3 research outputs found
Defending Continuous Variable Teleportation: Why a laser is a clock, not a quantum channel
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
can synchronize independent clocks each with a mean-square error of
radians.Comment: 22 pages, to be published in a special issue of J. Opt. B. This is an
extended version of quant-ph/0303116 (the SPIE conference paper