We are witnessing rapid progress in the fields of quantum computing with superconducting circuits on the one hand, and long-distance optical quantum communication on the other. Meanwhile, there is currently no solution to interface these two domains of quantum technology in analogy to fiber optic modems in classical communication systems. Apart from close to unity efficiency and high bandwidth, such a quantum interface also needs to operate close to its quantum ground state with hardly any excess noise on either the electrical or the optical output—an important milestone that we demonstrate in this work.
We realize the electro-optic wavelength converter based on a mechanically polished crystalline lithium niobate whispering gallery mode resonator. In contrast to traditional modulators, the interaction is resonantly enhanced using a superconducting microwave cavity that matches the free spectral range and leads to an extremely efficient bidirectional conversion process. We show that this conversion works well despite the relatively high optical pump powers required. The microwave mode remains close to the quantum ground state at millikelvin temperatures where superconducting qubits operate.
The centimeter-sized device benefits from a large heat capacity and a good thermalization to the cold environment, resulting in an extremely slow observed heating rate compared to on-chip devices. Based on this, we estimate that pulsing the pump can boost the conversion efficiency by another 4 orders of magnitude without a significant increase of added noise. This would open the way for long-distance quantum networks utilizing superconducting processors for secure communication and distributed quantum computing.
Bidirectional electro-optic wavelength conversion in the quantum ground state W. Hease*, A. Rueda*, R. Sahu, M. Wulf, G. Arnold, H. G. L. Schwefel, J. M. Fink. PRX Quantum 1, 020315 (2020) PRXQ, arXiv
In superconducting circuits, superinductors are employed to suppress charge fluctuations and increase zero-point voltage, enabling features for hardware-protected qubits, metrological standards, and strongly coupled hybrid devices. Conventionally these devices are based on kinetic inductance, and can suffer from nonlinearity, additional complexity due to multiterminal structure, and the limited control and reliability of bottom-up fabrication. Making use of miniaturization and substrate engineering, the authors realize a geometrically defined, single-wavefunction superinductor—a high-performance, innovative circuit element that promises to expand the scope of quantum circuitry.
Our new paper shows chip-scale transduction of microwave and optical photons via radiation pressure at mK temperature. It achieves the lowest Vπ to date and we carefully analyze and model the conversion noise that currently hinders using it for quantum applications in continuous wave mode. Big thanks to the team around Georg, Matthias and Shabir as well as our funders: (ERC, HOΤ and IST)!
Our proof of principle demonstration of quantum radar is out now. It shows that an entangled source of microwave radiation can result in a relative object detection advantage compared to a classically correlated noise source.
Science Advances 6 eabb0451 (2020) ScienceAdvances, arXiv, MIT TechReview, PhysOrg, APA
Alfredo won the “IST Outstanding Scientific Achievement award 2019” for his experimental work on electro-optic frequency combs and his theoretical (and experimental) work on microwave optical state transfer last year.