Probing High Frequency Noise with Macroscopic Resonant Tunneling

We have developed a method for extracting the high-frequency noise spectral density of an rf-SQUID flux qubit from macroscopic resonant tunneling (MRT) rate measurements. The extracted noise spectral density is consistent with that of an ohmic environment up to frequencies ~ 4 GHz. We have also derived an expression for the MRT lineshape expected for a noise spectral density consisting of such a broadband ohmic component and an additional strongly peaked low-frequency component. This hybrid model provides an excellent fit to experimental data across a range of tunneling amplitudes and temperatures

Geometrical dependence of low frequency noise in superconducting flux qubits

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.Comment: 5 pages, 5 figure

A scalable readout system for a superconducting adiabatic quantum optimization system

We have designed, fabricated and tested an XY-addressable readout system that is specifically tailored for the reading of superconducting flux qubits in an integrated circuit that could enable adiabatic quantum optimization. In such a system, the flux qubits only need to be read at the end of an adiabatic evolution when quantum mechanical tunneling has been suppressed, thus simplifying many aspects of the readout process. The readout architecture for an $N$-qubit adiabatic quantum optimization system comprises $N$ hysteretic dc SQUIDs and $N$ rf SQUID latches controlled by $2\sqrt{N} + 2$ bias lines. The latching elements are coupled to the qubits and the dc SQUIDs are then coupled to the latching elements. This readout scheme provides two key advantages: First, the latching elements provide exceptional flux sensitivity that significantly exceeds what may be achieved by directly coupling the flux qubits to the dc SQUIDs using a practical mutual inductance. Second, the states of the latching elements are robust against the influence of ac currents generated by the switching of the hysteretic dc SQUIDs, thus allowing one to interrogate the latching elements repeatedly so as to mitigate the effects of stochastic switching of the dc SQUIDs. We demonstrate that it is possible to achieve single qubit read error rates of $<10^{-6}$ with this readout scheme. We have characterized the system-level performance of a 128-qubit readout system and have measured a readout error probability of $8\times10^{-5}$ in the presence of optimal latching element bias conditions.Comment: Updated for clarity, final versio

Deep-well ultrafast manipulation of a SQUID flux qubit

Superconducting devices based on the Josephson effect are effectively used for the implementation of qubits and quantum gates. The manipulation of superconducting qubits is generally performed by using microwave pulses with frequencies from 5 to 15 GHz, obtaining a typical operating clock from 100MHz to 1GHz. A manipulation based on simple pulses in the absence of microwaves is also possible. In our system a magnetic flux pulse modifies the potential of a double SQUID qubit from a symmetric double well to a single deep well condition. By using this scheme with a Nb/AlOx/Nb system we obtained coherent oscillations with sub-nanosecond period (tunable from 50ps to 200ps), very fast with respect to other manipulating procedures, and with a coherence time up to 10ns, of the order of what obtained with similar devices and technologies but using microwave manipulation. We introduce the ultrafast manipulation presenting experimental results, new issues related to this approach (such as the use of a feedback procedure for cancelling the effect of "slow" fluctuations), and open perspectives, such as the possible use of RSFQ logic for the qubit control.Comment: 9 pages, 7 figure

Dynamics of Josephson junctions and single-flux-quantum networks with superconductor-insulator-normal metal junction shunts

Within the framework of the microscopic model of tunneling, we modelled the behavior of the Josephson junction shunted by the Superconductor-Insulator-Normal metal (SIN) tunnel junction. We found that the electromagnetic impedance of the SIN junction yields both the frequency-dependent damping and dynamic reactance which leads to an increase in the effective capacitance of the circuit. We calculated the dc I-V curves and transient characteristics of these circuits and explained their quantitative differences to the curves obtained within the resistively shunted junction model. The correct operation of the basic single-flux-quanta circuits with such SIN-shunted junctions, i.e. the Josephson transmission line and the toggle flip-flop, have also been modelled.Comment: 8 pages incl. 7 figure