quantum integrated chips

Researchers in the U.K. and France have shown how to speed up the detection of quantum light by integrating a photonic chip with an electronic amplifier chip. [Image: University of Bristol]

Detecting light below the shot-noise limit can improve precision in all sorts of measurements, from the interception of gravitational waves to sensing tiny concentrations of biomolecules. But measuring such “squeezed light” can currently only be carried out over a fairly narrow range of frequencies, and therefore at limited speeds.

Now, researchers in the U.K. and France have shown how to expand the shot-noise bandwidth by coupling two different silicon chips together—one a photonic chip and the other a more conventional electronic circuit. They say that their integrated system paves the way to improved quantum communication and computation (Nat. Photonics, doi: 10.1038/s41566-020-00715-5).

The promise of squeezed light

Shot noise arises from fluctuations in the quantum vacuum that cause random variations in the arrival time of photons at a detector. Normally, this imposes a lower limit on the uncertainty in a laser beam’s phase. However, Heisenberg’s uncertainty principle provides some wiggle room. The principle dictates a combined uncertainty on a photon’s phase and amplitude, which means that uncertainty in the former can be “squeezed” lower by allowing it to rise in the latter.

Squeezed light has been used to enhance the sensitivity of the world’s leading gravitational-wave detectors since April 2019, and has also improved plasmonic biosensors and Raman spectroscopy. Looking further ahead, it could be exploited in certain forms of quantum cryptography to boost key transmission rates as well as inside optical quantum computers.

The very weak signals characteristic of squeezed light are generally picked up using homodyne detectors, which measure the interference between the signal and a reference laser beam using a pair of photodiodes. But the range of frequencies that can be detected in this way is limited by the relatively large capacitances of non-integrated electronic circuits—used in today’s devices—and the fairly slow response of large, non-integrated photodiodes.

Increasing bandwidth, reducing capacitance

Jonathan Matthews and colleagues from the University of Bristol, U.K., and Université Côte d’Azur, France, sought to improve that in the latest work by using a narrow gold wire to bond a commercial silicon-germanium electronic amplifier circuit to a custom-made CMOS-compatible silicon photonic circuit. The latter brings together signal and local-oscillator laser beams at a beamsplitter made from a Mach–Zehnder interferometer, and then couples the interferometer’s outputs to two germanium photodiodes integrated into waveguides.

With their setup, the researchers found they could generate a 3-dB bandwidth—which defines the frequency range over which the signal power drops by a half—of 1.7 GHz. This, they point out, is an order of magnitude broader than previously attained using an integrated photonic circuit and about 1.5 times as broad compared with the previous best free-space detector. More specifically, by hooking up their signal input to a source of broadband squeezed light based on a continuous-wave telecom pump laser, they showed they could detect that squeezed light across a spectrum of 9 GHz.

As group member Joel Tasker explains, they were able to achieve this increased bandwidth by reducing capacitances in their device. In part, he says, they did this by avoiding the need for a printed circuit board to connect detectors and amplifier. On top of this, they used just a single amplifier chip, reducing capacitance inside the amplifier.

Toward quantum computers

The researchers suggest that by integrating optical and electronic components within a single chip they could in future reach bandwidths of up to 100 GHz. What’s more, because the device is very small—the two chips having a combined area of less than 1mm2—arrays of such detectors could be built to measure weak light fields with many spatial degrees of freedom, which could potentially be exploited in large-scale quantum computers.

They add that it might even be possible to combine this silicon-based detector technology with silicon-nitride ring resonators to generate as well as measure squeezed light on a single chip—so reducing optical losses and allowing more squeezing to be measured.