Using lasers, researchers at the Niels Bohr Institute at the University of Copenhagen have developed a way to entangle electromagnetic fields from microwave radiation and optical beams. Creating entanglement between microwave and optical fields could help scientists solve the challenge of sharing entanglement between two distant quantum computers operating in the microwave regime. According to the researchers, one of today’s most advanced quantum systems is based on superconducting circuits, which work in the microwave regime. However, connecting these computers to provide a quantum network is challenging because microwaves can’t propagate far without loss, which is harmful to quantum computing tasks. One way to alleviate this problem, the researchers said, is to first entangle microwaves with optical fields, then use optical links, which have far lower loss, for long-distance communication. However, due to the differences in wavelengths (mm for microwaves and μm for light), this conversion is not easy. When an electromagnetic field such as a laser beam is reflected off a vibrating object, it can read out the vibration. An electromagnetic field is composed of photons, which bombard the object as light is bounced off it, leading to additional vibration known as quantum backaction. Reflection of two electromagnetic fields upon the same mechanical object provides an effective interaction between the fields. From left: David Mason, Junxin Chen, and Massimiliano Rossi from the quantum optomechanics group at the Niels Bohr Institute. Courtesy of Ola Joensen. The researchers entangled two laser beams by bouncing them off the same mechanical resonator (a tensile membrane). They used a 3.6-mm × 3.6-mm × 20-nm membrane made of silicon nitride, and pierced it with a pattern of holes that isolated the motion of the central pad, making the device sensitive enough to show quantum backaction. They shined two lasers on the membrane simultaneously, so that each laser was able to “see” the quantum backaction of the other. This generated strong correlations, leading to entanglement between the two lasers. “You could say that the two lasers ‘talk’ through the motion of the membrane,” researcher Junxin Chen said. “The membrane oscillator functions as an interaction media, because the lasers don’t talk to each other directly — the photons don’t interact themselves, only through the oscillator,” Chen said. “The interaction between photons and the membrane is wavelength-independent, allowing in principle microwave-optical entanglement.” Entanglement is preserved from the cryogenic mechanical mediator all the way to the laser beams analyzed in room-temperature homodyne detectors. This could make it possible for a class of hybrid quantum systems based on mechanical interfaces to harness entanglement between solid-state quantum systems, typically operating at low temperatures, and itinerant optical fields. Further experimental work will be necessary — in particular, operation of the membrane at a temperature close to absolute zero, the temperature at which superconducting quantum computers work today. Mechanically mediated microwave-optical entanglement, based on membrane electro-optomechanical systems, could deliver a much-needed resource for networks of quantum computers based on superconducting qubits. The research was published in Nature Communications (www.doi.org/10.1038/s41467-020-14768-1).
