Reducing Spontaneous Emission in Circuit Quantum Electrodynamics by a Combined Readout/Filter Technique

Abstract

Physical implementations of qubits can be extremely sensitive to environmental coupling, which can result in decoherence. While efforts are made for protection, coupling to the environment is necessary to measure and manipulate the state of the qubit. As such, the goal of having long qubit energy relaxation times is in competition with that of achieving high-fidelity qubit control and measurement. Here we propose a method that integrates filtering techniques for preserving superconducting qubit lifetimes together with the dispersive coupling of the qubit to a microwave resonator for control and measurement. The result is a compact circuit that protects qubits from spontaneous loss to the environment, while also retaining the ability to perform fast, high-fidelity readout. Importantly, we show the device operates in a regime that is attainable with current experimental parameters and provide a specific example for superconducting qubits in circuit quantum electrodynamics. 1051-8223 (c)

Enhanced qubit readout using locally generated squeezing and inbuilt Purcell-decay suppression

“…Remarkably, these are not issues in the ISTMS setup. This is the result of an interference effect involving the two cavities in the setup that cause it to act as its own, intrinsic Purcell filter, thus removing the necessity to construct an external filter [14,[34][35][36]. Heuristically, vacuum noise entering the right cavity can either drive the even or odd cavity normal mode before reaching the qubit (see figure 4(a)).…”

Quantum theory of a bandpass Purcell filter for qubit readout

“…3(b) we choose ω d so that in the steady statē n |g f =n |e f for driving the filter; this is the natural choice for decreasing the microwave power to be amplified. This occurs at ω d /2π = 6.80120, which is close to the expected value (ω |g r + ω |e r )/2, but not equal because of the asymmetry of the line shape (19). We choose the amplitudes to producen Fig.…”

“…This leads to a trade-off between the qubit relaxation and measurement time, whereas it is desirable to suppress the Purcell rate without compromising qubit measurement. Several proposals have been put forward for this purpose, which include employing a Purcell filter [16][17][18][19], engineering a Purcell-protected qubit [20,21], or using a tunable coupler that decouples the transmission line from the resonator during the qubit-resonator interaction, thereby avoiding the Purcell effect altogether [22].…”

Equation of motion approach to black-box quantization: Taming the multimode Jaynes-Cummings model

“…corresponding to the frequency shift of the qubit, the frequency shift of the resonator, and the coherent coupling rate. Transforming the quantum Stratonovich stochastic differential equations (27) and (28) to an equivalent Lindblad equation yieldṡ…”