Isolation of a Josephson qubit from the electromagnetic environment

Abstract

We consider two aspects of isolation of a Josephson flux (charge-flux) qubit from the external dissipative electromagnetic environment: (i) selecting an optimal topology of the superconducting qubit circuit and (ii) passive filtering of Planck radiation at the input of the qubit-state detection circuit. When reading the state of a macroscopic quantum object (“Schrödinger's cat”) with the weak continuous measurement technique, the coupling to the environment, both direct and through the connected circuits, is the cause of the rapid loss of coherence of the superposition states. The coefficients of coupling to the external electromagnetic environment are discussed, as well as the problem of their minimization for flat (2D) and bulk (3D) designs of the qubit quantization loops. The analysis of the characteristics of low-temperature combined broadband filters designed to effectively reduce the electromagnetic noise in the control and measurement circuits is carried out. It is shown experimentally that a cryogenic Cu–CuO powder filter installed directly into the measuring circuit results in a significant suppression of the back action of a cooled HEMT amplifier to the qubit.

Cooled ferromagnetic shield as a part hybrid system for isolation of a flux qubit from electromagnetic environment

“…Б.І. Вєркіна НАН України, 47 Проспект Науки, Харків 61103, Україна 2 Інститут металофізики імені Г.В. Курдюмова НАН України, 03142, Київ, бульвар Академіка Вернадського, 36 3 Київський академічний університет, бульвар Академіка Вернадського, 36, Київ 03142, Україна Наразі квантові схеми для квантової інформатики, комунікаційного та вимірювального обладнання, що містить надпровідні потокові кубіти в планарному дизайні створюються методами квантової інженерії.…”

“…Thus, to let the qubit exhibit its quantum behavior, it is necessary, in addition to low temperatures ~10 mK, to provide low enough and stable magnetic fields, i.e. the question arises of the magnetic shielding and, generally speaking, of the isolation of the qubit from the electromagnetic environment [2]. Earlier, we proposed [3] a scheme for a single-photon counter in the microwave range based on a flux qubit.…”

“…Coaxially located ferromagnetic and superconducting screens not only have a higher magnetic field attenuation coefficient than taken alone, but also significantly reduce the spatial field variations. In subtle experiments with superconducting interferometers and qubits during quantum measurements, this is of decisive importance, along with careful filtering of the input circuits, and provide necessary isolation of the quantum object from the electromagnetic environment [2].…”