Multi-terminal Josephson Microwave Nanophotonic Devices

Nontechnical description Quantum computers built from superconducting circuits are among the most promising technologies for solving problems beyond the reach of today's classical machines, with potential impact on materials and drug discovery, secure communication, and national security. These machines face a fundamental scaling bottleneck: each additional quantum bit (qubit) requires bulky microwave cabling to control and read out its state, and this cabling carries heat into the ultra-cold environment the qubits need to operate. As processors grow, this heat load threatens to stall further progress. A promising remedy is to move the control and measurement electronics directly onto the cold chip beside the qubits, an approach known as "cold control", but existing on-chip devices are not yet accurate enough to be useful. This project develops a new class of compact, on-chip devices based on multi-terminal Josephson circuits that exploit the unusual physics of superconductors to serve as both highly sensitive detectors and low-noise sources of microwave signals. Success would remove a key barrier to scaling superconducting quantum computers, directly advancing U.S. leadership in quantum information science. Beyond quantum computing, the same devices could act as ultra-sensitive sensors and amplifiers for probing new materials and managing heat in cryogenic systems. The project also builds the emerging quantum workforce: it supports a graduate student and a postdoctoral researcher, provides hands-on research experiences for undergraduate and high school students, and channels real experimental data into new quantum engineering courses and programs at Syracuse University, coordinated with the institution's NSF-supported quantum workforce initiatives. Technical description Scaling superconducting quantum processors is increasingly limited by the passive heat load of the microwave cabling required for qubit control and readout. On-chip "cold control" can reduce this overhead, but present devices fall short: dc-biased Josephson emitters exhibit emission linewidths limited to the megahertz range by the thermodynamic uncertainty relation, and on-chip nanobolometers suffer recovery times that degrade effective readout fidelity. This project addresses both limitations by exploiting the enhanced harmonic content and tunable nonlinearity of multi-terminal Josephson circuits (MTJCs), whose higher-dimensional phase-space energy landscape supports dynamics inaccessible to conventional two-terminal junctions. The research pursues two complementary thrusts. The first develops MTJC-based Josephson parametric amplifier bolometers in which the engineered Kerr nonlinearity is mapped via dual-drive Stark-shift measurements and tuned through current biasing and electrostatic gating of graphene weak links. By operating at bias points of maximal responsivity, the goal is microwave single-photon detection in the 4–8 GHz qubit band with nanosecond-scale reset, improving on the sensitivity and gain demonstrated in two-terminal graphene devices. The second thrust characterizes coherent microwave emission from dynamically stabilized multiplet supercurrents, including quartet and nulltet states, measuring the harmonic content and phase-noise spectrum of the radiation to test whether the native phase-locking of these states yields emission with phase noise below the conventional thermodynamic limit. Temperature-dependent phase-noise measurements, benchmarked against extended resistively- and capacitively-shunted-junction modeling, distinguish thermally limited from dynamically stabilized behavior. As a longer-term objective, the project explores integrating the emitter and bolometer in a cryogenic transmit–receive geometry to demonstrate fully on-chip power measurement. Collectively, the work seeks to establish MTJCs as a platform for high-fidelity, low-heat-load cold control and readout of superconducting qubits, while advancing fundamental understanding of nonlinear and multiplet dynamics in multi-terminal Josephson systems. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.