Synopsis

A smoother quantum measurement

Physics 3, s33
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.
Illustration: Alan Stonebraker

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).


Subject Areas

Quantum InformationOptics

Related Articles

Photons Get Slippery
Superfluidity

Photons Get Slippery

Researchers have turned light into a superfluid by using a “synthetic” dimension, which is created by using temporal degrees of freedom to mimic spatial degrees of freedom. Read More »

Far Fewer Qubits Required for “Quantum Memory” Quantum Computers
Quantum Information

Far Fewer Qubits Required for “Quantum Memory” Quantum Computers

Incorporating storage units for quantum information into quantum computers may allow researchers to build such devices with several orders of magnitude fewer qubits in their processors. Read More »

Turning On a Light Beam with a Single Molecule
Quantum Information

Turning On a Light Beam with a Single Molecule

A single molecule can switch a beam of photons on or off, a potentially useful function for a quantum computer. Read More »

More Articles