Geometrical dependence of the low-frequency noise in superconducting flux qubits

Abstract

A general method for directly measuring the low-frequency flux noise (below 10 Hz) in compound Josephson junction superconducting flux qubits has been used to study a series of 85 devices of varying design. The variation in flux noise across sets of qubits with identical designs was observed to be small. However, the levels of flux noise systematically varied between qubit designs with strong dependence upon qubit wiring length and wiring width. Furthermore, qubits fabricated above a superconducting ground plane yielded lower noise than qubits without such a layer. These results support the hypothesis that localized magnetic impurities in the vicinity of the qubit wiring are a key source of low frequency flux noise in superconducting devices.Qubits implemented in superconducting integrated circuits show considerable promise as building blocks of scalable quantum processors. However, low frequency noise in superconducting devices places fundamental limitations on their use in quantum information processing [1,2,3,4]. Recent theoretical work has highlighted several potential sources for low frequency noise. These include ensembles of two level systems (TLS) that could be associated with dielectric defects [5,6,7], magnetic impurities in surface oxides on superconducting wiring [8] and flux noise induced by spin flips at dielectric interfaces [9]. Characterizing low frequency noise is an essential step in understanding its mechanism and in developing fabrication strategies to minimize its amplitude. Several techniques have been exploited to indirectly measure low frequency noise in superconducting qubits [10,11]. This article describes a technique for directly measuring low frequency noise in RF-SQUID flux qubits. We present measurements performed on a series of qubits of varying wiring lengths and widths and qubits with and without superconducting shielding layers.The devices described in this paper were fabricated on an oxidized Si wafer with a Nb/Al/Al 2 O 3 /Nb trilayer process. There were two additional wiring layers, WIRA, and WIRB, above the trilayer (see Fig 1a). All wiring layers were insulated from each other with layers of sputtered SiO 2 . Eighty-five qubits with a range of geometries (wiring length, wiring width, and the presence or absence of shielding planes) were tested. Qubit wiring lengths ranged from 350 µm to 2.1 mm and wiring widths ranged from 1.4 µm to 3.5 µm. Moreover, the qubits were drawn from several wafers to control for variability in fabrication process conditions.The compound Josephson junction (CJJ) RF-SQUID is shown schematically in Fig. 1b and consists the Hamiltonian for an isolated device can be approximately expressed as [12]:where Φ q is the total flux, Q is the charge stored in the net capacitance C q across the junctions, E J = Φ 0 I q c /2π and Φ 0 = h/2e. The potential energy U (Φ q ) is monostable when β = 2πL q I c cos(πΦ cjj x /Φ 0 )/Φ 0 < 1 and classically bistable, with two counter-circulating persistent current states (denoted as |0 and |1 ) possessing persistent cur...

Flux qubit noise spectroscopy using Rabi oscillations under strong driving conditions

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Optimal cooling of a driven artificial atom in dissipative environment

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“…Such a Hamiltonian will be drawn from one of the Gaussian ensembles of random matrix theory. Recent measurements of the low frequency flux noise in superconducting flux qubits exhibit a coloured noise spectrum [15]. We therefore assume that the noise term, δh(λ), evolves in time as a Ornstein-Uhlenbeck type process, which is a simple example of a random process with a coloured spectrum:…”