ISSN:0950-0340    

DOI:10.1080/09500340008244030

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

I consider some promising future directions for quantum information theory that could influence the development of 21st century physics. Advances in the theory of the distinguishability of superoperators may lead to new strategies for improving the precision of quantum-limited measurements. A better grasp of the properties of multi-partite quantum entanglement may lead to deeper understanding of strongly-coupled dynamics in quantum many-body systems, quantum field theory, and quantum gravity.

ISSN:0950-0340    

DOI:10.1080/09500340008244031

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Two observers (Alice and Bob) independently prepare two sets of singlets. They test one particle of each singlet along an arbitrarily chosen direction and send the other particle to a third observer, Eve. At a later time, Eve performs joint tests on pairs of particles (one from Alice and one from Bob). According to Eve's choice of test and to her results, Alice and Bob can sort into subsets the samples that they have already tested, and they can verify that each subset behaves as if it consisted of entangled pairs of distant particles that have never communicated in the past, even indirectly via other particles.

ISSN:0950-0340    

DOI:10.1080/09500340008244032

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We propose an experiment to demonstrate the intrinsic difference between quantum and classical information by applying Hardy's theorem to two ions trapped in a Paul trap.

ISSN:0950-0340    

DOI:10.1080/09500340008244033

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We describe some applications of quantum information theory to the analysis of quantum limits on measurement sensitivity. A measurement of a weak force acting on a quantum system is a determination of a classical parameter appearing in the master equation that governs the evolution of the system; limitations on measurement accuracy arise because it is not possible to distinguish perfectly among the different possible values of this parameter. Tools developed in the study of quantum information and computation can be exploited to improve the precision of physics experiments; examples include superdense coding, fast database search, and the quantum Fourier transform.

ISSN:0950-0340    

DOI:10.1080/09500340008244034

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We study a 2-spin quantum Turing architecture, in which discrete local rotations {αm} of the Turing-head spin alternate with quantum controlled NOT operations. We show that a single chaotic parameter input {αm} leads to a chaotic dynamics in the entire Hilbert space. The instability of periodic orbits on the Turing head and ‘chaos swapping’ onto the Turing tape are demonstrated explicitly as well as exponential parameter sensitivity of the Bures metric.

ISSN:0950-0340    

DOI:10.1080/09500340008244035

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A family of asymmetric cloning machines forN-dimensional quantum states is introduced. These machines produce two imperfect copies of a single state that emerge from non-identical Heisenberg channels. The trade-off between the quality of the copies imposed by quantum mechanics is shown to result from a complementarity akin to the Heisenberg uncertainty principle. More specifically, the probability distributions of the error operators affecting the two copies are the square modulus of two functions related by a Fourier transform. A no-cloning inequality is derived for the special case of isotropic cloners, quantifying the impossibility of perfect cloning: if πaand πbare the depolarizing fractions associated with the two copies, the domain in (π1/2a, π1/2b)-space located inside a particular ellipse representing close-to-perfect cloning is forbidden. More generally, an entropic no-cloning uncertainty relation is also discussed. Finally, the class of asymmetric cloning machines for quantum bits is investigated in detail, and a connection with the capacity of the Pauli channel is displayed.

ISSN:0950-0340    

DOI:10.1080/09500340008244036

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The action of a NOT gate on a classical bit results in a change of its value from a 0 to a 1 andvice versa. The action of the classical NOT gate is in principle perfect because with fidelity equal to unity it complements the value of a bit. The action of the quantum NOT gate in a computational basis |0⟩ and |1⟩ is very similar to the action of the classical NOT gate. However, a more general quantum mechanical operation which corresponds to a classical NOT gate would take a qubit in anarbitrarystate |Ψ⟩ and produce a qubit in the state |Ψ⊥⟩ orthogonal to |Ψ⟩. This operation is anti-unitary and therefore, cannot be realized exactly.So how well we can do?We find a unitary transformation acting on an input qubit and some auxiliary qubits, which represent degrees of freedom of the quantum NOT gate itself, which approximately realizes the NOT operation on the state of the original qubit. We call this ‘device’ auniversal-NOT gate because the size of the error it produces is independent of the input state. We show that anoptimalU-NOT gate which has as its inputNidentical qubits and producesMoutputs achieves a fidelity ofF= (N+ 1)/ (N+ 2), which is equal to the fidelity of estimation of the input qubits. We also show that whena prioriinformation about the state of the input qubit is available, the fidelity of a quantum NOT gate can be much better than the fidelity of estimation.

ISSN:0950-0340    

DOI:10.1080/09500340008244037

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We present ways of realizing quantum cloning via stimulated emission. Universality of the cloning procedure is achieved by choosing systems that have appropriate symmetries. We first discuss a scheme based on certain three-level systems, e.g. atoms in a cavity. Our numerical results show that this scheme approaches optimal cloning for short interaction times. Then we demonstrate that optimal universal cloning can be realized using parametric down-conversion. At the same time, our down-conversion scheme also implements the optimal universal NOT operation. We conclude with some remarks on cloning and superluminal signalling, using our cloner as an illustrative example.

ISSN:0950-0340    

DOI:10.1080/09500340008244038

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We consider the problem of multiparty teleportation, whereNcopies of a qubit are teleported toNdifferent locations. We show how to achieve this by measuring an appropriate POVM and using the multiparticle entangled state introduced in the context of telecloning. Our procedure also gives the optimalN←Mtelecloning.

ISSN:0950-0340    

DOI:10.1080/09500340008244039

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor