Moving Target Measured with Squeezed Light

A novel quantum mechanical squeezing technique that precisely tracks the phase of optical waveforms in motion has broken the standard limits for ultraprecise measurement by exploiting quantum lightwaves in a different way.

In optical communications, information is often stored in a waveform or light pulse. Yet noise and fluctuations arise, causing random jitter in the phase and amplitude of optical pulses, making it difficult to keep track of waveform phase.

Squeezed light can be used to make measurements of very small distances, and now scientists at the University of Tokyo and Griffith University in Australia have demonstrated that it is possible to take measurements even while the target is in motion.

Photo of the experimental setup. (Image: Hidehiro Yonezawa)

“At the heart of all scientific endeavor is the necessity to be able to measure things precisely,” said professor Howard Wiseman of Griffith University’s Centre for Quantum Dynamics. “By using squeezed light, we have broken the standard limits for precision phase tracking, making a fundamental contribution to science.”

To achieve this “optical-phase tracking method,” professor Akira Furusawa and project lecturer Hidehiro Yonezawa, both from the School of Engineering at the University of Tokyo, exploited phase squeezed light (the phase noise of which is smaller than that of a laser beam) and a feedback control technique. Their findings beat the classical mechanical boundary of precision; more importantly, their results reveal that, because of Heisenberg’s uncertainty principle, a quantum mechanical limitation exists in highly precise optical-phase tracking.

“Because the phase of a light beam changes whenever it passes through or bounces off an object, being able to measure that change is a very powerful tool,” Wiseman said. Using squeezed light has enabled the researchers to push the boundaries of precision phase tracking. “But, we have also shown that too much squeezing can actually hurt,” he said.

“Curiously, we found that it is possible to have too much of a good thing,” said professor Elanor Huntington from UNSW Canberra, the director of the Australian contribution to the experiment and Wiseman’s colleague in the Centre for Quantum Computation and Communication Technology. “Squeezing beyond a certain point actually degrades the performance of the measurement, making it less precise than if we had used light with no squeezing.”

Wiseman has been working with Dr. Dominic Berry of Macquarie University on the theory of this problem for the past several years.

“The key to this experiment has been to combine ‘phase squeezing’ of lightwaves with feedback control to track a moving phase better than previously possible,” Berry said. “Ultraprecise quantum-enhanced measurement has been done before, but only with small phase changes. Now we have shown we can track large phase changes as well.”

The ultrahigh-precision phase measurements could stimulate many applied research applications such as ultraprecision length measurement, ultrahigh-capacity coherent optical communication and secure quantum cryptography.

The Tokyo-based research was conducted in collaboration with The University of New South Wales, University of Queensland and Macquarie University, all in Australia. It was partly supported by Project for Developing Innovation Systems.

The research appeared Sept. 21 in Science.