In superconducting qubits, suppression of spontaneous emission is essential to achieve fast dispersive measurement and reset without sacrificing qubit lifetime. We show that resonator-mediateddecay of the qubit mode to the feedline can be suppressed using destructive interference, where the readout resonator is coupled to the feedline at two points. This „interferometric Purcell filter“ does not require dedicated filter components or impedance mismatch in the feedline, making it suitable for applications such as all-pass readout. We design and fabricate a device with the proposed scheme and demonstrate suppression of resonator-mediated decay that exceeds 2 orders of magnitude over a bandwidth of 400 MHz.
Robust and scalable multiplexed qubit readout will be essential to the realization of a fault-tolerant quantum computer. To this end, we propose and demonstrate transmission-based dispersivereadout of a superconducting qubit using an all-pass resonator that preferentially emits readout photons in one direction. This is in contrast to typical readout schemes, which intentionally mismatch the feedline at one end so that the readout signal preferentially decays toward the output. We show that this intentional mismatch creates scaling challenges, including larger spread of effective resonator linewidths due to non-ideal impedance environments and added infrastructure for impedance matching. Our proposed „all-pass readout“ architecture avoids the need for intentional mismatch and aims to enable reliable, modular design of multiplexed qubit readout, thus improving the scaling prospects of quantum computers. We design and fabricate an all-pass readout resonator that demonstrates insertion loss below 1.17 dB at the readout frequency and a maximum insertion loss of 1.53 dB across its full bandwidth for the lowest three states of a transmon qubit. We demonstrate qubit readout with an average single-shot fidelity of 98.1% in 600 ns; to assess the effect of larger dispersive shift, we implement a shelving protocol and achieve a fidelity of 99.0% in 300 ns.
We propose and demonstrate transmission-based dispersive readout of a superconducting qubit using an all-pass resonator, which preferentially emits readout photons toward the output.This is in contrast to typical readout schemes, which intentionally mismatch the feedline at one end so that the readout signal preferentially decays toward the output. We show that this intentional mismatch creates scaling challenges, including larger spread of effective resonator linewidths due to non-ideal impedance environments and added infrastructure for impedance matching. A future architecture using multiplexed all-pass readout resonators would avoid the need for intentional mismatch and potentially improve the scaling prospects of quantum computers. As a proof-of-concept demonstration of „all-pass readout,“ we design and fabricate an all-pass readout resonator that demonstrates insertion loss below 1.17 dB at the readout frequency and a maximum insertion loss of 1.53 dB across its full bandwidth for the lowest three states of a transmon qubit. We demonstrate qubit readout with an average single-shot fidelity of 98.1% in 600 ns; to assess the effect of larger dispersive shift, we implement a shelving protocol and achieve a fidelity of 99.0% in 300 ns.
We present an efficient, accurate, and comprehensive analysis framework for generic, multi-port nonlinear parametric circuits, in the presence of dissipation from lossy circuit components,based on „quantum-adapted“ X-parameters. We apply this method to Josephson traveling-wave parametric amplifiers (JTWPAs) – a key component in superconducting and spin qubit quantum computing architectures – which are challenging to model accurately due to their thousands of linear and nonlinear circuit components. X-parameters are generated from a harmonic balance solution of the classical nonlinear circuit and then mapped to the field ladder operator basis, so that the energy associated with each of the multiple interacting modes corresponds to photon occupancy, rather than classical power waves. Explicit relations for the quantum efficiency of a generic, multi-port, multi-frequency parametric circuit are presented and evaluated for two distinct JTWPA designs. The gain and quantum efficiency are consistent with those obtained from Fourier analysis of time-domain solutions, but with enhanced accuracy, speed, and the ability to include real-world impairments, statistical variations, parasitic effects, and impedance mismatches (in- and out-of-band) seamlessly. The unified flow is implemented in Keysight’s PathWave Advanced Design System (ADS) and independently in an open-source simulation code, JosephsonCircuits.jl, from the MIT authors.
Broadband quantum-limited amplifiers would advance applications in quantum information processing, metrology, and astronomy. However, conventional traveling-wave parametric amplifiers(TWPAs) support broadband amplification at the cost of increased added noise. In this work, we develop and apply a multi-mode, quantum input-output theory to quantitatively identify the sidebands as a primary noise mechanism in all conventional TWPAs. We then propose an adiabatic Floquet mode scheme that effectively eliminates the sideband-induced noise and subsequently overcomes the trade-off between quantum efficiency (QE) and bandwidth. We then show that a Floquet mode Josephson traveling-wave parametric amplifier implementation can simultaneously achieve >20dB gain and a QE of η/ηideal>99.9% of the quantum limit over more than an octave of bandwidth. Crucially, Floquet mode TWPAs also strongly suppress the nonlinear forward-backward wave coupling and are therefore genuinely directional. Floquet mode TWPAs can thus be directly integrated on-chip without isolators, making near-perfect measurement efficiency possible. The proposed Floquet scheme is also widely applicable to other platforms such as kinetic inductance traveling-wave amplifiers and optical parametric amplifiers.
We propose a traveling wave scheme for broadband microwave isolation using parametric mode conversion in conjunction with adiabatic phase matching technique in a pair of coupled nonlineartransmission lines. This scheme is compatible with the circuit quantum electrodynamics architecture (cQED) and provides isolation without introducing additional quantum noise. We first present the scheme in a general setting then propose an implementation with Josephson junction transmission lines. Numerical simulation shows more than 20 dB isolation over an octave bandwidth (4-8\,GHz) in a 2000 unit cell device with less than 0.05 dB insertion loss dominated by dielectric loss.