Mediated tunable coupling of flux qubits

It is sketched how a monostable rf- or dc-SQUID can mediate an inductive coupling between two adjacent flux qubits. The nontrivial dependence of the SQUID's susceptibility on external flux makes it possible to continuously tune the induced coupling from antiferromagnetic (AF) to ferromagnetic (FM). In particular, for suitable parameters, the induced FM coupling can be sufficiently large to overcome any possible direct AF inductive coupling between the qubits. The main features follow from a classical analysis of the multi-qubit potential. A fully quantum treatment yields similar results, but with a modified expression for the SQUID susceptibility. Since the latter is exact, it can also be used to evaluate the susceptibility--or, equivalently, energy-level curvature--of an isolated rf-SQUID for larger shielding and at degenerate flux bias, i.e., a (bistable) qubit. The result is compared to the standard two-level (pseudospin) treatment of the anticrossing, and the ensuing conclusions are verified numerically.Comment: REVTeX 4, 16 pp., 4 EPS figures. N.B.: "Alec" is my first, and "Maassen van den Brink" my family name. v2: major expansion and rewriting, new title and co-author; to appear in New Journal of Physics special issue (R. Fazio, ed.

Microwave-induced thermal escape in Josephson junctions

We investigate, by experiments and numerical simulations, thermal activation processes of Josephson tunnel junctions in the presence of microwave radiation. When the applied signal resonates with the Josephson plasma frequency oscillations, the switching current may become multi-valued in a temperature range far exceeding the classical to quantum crossover temperature. Plots of the switching currents traced as a function of the applied signal frequency show very good agreement with the functional forms expected from Josephson plasma frequency dependencies on the bias current. Throughout, numerical simulations of the corresponding thermally driven classical Josephson junction model show very good agreement with the experimental data.Comment: 10 pages and 4 figure

Resonant effects in a SQUID qubit subjected to non adiabatic changes

By quickly modifying the shape of the effective potential of a double SQUID flux qubit from a single-well to a double-well condition, we experimentally observe an anomalous behavior, namely an alternance of resonance peaks, in the probability to find the qubit in a given flux state. The occurrence of Landau-Zener transitions as well as resonant tunneling between degenerate levels in the two wells may be invoked to partially justify the experimental results. A quantum simulation of the time evolution of the system indeed suggests that the observed anomalous behavior can be imputable to quantum coherence effects. The interplay among all these mechanisms has a practical implication for quantum computing purposes, giving a direct measurement of the limits on the sweeping rates possible for a correct manipulation of the qubit state by means of fast flux pulses, avoiding transitions to non-computational states.Comment: 6 pages and 6 figures. The paper, as it is, has been accepted for publication on PRB on March 201

New application of superconductors: high sensitivity cryogenic light detectors

In this paper we describe the current status of the CALDER project, which is developing ultra-sensitive light detectors based on superconductors for cryogenic applications. When we apply an AC current to a superconductor, the Cooper pairs oscillate and acquire kinetic inductance, that can be measured by inserting the superconductor in a LC circuit with high merit factor. Interactions in the superconductor can break the Cooper pairs, causing sizable variations in the kinetic inductance and, thus, in the response of the LC circuit. The continuous monitoring of the amplitude and frequency modulation allows to reconstruct the incident energy with excellent sensitivity. This concept is at the basis of Kinetic Inductance Detectors (KIDs), that are characterized by natural aptitude to multiplexed read-out (several sensors can be tuned to different resonant frequencies and coupled to the same line), resolution of few eV, stable behavior over a wide temperature range, and ease in fabrication. We present the results obtained by the CALDER collaboration with 2x2 cm2 substrates sampled by 1 or 4 Aluminum KIDs. We show that the performances of the first prototypes are already competitive with those of other commonly used light detectors, and we discuss the strategies for a further improvement

Characterization of the KID-Based Light Detectors of CALDER

The aim of the Cryogenic wide-Area Light Detectors with Excellent Resolution (CALDER) project is the development of light detectors with active area of $5\times5$ cm$^2$ and noise energy resolution smaller than 20 eV RMS, implementing phonon-mediated kinetic inductance detectors. The detectors are developed to improve the background suppression in large-mass bolometric experiments such as CUORE, via the double read-out of the light and the heat released by particles interacting in the bolometers. In this work, we present the characterization of the first light detectors developed by CALDER. We describe the analysis tools to evaluate the resonator parameters (resonant frequency and quality factors) taking into account simultaneously all the resonance distortions introduced by the read-out chain (as the feed-line impedance and its mismatch) and by the power stored in the resonator itself. We detail the method for the selection of the optimal point for the detector operation (maximizing the signal-to-noise ratio). Finally, we present the response of the detector to optical pulses in the energy range of 0-30 keV

High sensitivity phonon-mediated kinetic inductance detector with combined amplitude and phase read-out

The development of wide-area cryogenic light detectors with good energy resolution is one of the priorities of next generation bolometric experiments searching for rare interactions, as the simultaneous read-out of the light and heat signals enables background suppression through particle identification. Among the proposed technological approaches for the phonon sensor, the naturally-multiplexed Kinetic Inductance Detectors (KIDs) stand out for their excellent intrinsic energy resolution and reproducibility. To satisfy the large surface requirement (several cm$^2$) KIDs are deposited on an insulating substrate that converts the impinging photons into phonons. A fraction of phonons is absorbed by the KID, producing a signal proportional to the energy of the original photons. The potential of this technique was proved by the CALDER project, that reached a baseline resolution of 154$\pm$7 eV RMS by sampling a 2$\times$2 cm$^2$ Silicon substrate with 4 Aluminum KIDs. In this paper we present a prototype of Aluminum KID with improved geometry and quality factor. The design improvement, as well as the combined analysis of amplitude and phase signals, allowed to reach a baseline resolution of 82$\pm$4 eV by sampling the same substrate with a single Aluminum KID

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