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.
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.
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.
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.
Qubit initialization is critical for many quantum algorithms and error correction schemes, and extensive efforts have been made to achieve this with high speed and efficiency. Herewe experimentally demonstrate a fast and high fidelity reset scheme for tunable superconducting qubits. A rapid decay channel is constructed by modulating the flux through a transmon qubit and realizing a swap between the qubit and its readout resonator. The residual excited population can be suppressed to 0.08% ± 0.08% within 34 ns, and the scheme requires no additional chip architecture, projective measurements, or feedback loops. In addition, the scheme has negligible effects on neighboring qubits, and is therefore suitable for large-scale multi-qubit systems. Our method also offers a way of entangling the qubit state with an itinerant single photon, particularly useful in quantum communication and quantum network applications.
Single-photon sources are of great interest because they are key elements in different promising applications of quantum technologies. Here we demonstrate a highly efficient tunableon-demand microwave single-photon source based on a transmon qubit with the intrinsic emission efficiency more than 99%. To confirm the single-photon property of the source, we study the single-photon interference in a Hanbury-Brown-Twiss (HBT) type setup and measure the correlation functions of the emission field using linear detectors with a GPU-enhanced signal processing technique. The antibunching in the second-order correlation function is clearly observed. The theoretical calculations agree well with the experimental results. Such a high-quality single-photon source can be used as a building block of devices for quantum communication, simulations and information processing in the microwave regime.
By driving a 3D transmon with microwave fields, we generate an effective avoided energy-level crossing. Then we chirp microwave frequency, which is equivalent to driving the systemthrough the avoided energy-level crossing by sweeping the avoided crossing. A double-passage chirp produces Landau-Zener-St\“uckelberg-Majorana interference that agree well with the numerical results. Our method is fully applicable to other quantum systems that contain no intrinsic avoided level crossing, providing an alternative approach for quantum control and quantum simulation.