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

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

2003-03-19

arxiv.org/abs/quant-ph/0303116v1

Proceedings of SPIE Vol. 5111 Fluctuations and Noise in Photonics and Quantum Optics, Eds Derek Abbott, Jeffrey H. Shapiro, Yo

Physics

Quantum Physics

14 pages, no figures, some equations this time. For proceedings of SPIE conference "Fluctuations and Noise 2003"

Scientific paper

10.1117/12.497090

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 on a scale 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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