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arXiv:quant-ph/0303116 · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Optical coherence and teleportation: why a laser is a clock, and not a quantum channel

Howard M. Wiseman

Published May 16, 2003• 7 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1117/12.497090

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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 .

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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 $\\mu$ can synchronize $M$ independent clocks each with a mean-square error of $\\sqrt{M}/4\\mu$ radians$^2$.

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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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Comment on ``Requirement of optical coherence for continuous--variable quantum teleportation'' by Terry Rudolph and Barry C. Sanders

Howard M. Wiseman

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 $t$ with implied resolution beyond that of direct human experience.

arXiv:quant-ph/0104004•Apr 2, 2001•0 citations

Continuous variable teleportation requires optical coherence

Terry Rudolph, Barry C. Sanders

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.

Quantum Electronics and Laser Science Conference•Nov 13, 2002•0 citations
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.
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 [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 [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.
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.
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.