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arXiv:quant-ph/9906018 · Journal of Experimental and Theoretical Physics

Teleportation of a continuous variable

S. N. Molotkov, S. S. Nazin

Published Sep 1, 1999• 4 citations
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Abstract

Measurements used in quantum teleportation are examined from the standpoint of the general theory of quantum-mechanical measurements. It is shown that in order to find a teleported state, it is sufficient to know only the resolution of the identity operator (positive operator-valued measure) generated by the respective instrument (the quantum operation determining the change in the state of the system as a result of the measurement) in the state space of the system, rather than the instrument itself. A protocol for quantum teleportation of the state of a system with a nondegenerate continuous spectrum based on a measurement which corresponds to a certain nonorthogonal resolution of the identity operator is proposed.

DOI
10.1134/1.558998

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Measurements used in quantum teleportation are examined from the standpoint of the general theory of quantum-mechanical measurements. It is shown that in order to find a teleported state, it is sufficient to know only the resolution of the identity operator (positive operator-valued measure) generated by the respective instrument (the quantum operation determining the change in the state of the system as a result of the measurement) in the state space of the system, rather than the instrument itself. A protocol for quantum teleportation of the state of a system with a nondegenerate continuous spectrum based on a measurement which corresponds to a certain nonorthogonal resolution of the identity operator is proposed.
The measurement procedures used in quantum teleportation are analyzed from the viewpoint of the general theory of quantum-mechanical measurements. It is shown that to find the teleported state one should only know the identity resolution (positive operator-valued measure) generated by the corresponding instrument (quantum operation describing the system state change caused by the measurement) rather than the instrument itself. A quantum teleportation protocol based on a measurement associated with a non-orthogonal identity resolution is proposed for a system with non-degenerate continuous spectrum.
The purpose of this paper is to provide a basis of theory of measurements of continuous observables. We generalize von Neumann’s description of measuring processes of discrete quantum observables in terms of interaction between the measured system and the apparatus to continuous observables, and show how every such measuring process determines the state change caused by the measurement. We establish a one-to-one correspondence between completely positive instruments in the sense of Davies and Lewis and the state changes determined by the measuring processes. We also prove that there are no weakly repeatable completely positive instruments of nondiscrete observables in the standard formulation of quantum mechanics, so that there are no measuring processes of nondiscrete observables whose state changes satisfy the repeatability hypothesis. A proof of the Wigner–Araki–Yanase theorem on the nonexistence of repeatable measurements of observables not commuting conserved quantities is given in our framework. We also discuss the implication of these results for the recent results due to Srinivas and due to Mercer on measurements of continuous observables.
Ideally, quantum teleportation should transfer a quantum state without distortion and without providing any information about that state. However, quantum teleportation of continuous electromagnetic-field variables introduces additional noise, limiting the fidelity of the quantum-state transfer. In this article, the operator describing the quantum-state transfer is derived. The transfer operator modifies the probability amplitudes of the quantum state in a shifted photon-number basis by enhancing low photon numbers and suppressing high photon numbers. This modification of the statistical weight corresponds to a measurement of finite resolution performed on the original quantum state. The limited fidelity of quantum teleportation is thus shown to be a direct consequence of the information obtained in the measurement.
Ideally, quantum teleportation should transfer a quantum state without distortion and without providing any information about that state. However, quantum teleportation of continuous electromagnetic-field variables introduces additional noise, limiting the fidelity of the quantum-state transfer. In this article, the operator describing the quantum-state transfer is derived. The transfer operator modifies the probability amplitudes of the quantum state in a shifted photon-number basis by enhancing low photon numbers and suppressing high photon numbers. This modification of the statistical weight corresponds to a measurement of finite resolution performed on the original quantum state. The limited fidelity of quantum teleportation is thus shown to be a direct consequence of the information obtained in the measurement.
Two different teleportation protocols are investigated by using two pairs of canonically conjugated observables such as position-momentum and phase-number. The discrete variable and continuous variable quantum teleportation are unified in the framework of canonical quantum teleportation. Canonically conjugated observables are found to play a 3-fold role in the quantum teleportation. Firstly, the common eigenstate of two commuting canonical observables like phase-difference and number-sum provides the quantum channel between two systems. Secondly, a similar pair of canonical observables from another two systems is measured in the Bell operator measurements. Finally, two translations generated by the canonically conjugated observables of a single system constitute the local unitary operation to recover the unknown state.