Low-Noise Transducers to Bridge the Gap Between Microwave and Optical Qubits

In the effort to build superconducting quantum computers, researchers around the world are working to develop electrical circuits that operate in the microwave domain using individual particles of microwave radiation, or microwave photons, as qubits—the basic building blocks of quantum computing. These microwave qubits are a leading approach to building quantum computers due to their ease of control and scalable fabrication. These devices need to be cooled to ultralow temperatures around 30 milliKelvin (-459.6 degrees Fahrenheit) to keep background noise low enough so that the individual microwave photons can be detected and worked with.

But microwave photons quickly lose their quantum information, or decohere, at room temperature. In order to transmit those qubits at room temperature via optical cables like those used for the current internet, the microwave photons would need to be converted into higher-energy optical photons. Such a conversion could enable the construction of large-scale distributed superconducting quantum computers.

Now a team of researchers led by Mohammad Mirhosseini, assistant professor of electrical engineering and applied physics at Caltech, has developed an on-chip transducer to help bridge that significant energy gap. The silicon device performs a stepwise transformation to convert microwave photons to optical photons. The work is described online in the journal Nature Nanotechnology.

The new device involves a tiny silicon beam that vibrates at 5 gigahertz and couples to a microwave resonator—essentially a nanoscale box in which photons bounce around, also at 5 GHz. Using a technique called electrostatic actuation, developed previously by the Mirhosseini lab for quantum applications, a microwave photon gets converted within that box to a mechanical vibration of the beam, and that mechanical oscillation, with the help of laser light, gets converted by the resonator into an optical photon.

"Even though it's hard to get direct coupling between microwaves and optical photons, it's relatively easy to get large coupling between microwave photons and mechanics, and then mechanics and optical photons," says William Chen, Caltech electrical engineering graduate student and co-lead author of the paper along with former Caltech postdoctoral scholar Han Zhao.

Engineers use several metrics to analyze the effectiveness of such a conversion technique. Most important among those metrics is keeping noise, or the introduction of false signals, to a minimum. "Our method is agnostic to the exact material that our mechanical oscillator is built from," Chen explains, "so we were able to build the transducer from silicon, which has been shown to have very little heating under laser illumination. This enables us to get the low noise level that we have been able to achieve in this work."

Another important metric is the product of a method's efficiency, that is, its ability to convert a microwave photon into an optical photon, and its turnover rate, or how fast the device can be reused. The Caltech device is able to convert microwave photons to optical photons about 100 times better than previous state-of-the-art systems with the same amount of noise.

"Leading up to this, gradual progress has been made in achieving higher and higher efficiencies with lower and lower noise. Now we have made a system that can achieve a really high efficiency compared to what was out there," Mirhosseini says. "Our device is also much simpler to fabricate on larger scales, so we are excited that it has the potential to allow us to do demonstrations that were not within reach before."

Caltech graduate student Abhishek Kejriwal is also an author of the paper, "Quantum-enabled microwave-to-optical transduction via silicon nanomechanics." The work was supported by the Cross Quantum Technology Systems program of the US Army Research Office and 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 supported by the Kavli Nanoscience Institute at Caltech.