In the pursuit of constructing superconducting quantum computers, scientists globally are striving to create electrical circuits that function within the microwave spectrum, employing individual units of microwave radiation, or microwave photons, as qubits—the fundamental components of quantum computing. These microwave qubits represent a prominent strategy for developing quantum computers due to their manageable control and scalable manufacturing. Such devices must be cooled to extremely low temperatures near 30 milliKelvin (-459.6 degrees Fahrenheit) to maintain background noise at a sufficiently minimal level allowing the detection and manipulation of individual microwave photons.

However, microwave photons rapidly deteriorate their quantum information, or undergo decoherence, at ambient temperature. To transmit these qubits at room temperature via optical fibres akin to those utilized in the present internet, it would be necessary to convert the microwave photons into higher-energy optical photons. This transformation could facilitate the establishment of widespread distributed superconducting quantum computers.

Currently, a group of researchers headed by Mohammad Mirhosseini, an assistant professor of electrical engineering and applied physics at Caltech, has created an on-chip transducer aimed at bridging that crucial energy chasm. The silicon device executes a gradual transformation to convert microwave photons into optical photons. The findings are detailed online in the journal Nature Nanotechnology.

The novel device comprises a minuscule silicon beam that oscillates at 5 gigahertz and connects with a microwave resonator—essentially a nanoscale enclosure where photons circulate, also at 5 GHz. By utilizing a method known as electrostatic actuation, previously developed by the Mirhosseini lab for quantum purposes, a microwave photon is transformed within that enclosure into a mechanical vibration of the beam, and this mechanical oscillation, aided by laser light, is transformed by the resonator into an optical photon.

“Despite the challenges in achieving direct linkage between microwaves and optical photons, establishing significant coupling between microwave photons and mechanical systems, and subsequently between mechanical systems and optical photons, is comparatively straightforward,” expresses William Chen, a Caltech electrical engineering graduate student and co-lead author of the study alongside former Caltech postdoctoral researcher Han Zhao.

Engineers utilize various parameters to evaluate the efficacy of such a conversion technique. The most crucial among these parameters is the minimization of noise, or the introduction of erroneous signals. “Our approach is indifferent to the specific material used for our mechanical oscillator,” Chen clarifies, “therefore, we were able to fabricate the transducer from silicon, which has demonstrated minimal heating under laser exposure. This capability enables us to achieve the low noise levels we have realized in this research.”

Another significant parameter is the product of a method’s efficiency—its ability to convert a microwave photon into an optical photon—and its turnover rate, or the speed at which the device can be reused. The Caltech device achieves a conversion of microwave photons to optical photons approximately 100 times superior to previous leading systems while maintaining the same noise levels.

“Prior to this, steady advancements were made in attaining increasingly higher efficiencies with progressively lower noise. Now we have developed a system capable of achieving an exceptionally high efficiency relative to prior technologies,” notes Mirhosseini. “Our device is also considerably easier to manufacture on larger scales, so we are enthusiastic about its potential to facilitate demonstrations previously thought unattainable.”

Caltech graduate student Abhishek Kejriwal also contributed as an author of the paper, “Quantum-enabled microwave-to-optical transduction via silicon nanomechanics.” This research was supported by the Cross Quantum Technology Systems initiative of the US Army Research Office, the National Security Agency’s Laboratory for Physical Sciences, the US Department of Energy Office of Science’s National Quantum Information Science Research Centers, and the National Science Foundation. Fabrication was aided by the Kavli Nanoscience Institute at Caltech.