two qubits.

ISSN:0950-0340    

DOI:10.1080/09500340008244045

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The performance of fault-tolerant quantum computation with concatenated codes using local gates in small numbers of spatial dimensions is discussed. It is shown that a threshold result still exists in three, two, or one spatial dimensions when next-to-nearest-neighbour gates are available, and explicit constructions are presented. In two or three dimensions, it is also shown how nearest-neighbour gates can give a threshold result. In all cases, it is simply demonstrated that a threshold exists, and no attempt to optimize the error correction circuit or to determine the exact value of the threshold is made. The additional overhead due to the fault-tolerance in both space and time is polylogarithmic in the error rate per logical gate.

ISSN:0950-0340    

DOI:10.1080/09500340008244046

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We define the binding entanglement channel as the quantum channel through which quantum information cannot be reliably transmitted, but which can be used to share bound entanglement. We provide a characterization of this class of channels. We also show that any bound entangled state can be used for the construction of a map corresponding to the binding entanglement channel.

ISSN:0950-0340    

DOI:10.1080/09500340008244047

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In the context of quantifying entanglement we study those functions of a multipartite state which do not increase under the set of local transformations. A mathematical characterization of these monotone magnitudes is presented. They are then related to optimal strategies of conversion of shared states. More detailed results are presented for pure states of bipartite systems. It is shown that more than one measure is required simultaneously in order to quantify completely the non-local resources contained in a bipartite pure state, while examining how this fact does not hold in the so-called asymptotic limit. Finally, monotonicity under local transformations is proposed as the only natural requirement for measures of entanglement.

ISSN:0950-0340    

DOI:10.1080/09500340008244048

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Two families of bipartite mixed quantum states are studied for which it is proved that the number of members in the optimal-decomposition ensemble—the ensemble realizing the entanglement of formation—is greater than the rank of the mixed state. We find examples for which the number of states in this optimal ensemble can be larger than the rank by an arbitrarily large factor. In one case the proof relies on the fact that the partial transpose of the mixed state has zero eigenvalues; in the other case the result arises from the properties of product bases that are completable only by embedding in a larger Hilbert space.

ISSN:0950-0340    

DOI:10.1080/09500340008244049

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor