Subgap quasiparticle transport and low frequency critical current fluctuations in niobium/aluminum oxide/niobium Josephson junctions

Abstract

An ideal qubit for quantum computation is one that follows the DiVincenzo criteria[1]. Superconducting rf-SQUIDs meet part of the ideal qubit criteria in their ability for control and measurement, and scalability. But, it is the mechanisms that cause decoherence in the rf-SQUID qubit that are of greater importance for their utilization as a qubit in quantum computation applications.

The decoherence time of coherent quantum states in flux based superconducting qubits has inherent limitations related to the quality of the Josephson junction(s) in the qubit. Critical current fluctuations and the low-voltage subgap resistance of the junction(s) impose these limits through the Josephson energy and damping of the qubit[2, 3, 4]. These limits suggest that with a decay time from the excited qubit state of τ_1/e ∼ 2 μs at the operating temperature of the qubit having a Josephson junction with area of 4.5 μm ², critical current of 2.25 μA and a level spacing δ ∼ 10⁹ s^–1 between the coherent states, the normalized critical current fluctuations should be S_IcN (1 Hz) ∼ 10^–22 (A²/Hz)(μm/μA) ² or less and the subgap resistance should be 10⁹ Ω or greater.

The quasiparticle tunneling current (subgap leakage) and the low frequency critical current fluctuations in Josephson junctions fabricated at Stony Brook have been studied. The devices are fabricated from a Nb/AlO_x /Nb trilayer using a lift-off process and employing optical and electron beam lithography.

Subgap leakage measurements were performed down to 360 mK, and a subgap resistance greater than 1 GΩ is demonstrated. Low frequency current noise measurements of externally shunted junctions and unshunted junctions were acquired via a bridge circuit with a SQUID null current detector. Excess 1/f noise at low frequency was observed to decrease linearly from 4.2 K down to 410 mK, and the normalized critical current fluctuations measured at 1 Hz was 2.2 · 10^–24 A²/Hz·(μm/μA) ² at 4.2 K.