High Kinetic Inductance NbN Nanowire Superinductors

We demonstrate that a high kinetic inductance disordered superconductor can realize a low microwave loss, non-dissipative circuit element with an impedance greater than the quantum resistance ($R_Q = h/4e^2 \simeq 6.5k\Omega$). This element, known as a superinductor, can produce a quantum circuit where charge fluctuations are suppressed. The superinductor consists of a 40 nm wide niobium nitride nanowire and exhibits a single photon quality factor of $2.5 \times 10^4$. Furthermore, by examining loss rates, we demonstrate that the dissipation of our nanowire devices can be fully understood in the framework of two-level system loss

Line Widths of Single-Electron Tunneling Oscillations: Experiment and Numerical Simulations

We present experimental and numerical results from a real-time detection of time-correlated single-electron tunneling oscillations in a one-dimensional series array of small tunnel junctions. The electrons tunnel with a frequency f=I/e, where I is the current and e is the electron charge. Experimentally, we have connected a single-electron transistor to the last array island, and in this way measured currents from 5 fA to 1 pA by counting the single electrons. We find that the line width of the oscillation is proportional to the frequency f. The experimental data agrees well with numerical simulations.Comment: 2 pages, 1 figure. Submitted to the 24th International Conference on Low Temperature Physics (LT24), Orlando, FL, USA, Aug. 2005; to be published in the AIP Conference Proceedings serie

Direct observation of time correlated single-electron tunneling

We report a direct detection of time correlated single-electron tunneling oscillations in a series array of small tunnel junctions. Here the current, I, is made up of a lattice of charge solitons moving throughout the array by time correlated tunneling with the frequency f=I/e, where e is the electron charge. To detect the single charges, we have integrated the array with a radio-frequency single-electron transistor (RF-SET) and employed two different methods to couple the array to the SET input: by direct injection through a tunnel junction, and by capacitive coupling. In this paper we report the results from the latter type of charge input, where we have observed the oscillations in the frequency domain and measured currents from 50 to 250 fA by means of electron counting.Comment: 2 pages, 1 figure; submitted to the 10th International Superconductive Electronics Conference (ISEC'05), the Netherlands, Sept. 200

Noise and loss of superconducting aluminium resonators at single photon energies

The loss and noise mechanisms of superconducting resonators are useful tools for understanding decoherence in superconducting circuits. While the loss mechanisms have been heavily studied, noise in superconducting resonators has only recently been investigated. In particular, there is an absence of literature on noise in the single photon limit. Here, we measure the loss and noise of an aluminium on silicon quarter-wavelength ($\lambda/4$) resonator in the single photon regime.Comment: LT28 Conference proceeding, to be published in IOP Conference Serie

Improved content mastery and written communication through a lab-report assignment with peer review: an example from a quantum engineering course

The promotion of high-quality written communication in the disciplines is an important learning outcome in higher education. Given the time invested by students and teachers alike, it is crucial that writing assignments also promote engagement and content learning. But is it worth the time for university teachers to invest in such \u27writing-to-learn\u27 activityes? We find that it can be, and present an improved design for an experimental lab-report writing assignment in an English medium instruction environment, where English is an additional language. Our context is assignment development for formative assessment in master\u27s-level physics, but the method is broadly applicable within the science-technology-engineering-math disciplines. Our first experience with the assignment resulted in substandard lab reports, suggesting insufficient subject understanding and prompting this assignment design. We therefore focused on communicating the alignment of aims, learning objectives, instruction, assessment criteria, and feedback design, and developed simplified rubrics facilitating assessment fairness and efficiency. The revised assignment enhanced the learning of the subject matter and the writing quality over the four years of the study, indicated by clearly improved reports and relevant peer feedback comments. The learning activity also had an observable but less distinct effect on the students\u27 exam performance

Time-Reversal Symmetry and Universal Conductance Fluctuations in a Driven Two-Level System

In the presence of time-reversal symmetry, quantum interference gives strong corrections to the electric conductivity of disordered systems. The self-interference of an electron wavefunction traveling time-reversed paths leads to effects such as weak localization and universal conductance fluctuations. Here, we investigate the effects of broken time-reversal symmetry in a driven artificial two-level system. Using a superconducting flux qubit, we implement scattering events as multiple Landau-Zener transitions by driving the qubit periodically back and forth through an avoided crossing. Interference between different qubit trajectories give rise to a speckle pattern in the qubit transition rate, similar to the interference patterns created when coherent light is scattered off a disordered potential. Since the scattering events are imposed by the driving protocol, we can control the time-reversal symmetry of the system by making the drive waveform symmetric or asymmetric in time. We find that the fluctuations of the transition rate exhibit a sharp peak when the drive is time-symmetric, similar to universal conductance fluctuations in electronic transport through mesoscopic systems

Period-tripling subharmonic oscillations in a driven superconducting resonator

We have observed period-tripling subharmonic oscillations, in a superconducting coplanar waveguide resonator operated in the quantum regime, $k_B T \ll \hbar\omega$. The resonator is terminated by a tunable inductance that provides a Kerr-type nonlinearity. We detected the output field quadratures at frequencies near the fundamental mode, $\omega/2\pi \sim 5\,$GHz, when the resonator was driven by a current at $3\omega$ with an amplitude exceeding an instability threshold. The output radiation was red-detuned from the fundamental mode. We observed three stable radiative states with equal amplitudes and phase-shifted by $120^\circ$. The downconversion from $3\omega$ to $\omega$ is strongly enhanced by resonant excitation of the second mode of the resonator, and the cross-Kerr effect. Our experimental results are in quantitative agreement with a model for the driven dynamics of two coupled modes

Decoherence benchmarking of superconducting qubits

We benchmark the decoherence of superconducting qubits to examine the temporal stability of energy-relaxation and dephasing. By collecting statistics during measurements spanning multiple days, we find the mean parameters $\overline{T_{1}}$ = 49 $\mu$s and $\overline{T_{2}^{*}}$ = 95 $\mu$s, however, both of these quantities fluctuate explaining the need for frequent re-calibration in qubit setups. Our main finding is that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems (TLS). Through statistical analysis, we determine switching rates of these TLS and observe the coherent coupling between an individual TLS and a transmon qubit. Finally, we find evidence that the qubit's frequency stability is limited by capacitance noise. Importantly, this produces a 0.8 ms limit on the pure dephasing which we also observe. Collectively, these findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity.Comment: 15 pages ArXiv version rev

Noise correlations in a flux qubit with tunable tunnel coupling

We have measured flux-noise correlations in a tunable superconducting flux qubit. The device consists of two loops that independently control the qubit's energy splitting and tunnel coupling. Low frequency flux noise in the loops causes fluctuations of the qubit frequency and leads to dephasing. Since the noises in the two loops couple to different terms of the qubit Hamiltonian, a measurement of the dephasing rate at different bias points provides a way to extract both the amplitude and the sign of the noise correlations. We find that the flux fluctuations in the two loops are anti-correlated, consistent with a model where the flux noise is generated by randomly oriented unpaired spins on the metal surface.Comment: 7 pages, including supplementary materia

Dynamical decoupling and dephasing in interacting two-level systems

We implement dynamical decoupling techniques to mitigate noise and enhance the lifetime of an entangled state that is formed in a superconducting flux qubit coupled to a microscopic two-level system. By rapidly changing the qubit's transition frequency relative to the two-level system, we realize a refocusing pulse that reduces dephasing due to fluctuations in the transition frequencies, thereby improving the coherence time of the entangled state. The coupling coherence is further enhanced when applying multiple refocusing pulses, in agreement with our $1/f$ noise model. The results are applicable to any two-qubit system with transverse coupling, and they highlight the potential of decoupling techniques for improving two-qubit gate fidelities, an essential prerequisite for implementing fault-tolerant quantum computing