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arXiv:quant-ph/0103147 · Physical Review Letters

Requirement of Optical Coherence for Continuous-Variable Quantum Teleportation

Terry Rudolph, Barry C. Sanders

Published Jul 31, 2001• 80 citations
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Abstract

We show that the sender and the receiver each require coherent devices in order to achieve unconditional continuous variable quantum teleportation (CVQT), and this requirement cannot be achieved with conventional laser sources, linear optics, ideal photon detectors, and perfect Fock state sources. The appearance of successful CVQT in recent experiments is due to interpreting the measurement record fallaciously in terms of one preferred ensemble (or decomposition) of the correct density matrix describing the state. Our analysis is unrelated to technical problems such as laser phase drift or finite squeezing bandwidth.

DOI
10.1103/physrevlett.87.077903

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arXiv:quant-ph/0203031•Mar 7, 2002•0 citations
We show that the sender and the receiver each require coherent devices in order to achieve unconditional continuous variable quantum teleportation (CVQT), and this requirement cannot be achieved with conventional laser sources, linear optics, ideal photon detectors, and perfect Fock state sources. The appearance of successful CVQT in recent experiments is due to interpreting the measurement record fallaciously in terms of one preferred ensemble (or decomposition) of the correct density matrix describing the state. Our analysis is unrelated to technical problems such as laser phase drift or finite squeezing bandwidth.
Summary form only given. Quantum teleportation, first proposed as a method for teleporting an unknown spin-1/2 quantum state from a sender, "Alice", to a receiver, "Bob", has been extended to continuous-variable quantum teleportation (CVQT). In this variant of quantum teleportation, Bob and Alice share a nonlocal entangled two-mode field which, ideally, should have perfect correlations between in-phase quadrature ('position' x) and out-of-phase quadrature ('momentum' p), although the requirement of perfect correlations can be relaxed somewhat. The experimental success of CVQT has recently been reported, with two-mode squeezed light providing the nonlocal, entangled source shared between Alice and Bob. The objective here is to consider whether CVQT has indeed been achieved, or could ever be achieved, in optical experiments employing conventional laser sources. In particular we show that, in the context of this experiment, Alice and Bob may not employ independent laser sources for their operations if the state is to be teleported, yet this requires them to share more than just the Einstein-Podolsky-Rosen (EPR) resource.
We give a description of balanced homodyne detection (BHD) using a conventional laser as a local oscillator (LO), where the laser field outside the cavity is a mixed state whose phase is completely unknown. We apply our description to continuous-variable quantum teleportation (CVQT) with a laser, and show that CVQT with a laser is valid only if the unknown phase of the laser field is shared among sender's LOs, the EPR state, and receiver's LO. We also propose a method to generate a strongly phase-correlated quantum state necessary for CVQT without an additional optical path.
It has been argued [T.Rudolph and B.C. Sanders, Phys.Rev. Lett.87, 077903 (2001)] that continuous-variable quantum teleportation at optical frequencies has not been achieved because the source used (a laser) was not 'truly coherent'.Here I show that 'true coherence' is always illusory, as the concept of absolute time on a scale beyond direct human experience is meaningless.A laser is as good a clock as any other, even in principle, and this objection to teleportation experiments is baseless.
Almost all novel observable phenomena in quantum optics are related to the quantum coherence. The coherence here is determined by the relative phase inside a state. Unfortunately, so far all the relevant experimental results in quantum optics are insensitive to the phase information of the coherent state. Lack of phase information may cause serious consequences in many problems in quantum optics. For example, an ensemble of two mode squeezed states is a classical ensemble if the phase in each state is totally random; but it is a non-classical ensemble if the phase in each state is fixed. As a timly application, verification of this type of phase information in an ensemble of two mode squeezed states from the conventional laser is crucial to the validity of the continuous variavable quantum teleportation(CVQT) experiment. Here we give a simple scheme to distinguish two different ensemble of states: the Rudolph-Sanders ensemble, by which each squeezed states emitted has a uniform distribution from 0−2π0-2π0−2π on the phase value; and the van Enk-Fuchs ensemble, which emmits identical states with a fixed(but unknown) phase for every state. We believe our proposal can help to give a clear picture on whether the existing two mode squeezed states so far are indeed non-classical states which can be used as the entanglement resource.