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Quantum Information and Measurement (QIM)

Towards single-shot readout in double-sided coaxial circuit-QED

Martina Esposito, Joseph Rahamim, A. Patterson, James Wills, Giulio Campanaro, Takahiro Tsunoda, Peter Spring, Matthias Mergenthaler, Sophia Sosnina, Salha Jebari, Kitti Ratter, Giovanna Tancredi, Brian Vlastakis, Peter Leek

Published Jan 1, 2019• 0 citations
Publisher

Abstract

We developed and characterized a Josephson parametric amplifier based on an LC resonator and an array of superconducting quantum interference devices (SQUIDs). We performed preliminary experiments of qubit readout in a double-sided coaxial circuit-QED architecture.
DOI
10.1364/qim.2019.f5a.15

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Quantum Information and Measurement (QIM)•Jan 1, 2017•0 citations
Superconducting circuits are well established as a strong candidate platform for the development of quantum computing. In order to advance to a practically useful level, architectures are needed which combine arrays of many qubits with selective qubit control and readout, without compromising on coherence. Here, we present a coaxial circuit quantum electrodynamics architecture in which qubit and resonator are fabricated on opposing sides of a single chip, and control and readout wiring are provided by coaxial wiring running perpendicular to the chip plane. We present characterization measurements of a fabricated device in good agreement with simulated parameters and demonstrating energy relaxation and dephasing times of T1 = 4.1 μs and T2 = 5.7 μs, respectively. The architecture allows for scaling to large arrays of selectively controlled and measured qubits with the advantage of all wiring being out of the plane.
Josephson parametric amplification is a tool of paramount importance in circuit-QED especially for the quantum-noise-limited single-shot read-out of superconducting qubits. We developed a Josephson parametric amplifier (JPA) based on a lumped-element LC resonator, in which the inductance L is composed by a geometric inductance and an array of 4 superconducting quantum interference devices (SQUIDs). We characterized the main figures of merit of the device, obtaining a −3 dB bandwidth BW = 15 MHz for a gain G = 21 dB and a 1 dB compression point P1dB = −115 dBm. The obtained results are promising for the future use of such JPA as the first stage of amplification for single-shot readout of superconducting qubits.
Superconducting circuit QED is a promising approach for building a quantum computer. In order to realise superconducting circuits at a sufficient scale for useful near-term applications, an architecture with an extensible design is required which implements good connectivity between qubits, and allows for selective readout and control of the qubits without introducing detrimental crosstalk or decoherence. This thesis describes the development of a new coaxial circuit QED architecture that fulfils this requirement of extensibility by incorporating out-of-plane wiring into the sample holder. Single-qubit unit cells consisting of a transmon qubit and readout resonator with coaxial geometries are fabricated on opposing sides of a substrate, and selective control and readout of the qubits is achieved via a capacitance to coaxial wiring built into the device enclosure. Unit cells of qubit and resonator can be arranged in a 2D array without modification of the wiring scheme. A single-qubit unit cell of this architecture is used to implement dispersive circuit QED, and a full characterisation of the Hamiltonian is performed. The device is shown to have parameters comparable to those found in other approaches, such as a coupling between qubit-resonator of ~100 MHz and a coherence time of order ~10 µs. The extension of this scheme to 2D arrays of qubits is then presented, and realizations of two-qubits gates are demonstrated with fidelities all above 87% on a four qubit device. Further evaluations are performed on multi-qubit devices, including a characterisation of the drive isolation of the mode-matched drive ports, finding values in the range of 50 dB and 30 dB for measurement and control respectively. Similarly, the cross coupling between circuits is shown to have values ~2% of the coupling within a unit cell. The effective circuit temperatures are measured, finding typical values of ~100 mK, and the techniques of spin-locking and T2 spectroscopy are employed to probe the noise environment. Finally the architecture is extended to incorporate frequency tuning of qubits with gradiometric SQUID loops by way of off-chip flux bias lines (FBLs). These lines are used to tune qubits with a signal isolation of >99%. Furthermore, the ability of these FBLs to dynamically control the qubit frequency is shown by demonstrating switching of the frequency on a nanosecond time-scale, and parametric driving over a frequency range of gigahertz.
Circuit quantum electrodynamics (cQED) is a prominent platform for quantum information processing, in which microwave photons are confined into resonant cavities coupled to superconducting quantum bits (qubits). The large effective dipole moment of the qubit, in combination with the high energy density of the quasi 1-D resonator allow these systems to enter the so-called strong coupling regime. The quantum state of the qubit can then be assessed by probing the frequency of the resonator, protecting the qubit from otherwise losing its energy to the environment. However, eventhough this so-called dispersive readout technique has proven useful, it is often in itself insufficient to render single-shot readout performance | one of the crucial tasks required for realizing a quantum computer. This thesis describes the demonstration of a single-shot readout technique forsuperconducting quantum bits, based on coupling the qubit to a frequency-tunable resonator. The backbone of our device is a 5 GHz quarter-wavelength coplanar waveguide resonator, terminated at one end by a non-linear inductance provided by a superconducting quantum interference device (SQUID). The SQUID acts as a flux-controlled boundary condition, which effectively changes the electrical length of the resonator. This enables the modulation of the resonant frequency by coupling microwave magnetic flux into the SQUID, using an on-chip transmission line. The modulation occurs on a timescale much faster than the photon loss out of the resonator and if the pump strength exceeds a threshold value set by the damping rate of the resonator, an intense photon field will build up inside the resonator | known as "parametric oscillations". By heterodyne detection of the output signal from the Josephson parametric oscillator (JPO), we first extracted two leading nonlinear effects of the system (dominating in different limits of applied magnetic flux). Next, we couple a qubit to the JPO and demonstrate that we can encode its quantum state onto the strong output field of the parametric oscillator (or lack thereof), rendering a signal-to-noise ratio sufficient for single-shot state detection and therefore also obviating a quantum limited parametric amplifier. The thesis also contains results from microscopic studies of the Josephson junctions, which we use to provide the necessary nonlinearities in the above mentioned devices. In particular, we show how the microstructure of the barrier layer and its atomic composition can be used to infer important electrical properties of the junctions. This knowledge allows us to better tailor the properties of Josephson-based devices.
We propose and demonstrate a frequency-multiplexed readout scheme in 3D circuit-quantum electrodynamics (cQED) architecture. We use four transmon qubits coupled to individual rectangular cavities which are aperture-coupled to a common rectangular waveguide feedline. A coaxial to waveguide transformer at the other end of the feedline allows one to launch and collect the multiplexed signal. The reflected readout signal is amplified by an impedance-engineered broadband parametric amplifier with 380 MHz bandwidth. This provides us high fidelity single-shot readout of multiple qubits using compact microwave circuitry, an efficient way for scaling up to more qubits in 3D cQED.