The scalability of the quantum processor technology is elemental in reaching fault-tolerant quantum computing. Owing to the maturity of silicon microelectronics, quantum bits (qubits) realized with spins in silicon quantum dots are considered among the most promising technologies for building scalable quantum computers. However, to realize quantum-dot-based high-fidelity quantum processing units several challenges need to be solved. In this respect, improving the charge noise environment of silicon quantum dot-based qubits and the development of ultra-low-power on-chip cryogenic classical complementary metal oxide semiconductor (CMOS) electronics for the manipulation and interfacing of the qubits are important milestones. We report scalable interfacing of highly tunable and ultra-low charge noise electron and hole quantum dots embedded in a 64-channel cryogenic CMOS multiplexer, which has less-than-detectable static power dissipation (< 1 pW) even at sub-1 K temperatures. Our integrated hybrid quantum-dot CMOS technology provides a route to scalable interfacing of up to millions of high-quality quantum dots enabling, for example, straightforward variability analysis and qubit geometry optimization, which are essential prerequisites for building fault-tolerant large-scale silicon-based quantum computers. At 5.6 K temperature, we find unprecedentedly low charge noise of 22 and 28 μeV/sqrt(Hz) at 1 Hz in the electrostatically defined few-electron and few-hole quantum dots, respectively. The low-noise quantum dots are realized by harnessing a custom CMOS process that utilizes a conventional doped-Poly-Si/SiO2/Si MOS stack. This approach provides lower charge noise background than high-k metal gate solutions and translates into higher spin qubit fidelities.

We have integrated single and coupled superconducting transmon qubits into flip-chip modules. Each module consists of two chips – one quantum chip and one control chip – that are bump-bonded together. We demonstrate time-averaged coherence times exceeding 90 μs, single-qubit gate fidelities exceeding 99.9%, and two-qubit gate fidelities above 98.6%. We also present device design methods and discuss the sensitivity of device parameters to variation in interchip spacing. Notably, the additional flip-chip fabrication steps do not degrade the qubit performance compared to our baseline state-of-the-art in single-chip, planar circuits. This integration technique can be extended to the realisation of quantum processors accommodating hundreds of qubits in one module as it offers adequate input/output wiring access to all qubits and couplers.

Refrigeration is an important enabler for quantum technology. The very low energy of the fundamental excitations typically utilized in quantum technology devices and systems requires temperature well below 1 K. Expensive cryostats are utilized in reaching sub-1 K regime and solid-state cooling solutions would revolutionize the field. New electronic micro-coolers based on phonon-blocked semiconductor-superconductor junctions could provide a viable route to such miniaturization. Here, we investigate the performance limits of these junction refrigerators.

We demonstrate highly transparent silicon-vanadium and silicon-aluminum tunnel junctions with relatively low sub-gap leakage current and discuss how a trade-off typically encountered between transparency and leakage affects their refrigeration performance. We theoretically investigate cascaded superconducting tunnel junction refrigerators with two or more refrigeration stages. In particular, we develop an approximate method that takes into account self-heating effects but still allows us to optimize the cascade a single stage at a time. We design a cascade consisting of energy-efficient refrigeration stages, which makes cooling of, e.g., quantum devices from above 1 K to below 100 mK a realistic experimental target.

We have used focused ion beam irradiation to progressively cause defects in annealed molybdenum silicide thin films. Without the treatment, the films are superconducting with critical temperature of about 1 K. We observe that both resistivity and critical temperature increase as the ion dose is increased. For resistivity, the increase is almost linear, whereas critical temperature changes abruptly at the smallest doses and then remains almost constant at 4 K. We believe that our results originate from amorphization of the polycrystalline molybdenum silicide films.

Quantum thermodynamics is emerging both as a topic of fundamental research and as means to understand and potentially improve the performance of quantum devices. A prominent platform for achieving the necessary manipulation of quantum states is superconducting circuit quantum electrodynamics (QED). In this platform, thermalization of a quantum system can be achieved by interfacing the circuit QED subsystem with a thermal reservoir of appropriate Hilbert dimensionality. Here we study heat transport through an assembly consisting of a superconducting qubit capacitively coupled between two nominally identical coplanar waveguide resonators, each equipped with a heat reservoir in the form of a normal-metal mesoscopic resistor termination. We report the observation of tunable photonic heat transport through the resonator-qubit-resonator assembly, showing that the reservoir-to-reservoir heat flux depends on the interplay between the qubit-resonator and the resonator-reservoir couplings, yielding qualitatively dissimilar results in different coupling regimes. Our quantum heat valve is relevant for the realisation of quantum heat engines and refrigerators, that can be obtained, for example, by exploiting the time-domain dynamics and coherence of driven superconducting qubits. This effort would ultimately bridge the gap between the fields of quantum information and thermodynamics of mesoscopic systems.

Superconductivity can be understood in terms of a phase transition from an uncorrelated electron gas to a condensate of Cooper pairs in which the relative phases of the constituent electrons are coherent over macroscopic length scales. The degree of correlation is quantified by a complex-valued order parameter, whose amplitude is proportional to the strength of the pairing potential in the condensate. Supercurrent-carrying states are associated with non-zero values of the spatial gradient of the phase. The pairing potential and several physical observables of the Cooper condensate can be manipulated by means of temperature, current bias, dishomogeneities in the chemical composition or application of a magnetic field. Here we show evidence of complete suppression of the energy gap in the local density of quasiparticle states (DOS) of a superconducting nanowire upon establishing a phase difference equal to pi over a length scale comparable to the superconducting coherence length. These observations are consistent with a complete collapse of the pairing potential in the center of the wire, in accordance with theoretical modeling based on the quasiclassical theory of superconductivity in diffusive systems. Our spectroscopic data, fully exploring the phase-biased states of the condensate, highlight the profound effect that extreme phase gradients exert on the amplitude of the pairing potential. Moreover, the sharp magnetic response observed near the onset of the superconducting gap collapse regime can be exploited to realize ultra-low noise magnetic flux detectors.