In quantum information processing, two primary research directions have emerged: one based on discrete variables (DV) and the other on the structure of quantum states in a continuous-variable(CV) space. It is increasingly recognized that integrating these two approaches could unlock new potentials, overcoming the inherent limitations of each. Here, we show that such a DV-CV hybrid approach, applied to superconducting Kerr parametric oscillators (KPOs), enables us to entangle a pair of Schrödinger’s cat states by two straightforward methods. The first method involves the entanglement-preserving and deterministic conversion between Bell states in the Fock-state basis (DV encoding) and those in the cat-state basis (CV encoding). This method would allow us to construct quantum networks in the cat-state basis using conventional schemes originally developed for the Fock-state basis. In the second method, the iSWAP‾‾‾‾‾‾‾√ gate operation is implemented between two cat states following the procedure used for Fock-state encoding. This DV-like gate operation on CV encoding not only completes the demonstration of a universal quantum gate set in a KPO system but also enables faster and simpler gate operations compared to previous SWAP gate implementations on bosonic modes. Our work offers a simple yet powerful application of DV-CV hybridization while also highlighting the scalability of this planar KPO system.

We report an experimental demonstration of resonance fluorescence in a two-level superconducting artificial atom under two driving fields coupled to a detuned cavity. One of the fieldsis classical and the other is varied from quantum (vacuum fluctuations) to classical one by controlling the photon number inside the cavity. The device consists of a transmon qubit strongly coupled to a one-dimensional transmission line and a coplanar waveguide resonator. We observe a sideband anti-crossing and asymmetry in the emission spectra of the system through a one-dimensional transmission line, which is fundamentally different from the weak coupling case. By changing the photon number inside the cavity, the emission spectrum of our doubly driven system approaches to the case when the atom is driven by two classical bichromatic fields. We also measure the dynamical evolution of the system through the transmission line and study the properties of the first-order correlation function, Rabi oscillations and energy relaxation in the system. The study of resonance fluorescence from an atom driven by two fields promotes understanding decoherence in superconducting quantum circuits and may find applications in superconducting quantum computing and quantum networks.

We develop a compact four-port superconducting switch with a tunable operating frequency in the range of 4.8 GHz — 7.3 GHz. Isolation between channel exceeds 20~dB over a bandwidthof several hundred megahertz, exceeding 40 dB at some frequencies. The footprint of the device is 80×420 μm. The tunability requires only a global flux bias without either permanent magnets or micro-electromechanical structures. As the switch is superconducting, the heat dissipation during operation is negligible. The device can operate at up to -80~dBm, which is equal to 2.5×106 photons at 6 GHz per microsecond. The device show a possibility to be operated as a beamsplitter with tunable splitting ratio.

We demonstrate control and readout of a superconducting artificial atom based on a transmon qubit using a compact lumped-element resonator. The resonator consists of a parallel-platecapacitor (PPC) with a wire geometric inductor. The footprint of the resonators is about 200 {\mu}m by 200 {\mu}m, which is similar to the standard transmon size and one or two orders of magnitude more compact in the occupied area comparing to coplanar waveguide resonators. We observe coherent Rabi oscillations and obtain time-domain properties of the transmon. The work opens a door to miniaturize essential components of superconducting circuits and to further scaling up quantum systems with superconducting transmons.

Quantum tunneling is the phenomenon that makes superconducting circuits „quantum“. Recently, there has been a renewed interest in using quantum tunneling in phase spaceof a Kerr parametric oscillator as a resource for quantum information processing. Here, we report a direct observation of quantum interference induced by such tunneling in a planar superconducting circuit. We experimentally elucidate all essential properties of this quantum interference, such as mapping from Fock states to cat states, a temporal oscillation induced by the pump detuning, as well as its characteristic Rabi oscillations and Ramsey fringes. Finally, we perform gate operations as manipulations of the observed quantum interference. Our findings lay the groundwork for further studies on quantum properties of Kerr parametric oscillators and their use in quantum information technologies.

The initialization of superconducting qubits is one of the essential techniques for the realization of quantum computation. In previous research, initialization above 99% fidelityhas been achieved at 280 ns. Here, we demonstrate the rapid initialization of a superconducting qubit with a quantum-circuit refrigerator (QCR). Photon-assisted tunneling of quasiparticles in the QCR can temporally increase the relaxation time of photons inside the resonator and helps release energy from the qubit to the environment. Experiments using this protocol have shown that 99\% of initialization time is reduced to 180 ns. This initialization time depends strongly on the relaxation rate of the resonator, and faster initialization is possible by reducing the resistance of the QCR, which limits the ON/OFF ratio, and by strengthening the coupling between the QCR and the resonator.

We address the scaling-up problem for superconducting quantum circuits by using lumped-element resonators based on a new fabrication method of aluminum — aluminum oxide —aluminum (Al/AlOx/Al) parallel-plate capacitors. The size of the resonators is only 0.04 mm2, which is more than one order smaller than the typical size of coplanar resonators (1 mm2). The fabrication method we developed easily fits into the standard superconducting qubits fabrication process. We have obtained capacitance per area 14 fF/μm2 and the internal quality factor 1×103−8×103 at the single-photon level. Our results show that such devices based on Al/AlOx/Al capacitors could be further applied to the qubit readout scheme, including resonators, filters, amplifiers, as well as microwave metamaterials and novel types of qubits, such as 0−π qubit.

We propose a design to realize integrated broadband nonreciprocal microwave isolators and circulators using superconducting circuit elements without any magnetic materials. To obtaina broadband response, we develop a waveguide-based design by temporal modulations. The corresponding compact traveling-wave structure is implemented with integrated superconducting composite right-/left-handed transmission lines. The calculations show that the bandwidth of 580 MHz can be realized over a nonreciprocal isolation of 20 dB in reflections. Such on-chip isolators and circulators are useful for cryogenic integrated microwave connections and measurements, such as protecting qubits from the amplified reflected signal in multiplexed readout.

We report an experimentally observed anomalous doubly split spectrum and its split-width fluctuation in an ultrastrongly coupled superconducting qubit and resonator. From an analysisof Rabimodel and circuit model Hamiltonians, we found that the doubly split spectrum and split-width fluctuation are caused by discrete charge hops due to quasiparticle tunnelings and a continuous background charge fluctuation in islands of a flux qubit. During 70 hours in the spectrum measurement, split width fluctuates but the middle frequency of the split is constant. This result indicates that quasiparticles in our device seem mainly tunnel one particular junction. The background offsetcharge obtained from split width has the 1/f noise characteristic.

Cluster states, a type of highly entangled state, are essential resources for quantum information processing. Here we demonstrated the generation of a time-domain linear cluster state(t-LCS) using a superconducting quantum circuit consisting of only two transmon qubits. By recycling the physical qubits, the t-LCS equivalent up to four physical qubits was validated by quantum state tomography with fidelity of 59%. We further confirmed the true generation of t-LCS by examining the expectation value of an entanglement witness. Our demonstrated protocol of t-LCS generation allows efficient use of physical qubits which could lead to resource-efficient execution of quantum circuits on large scale.