# Synopsis: A smoother quantum measurement

A method called quantum smoothing has been experimentally shown to provide a mean-square measurement error that is a factor of $2$ smaller than the quantum limit.

The precision of any measurement is fundamentally limited by the standard quantum limit. Often there are classical quantities related to the dynamical evolution of a quantum system one would like to measure, a process known as quantum parameter estimation. This kind of estimation is useful in delicate measurements ranging from gravitational wave detection to quantum computation. Recently, Tsang [1] considered the case of quantum estimation for dynamical systems and proposed a method called quantum smoothing that combines past observations with “future” measurements (that is, a signal is inferred from measurements both before and after a chosen point in time).

As reported in Physical Review Letters, Trevor Wheatley at the University of New South Wales in Canberra, Australia, and co-workers in Australia, Japan, and Canada now have experimentally tested these ideas by considering the problem of estimating the phase of a continuous optical field in the presence of classical noise. The authors use optical modulators to prepare a laser beam in a known state with a predetermined noise signature and then apply an adaptive measurement technique to estimate the optical phase. By including data obtained after time $t$ with data collected before $t$ along with Tsang’s theory, the researchers were able to estimate the phase at $t$ with a mean-square error more than a factor of $2$ smaller than the standard quantum limit. – David Voss

[1] M. Tsang, Phys. Rev. Lett. 102, 250403 (2009).

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