How to transmit microwave quantum information via fiber optics

Quantum systems are extremely susceptible to interference, especially when they consist of superconducting circuits. Therefore, most quantum processors must operate at temperatures near absolute zero in a vacuum environment. From the outside, such a quantum computer looks more like a refrigerator than a classical computer. Due to these technical challenges and the associated costs, long-distance networks seem to remain a distant prospect—at least for now.

Scientists are currently working hard to transport quantum information using standard communication methods such as fiber optics. These could easily transmit quantum signals over more than a hundred kilometers. However, the actual challenge is not transmission, but rather signal conversion. Superconducting processors typically operate at microwave frequencies, where quantum signals are extremely weak compared to ambient noise. A perfect converter would therefore have to convert signals between the electrical and optical ranges without noise or loss.

“Such converters are ubiquitous in classical communication networks, and they operate with incredible bandwidth and reliability. However, for signals at the quantum level, their efficiency would have to be significantly higher, and electronic noise would have to be strongly suppressed,” explains Professor Johannes Fink.

At IST Austria, Professor Johannes Fink and his team are researching quantum networks. Two scientists from his group have now investigated the conversion of microwave quantum information using two different approaches. The first approach offers one of the highest conversion efficiencies of any microwave optical converter to date. The second approach exhibits remarkably noise-free signals.

A Micromechanical Signal Converter

Georg Arnold (Fink Group) presents the first microwave optical converter on a microchip. Using a mechanical coupling element, this signal converter achieves high efficiency at unprecedentedly low input power levels. The researchers designed, manufactured, and tested a new type of hybrid component that operates at ultracold temperatures and combines the best aspects of various emerging technologies.

Georg Arnold summarizes his findings as follows: “We have created an integrated link between light and microwave signals with a footprint of 0.1 × 0.2 mm² by leveraging the advantages of compact size and the versatile coupling of mechanical elements. We use industry standards for our prototypes, which gives us great hope that an optimized version will find a future application. One remaining challenge is the local heat generation due to the tiny dimensions when the converter is operating at maximum power. This wouldn't be a problem with classical signals. However, with quantum signals, noise-free conversion is essential.”

Motion, or more precisely, vibrations, can link light, electronic signals, and even more complex physical systems. For the signal converter, the scientists utilize a phenomenon called radiation pressure to convert microwaves into optical light. Radiation pressure means that, under certain conditions, light can push and pull physical objects.

At the heart of the device is a photonic crystal resonator connected to two ultrathin silicon strands. The end of each line is either a microwave or a fiber optic input. Like playing a stringed instrument, the vibration of the incoming light causes the silicon wires to vibrate. The entire resonator oscillates, and the vibration travels from one end to the other, transforming into a microwave signal. An optical signal is thus converted into a microwave signal and vice versa.

“The device essentially works like a very sensitive mirror that is displaced by the incident light. This couples mechanical motion and optical radiation. We have optimized the system so that even the smallest unit of light, a single photon, can excite an oscillation. The microwave power works conceptually similarly, allowing us to convert optical photons to microwave frequencies by mapping them onto an intermediate mechanical oscillator,” says Georg Arnold.

This approach currently also amplifies the signal, an effect that is undesirable for the faithful conversion of quantum information but potentially useful for more power-efficient optical modulation techniques.

Conversion via Optical Nonlinearities

William Hease (Fink Group) approached the signal converter from a different angle. His system converts signals as cleanly as possible and without noise. He created a prototype that functions remarkably similarly to a laser.

“Our approach uses single-crystal resonators in which the light travels back and forth a thousand times before being absorbed. This enhances the interaction of light with microwave quantum signals. Furthermore, due to its size (~1 cm³), our system is quite insensitive to heating, which minimizes the noise resulting from the conversion,” explains William Hease.

The light from the optical fiber travels through a nonlinear crystal. Such a crystal changes its properties significantly when a voltage is applied. In this case, the microwave signal, which contains the quantum information to be converted, itself generates the voltage. The controlled change in the crystal then also affects the optical light passing through it. The quantum signal therefore controls the frequency and shape of the optical light. A receiver can translate these changes back into the microwave signal using the same principle. This is called a transceiver.

William Hease summarizes his results: “Our system utilizes nonlinear effects in lithium niobate to convert microwave signals into optical signals and vice versa. Unlike amplifiers based on transistor technologies (CMOS, etc.), nonlinear effects in lithium niobate result in an absolutely noise-free conversion, as long as the system can be kept at zero temperature. In real-world experiments, this isn't so easy due to optical heating, but we still manage to keep the noise at an extremely low level.”

This approach offers virtually noise-free quantum conversion—a crucial prerequisite for future applications of quantum communication.

The Future of the Quantum Converter

An ideal quantum converter transforms quantum information into optical light and vice versa without signal loss or adding noise. Both results from the Fink group attempt to address this challenge in different ways. Georg Arnold's micromechanical solution offers unprecedented efficiency in an extremely small component and is therefore scalable. On the other hand, William Hease's nonlinear optics approach enables flawless conversion with comparably high bandwidth.

Professor Johannes Fink says of these two results: “It is exciting to have two strong competitors in the race for a future quantum converter for superconducting processors. There is still much work ahead of us, but it is also becoming clear that we will be able to implement the first fundamental quantum communication protocols in the coming years.”

Publications

G. Arnold, M. Wulf, S. Barzanjeh, E. S. Redchenko, A. Rueda, W. J. Hease, F. Hassani, and J. M. Fink. 2020. Converting microwave and telecom photons with a silicon photonic nanomechanical interface. Nature Communications. DOI: 10.1038/s41467-020-18269-z

William Hease, Alfredo Rueda, Rishabh Sahu, Matthias Wulf, Georg Arnold, Harald G. L. Schwefel, and Johannes M. Fink. 2020. Bidirectional Electro-Optic Wavelength Conversion in the Quantum Ground State. PRX Quantum. DOI: 10.1103/PRXQuantum.1.020315