Scientists at Rice and Princeton University have made a breakthrough in the use of magnets to tune supercooled gallium arsenide semiconductors as an ultrasensitive microwave detector that could greatly improve the processing time of the next generation of computers.
The study team specialize in quantum transport in two-dimensional electron systems, and have had a long collaboration with Loren Pfeiffer at Princeton University, whose group produces extremely pure samples of gallium arsenide. For this research, published in Physical Review Letters [Dai et al. Phys Rev Lett (2010) doi: 10.1103/PhysRevLett.105.246802], the researchers cooled one of Pfeiffer's pure samples to below 4 K, the temperature of liquid helium, and then bombarded the sample with microwaves while simultaneously applying a weak magnetic field, measuring its response.
It was found that microwaves of a specific wavelength resonated strongly with the cooled sample, and that the magnet was able to tune this resonance to specific microwave frequencies. Rui-Rui Du, lead author of the study, commented “Tunable photon-detection technology in the microwave range is not well-developed. Single-photon detectors based on superconductors in the 10 GHz to 100 GHz range are available, but their resonance frequency has been difficult to tune. Our findings suggest that tunable single-photon detection may be within reach with ultrapure gallium arsenide.”
A few years ago the team discovered a new phenomenon, microwave-induced resistance oscillations (MIRO), in GaA samples prepared by Pfeiffer's lab by molecular beam epitaxy (MBE): a materials growth technique that allows for atomic precision in the growth of very clean crystals.
The new study found a very sharp spike on top of the MIRO in the purest GaA samples from Pfeiffer's lab, a peak of about 300 % in amplitude compared to MIRO. It was shown that the magnetic field position shifts to a high magnetic field if the microwave frequency is increased. The combination of this spike and its magnetic field position corresponding to a certain microwave frequency, is essentially a very high quality (Q) factor, tunable microwave detector.
Du admits that “One of the very attractive prospects is a tunable microwave detector that can detect a single photon. That will be very useful for quantum communication applications.”
Manufacturers of computer chips hope that the use of photons, rather than electrons, in devices that create, transmit and measure digital information will make computers faster and more powerful.
In terms of other uses that could emerge from the discovery, it is hoped that, since the phenomenon works only under very cold conditions, there may be research applications at the liquid helium temperature in astrophysics for example, and also for devices that can operate at higher temperatures.

Laurie Donaldson