We have designed, fabricated and operated a scalable system for applying independently programmable time-independent, and limited time-dependent flux biases to control superconducting devices in an integrated circuit. Here we report on the operation of a system designed to supply 64 flux biases to devices in a circuit designed to be a unit cell for a superconducting adiabatic quantum optimization (AQO) system. The system requires six digital address lines, two power lines, and a handful of global analog lines.PACS numbers: 85.25.Dq, 85.25.Hv, 03.67.Lx arXiv:0907.3757v2 [quant-ph]

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...

We report on the functionality of a Nb-based superconducting single flux quantum (SFQ) toggle flip-flop (TFF) circuit, comprising a complementary superconductor-ferromagnetsuperconductor (SFS) Josephson π-junction. The SFS junction was used as a phase shifting element inserted in the storage loop of the TFF. The fabricated circuits demonstrated correct functionality with the operation parameter ranges of ±20%. The application of SFS π-junctions makes the SFQ circuits very compact, may substantially improve their stability, and may also be suitable for integration with Josephson quantum circuits (qubits).

We have designed an experiment and performed extensive simulations and preliminary measurements to identify a set of realistic circuit parameters that should allow the observation of constantcurrent steps at I = 2ef in short arrays of small Josephson junctions under external AC drive of frequency f . Observation of these steps demonstrating phase lock of the Bloch oscillations with the external drive requires a high-impedance environment for the array, which is provided by on-chip resistors close to the junctions. We show that the width and shape of the steps crucially depend on the shape of the drive and the electron temperature in the resistors.

We study the applicability of superconducting NbN vacuum bridge bolometer arrays with room temperature readout electronics to passive THz imaging applications. We show that sufficient bandwidth for video-rate mechanical scanning in terms of stability and noise can be reached by exploiting the divergences of the bolometer noise temperature and the differential impedance at the curve minimum.Experimental electrical noise equivalent power is 9 fW Hz 1 2 .This would correspond to 10 times the photon noise in a bandwidth of 0.5 THz and is comparable to the expected clutter in passive THz images due to atmospheric fluctuations.