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Journal of Modern Optics

There is no unmet requirement of optical coherence for continuous-variable quantum teleportation

Howard M. Wiseman

Published Jan 1, 2003• 2 citations
Publisher

Abstract

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.

DOI
10.1080/0950034031000064843

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It has been argued that continuous-variable quantum teleportation at optical frequencies has not been achieved because the source used (a laser) was not ‘truly coherent’. Here it is shown 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.

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Requirement of optical coherence for continuous-variable quantum teleporation

Terry Rudolph, Barry C. Sanders

We show that unconditional quantum teleportation of continuous variables requires coherent devices for the sender (Alice) and the receiver (Bob) and that this requirement is not in principle achievable in experiments employing conventional laser sources.

Optical Fiber Communication Conference•Jan 1, 2001•0 citations
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.
It has been argued [T. Rudolph and B.C. Sanders, Phys. Rev. Lett. {\\bf 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'. Van Enk, and Fuchs [Phys. Rev. Lett, {\\bf 88}, 027902 (2002)], while arguing against Rudolph and Sanders, also accept that an `absolute phase' is achievable, even if it has not been achieved yet. I will argue to the contrary that `true coherence' or `absolute phase' is always illusory, as the concept of absolute time (at least for frequencies beyond direct human experience) is meaningless. All we can ever do is to use an agreed time standard. In this context, a laser beam is fundamentally as good a `clock' as any other. I explain in detail why this claim is true, and defend my argument against various objections. In the process I discuss super-selection rules, quantum channels, and the ultimate limits to the performance of a laser as a clock. For this last topic I use some earlier work by myself [Phys. Rev. A {\\bf 60}, 4083 (1999)] and Berry and myself [Phys. Rev. A {\\bf 65}, 043803 (2002)] to show that a Heisenberg-limited laser with a mean photon number mu\\mumu can synchronize MMM independent clocks each with a mean-square error of sqrtM/4mu\\sqrt{M}/4\\musqrtM/4mu radians2^22.
It has been argued (Rudolph and Sanders 2001 Phys. Rev. Lett. 87 077903) that continuous-variable quantum teleportation at optical frequencies has not been achieved because the source used (a laser) was not 'truly coherent'. Van Enk and Fuchs (2002 Phys. Rev. Lett. 88 027902), while arguing against Rudolph and Sanders, also accept that an 'absolute phase' is achievable, even if it has not been achieved yet. I will argue to the contrary that 'true coherence' or 'absolute phase' is always illusory, as the concept of absolute time (at least for frequencies beyond direct human experience) is meaningless. All we can ever do is to use an agreed time standard. In this context, a laser beam is fundamentally as good a 'clock' as any other. I explain in detail why this claim is true, and defend my argument against various objections. In the process I discuss super-selection rules, quantum channels, and the ultimate limits to the performance of a laser as a clock. For this last topic I use some earlier work by myself (1999 Phys. Rev. A 60 4083) and Berry and myself (2002 Phys. Rev. A 65 043803) to show that a Heisenberg-limited laser with a mean photon number μ can synchronize M independent clocks each with a mean-square error of .
It has been argued that continuous-variable quantum teleportation at optical frequencies has not been achieved because the source used was not 'truly coherent'. While arguing against Rudolph and Sanders, also accept that an 'absolute phase' is achievable, even if it has not been achieved yet. I will argue to the contrary that 'true coherence' or 'absolute phase' is always illusory, as the concept of absolute time on a scale beyond direct human experience is meaningless. All we can ever do is to use an agreed time standard. in this context, laser beam is fundamentally as good a 'clock' as any other. I explain in detail why this claim is true, and defend my argument against various objections. In the process I discuss super-selection rules, quantum work by myself and Berry and myself to show that a Heisenberg-limited laser with a mean photon number μ can synchronize M independent clocks each with a mean-square error of square root M/4μ radians2.
The argument of Rudolph and Sanders, while technically correct, raises conceptual problems. In particular, if carried to its logical conclusion, it would disallow the use in our theories of any time ttt with implied resolution beyond that of direct human experience.