The photon blockade breakdown in a continuously driven cavity QED system has been proposed as a prime example for a first-order driven-dissipative quantum phase transition. But thepredicted scaling from a microscopic system – dominated by quantum fluctuations – to a macroscopic one – characterized by stable phases – and the associated exponents and phase diagram have not been observed so far. In this work we couple a single transmon qubit with a fixed coupling strength g to an in-situ bandwidth κ tuneable superconducting cavity to controllably approach this thermodynamic limit. Even though the system remains microscopic, we observe its behavior to become more and more macroscopic as a function of g/κ. For the highest realized g/κ≈287 the system switches with a characteristic dwell time as high as 6 seconds between a bright coherent state with ≈8×103 intra-cavity photons and the vacuum state with equal probability. This exceeds the microscopic time scales by six orders of magnitude and approaches the near perfect hysteresis expected between two macroscopic attractors in the thermodynamic limit. These findings and interpretation are qualitatively supported by semi-classical theory and large-scale Quantum-Jump Monte Carlo simulations. Besides shedding more light on driven-dissipative physics in the limit of strong light-matter coupling, this system might also find applications in quantum sensing and metrology.
State-of-the-art transmon qubits rely on large capacitors which systematically improves their coherence due to reduced surface loss participation. However, this approach increases boththe footprint and the parasitic cross-coupling and is ultimately limited by radiation losses – a potential roadblock for scaling up quantum processors to millions of qubits. In this work we present transmon qubits with sizes as low as 36×39μm2 with ≳100\,nm wide vacuum gap capacitors that are micro-machined from commercial silicon-on-insulator wafers and shadow evaporated with aluminum. After the release in HF vapor we achieve a vacuum participation ratio up to 99.6\% in an in-plane design that is compatible with standard coplanar circuits. Qubit relaxation time measurements for small gaps with high vacuum electric fields of up to 22\,V/m reveal a double exponential decay indicating comparably strong coupling to long-lived two-level-systems (TLS). %We also show that the fast ‚initial‘ and slow ‚residual‘ decay strongly correlates with the measured sub-single-photon and high-drive-power quality factors of lumped element vacuum gap resonators, respectively. The exceptionally high selectivity of >20\,dB to the superconductor-vacuum surface allows to precisely back out the sub-single-photon dielectric loss tangent of aluminum oxide exposed to ambient conditions of tanδ=1.5×10−4 for a thickness of 3\,nm. %assuming 3\,nm thick. %the widely used aluminum oxide exposed to ambient conditions. In terms of future scaling potential we achieve a qubit quality factor by footprint area of 20μs−2, which is on par with the highest T1 devices relying on larger geometries and expected to improve substantially for lower loss superconductors like NbTiN, TiN or Ta.
Quantum illumination is a powerful sensing technique which employs entangled photons to boost the detection of low-reflectivity objects in environments with bright thermal noise. Thepromised advantage over classical strategies is particularly evident at low signal photon flux, a feature which makes the protocol an ideal prototype for non-invasive biomedical scanning or low-power short-range radar detection. In this work we experimentally demonstrate quantum illumination at microwave frequencies. We generate entangled fields using a Josephson parametric converter at millikelvin temperatures to illuminate a room-temperature object at a distance of 1 meter in a proof of principle bistatic radar setup. Using heterodyne detection and suitable data-processing at the receiver we observe an up to three times improved signal-to-noise ratio compared to the classical benchmark, the coherent-state transmitter, outperforming any classically-correlated radar source at the same signal power and bandwidth. Quantum illumination is a first room-temperature application of microwave quantum circuits demonstrating quantum supremacy in detection and sensing.
Nonreciprocal circuit elements form an integral part of modern measurement and communication systems. Mathematically they require breaking of time-reversal symmetry, typically achievedusing magnetic materials and more recently using the quantum Hall effect, parametric permittivity modulation or Josephson nonlinearities. Here, we demonstrate an on-chip magnetic-free circulator based on reservoir engineered optomechanical interactions. Directional circulation is achieved with controlled phase-sensitive interference of six distinct electro-mechanical signal conversion paths. The presented circulator is compact, its silicon-on-insulator platform is compatible with both superconducting qubits and silicon photonics, and its noise performance is close to the quantum limit. With a high dynamic range, a tunable bandwidth of up to 30 MHz and an in-situ reconfigurability as beam splitter or wavelength converter, it could pave the way for superconducting qubit processors with integrated and multiplexed on-chip signal processing and readout.