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Axions detection requires the ultimate sensitivity down to the single photon limit. In the microwave region this corresponds to energies in the yJ range. This extreme sensitivity has to be combined with an extremely low dark count rate, since the probability of axions conversion into microwave photons is supposed to be very low. To face this complicated task, we followed two promising approaches that both rely on the use of superconducting devices based on the Josephson effect. The first one is to use a single Josephson junction (JJ) as a switching detector (i.e. exploiting the superconducting to normal state transition in presence of microwave photons). We designed a device composed of a coplanar waveguide terminated on a current biased Josephson junction. We tested its efficiency to pulsed (pulse duration 10 ns) microwave signals, since this configuration is closer to an actual axions search experiment. We show how our device is able to reach detection capability of the order of 10 photons with frequency 8 GHz. The second approach is based on an intrinsically quantum device formed by two resonators coupled only via a superconducting qubit network (SQN). This approach relies on quantum nondemolition measurements of the resonator photons. We show that injecting RF power into the resonator, the frequency position of the resonant drop in the transmission coefficient (S21) can be modulated up to 4 MHz. We anticipate that, once optimized, both the devices have the potential to reach single photon sensitivity.

The AC Josephson effect predicted in 1962 and observed experimentally in 1963 as quantised voltage steps (the Shapiro steps) from photon assisted tunnelling of Cooper pairs is among the most fundamental phenomena of quantum mechanics and is vital for metrological quantum voltage standards. The physically dual effect, the AC coherent quantum phase slip (CQPS), photon assisted tunnelling of magnetic fluxes through a superconducting nanowire, is envisaged to reveal itself as quantised current steps. The basic physical significance of the AC CQPS is also complemented by practical importance in future current standards; a missing element for closing the Quantum Metrology Triangle. In 2012, the CQPS was demonstrated as superposition of magnetic flux quanta in superconducting nanowires. However the direct sharp current steps in superconductors; the only unavailable basic effect of superconductivity to date, was unattainable due to lack of appropriate materials and challenges in circuit engineering. Here we report the direct observation of the dual Shapiro steps in a superconducting nanowire. The sharp steps are clear up to 26 GHz frequency with current values 8.3 nA and limited by the present setup bandwidth. The current steps have been theoretically predicted in small Josephson junctions (JJs) 30 years ago. However, broadening unavoidable in JJs prevents their direct experimental observation. We solve this problem by placing a thin NbN nanowire in an inductive environment., Comment: 5 pages 4 figures

We present a blueprint for a maser with a single three-level transmon superconducting artificial atom. The system can be pumped coherently via a two-photon process, and to achieve high population inversion, the relaxation rate of the metastable state is increased via an auxiliary low-Q cavity coupled to a transition between the transmon excited states. We show numerically that such a maser can operate both in the intermediate coupling regime with super-Poissonian photon statistics and in the strong coupling regime, where the statistics is sub-Poissonian. For the former, the maser exhibits thresholdless behavior and for the latter, there is a well-defined pumping threshold. A useful side-effect of the auxiliary resonator is that it allows to overcome the photon blockade effect for the pump, which would otherwise prevent high photon population. Finally, we observe the bistability of the steady-state Wigner function and the self-quenching effect for some parameters.

In a continuous measurement scheme a spin-1/2 particle can be measured and simultaneously driven by an external resonant signal. When the driving is weak, it does not prevent the particle wave-function from collapsing and a detector randomly outputs two responses corresponding to the states of the particle. In contrast, when driving is strong, the detector returns a single response corresponding to the mean of the two single-state responses. This situation is similar to a motional averaging, observed in nuclear magnetic resonance spectroscopy. We study such quantum system, being periodically driven and probed, which consists of a qubit coupled to a quantum resonator. It is demonstrated that the transmission through the resonator is defined by the interplay between driving strength, qubit dissipation, and resonator linewidth. We demonstrate that our experimental results are in good agreement with numerical and analytical calculations., Comment: 7 pages, 4 figues

We demonstrate that the non-Hermitian Hamiltonian approach can be used as a universal tool to design and describe a performance of single photon quantum electrodynamical circuits(cQED). As an example of the validity of this method, we calculate a novel six port quantum router, constructed from 4 qubits and 3 open waveguides. We have got analytical expressions, which describe the transmission and reflection coefficients of a single photon in general form taking into account the non-uniform qubit's parameters. We show that, due to naturally derived interferences, it is possible to tune the probability of photon detection in different ports in-situ., Comment: 11 pages,9 figures, 3 tables

We study the influence of a strong off-resonant driving signal to the energy levels of a superconducting flux qubit both experimentally and theoretically. In the experiment, we carry out a three-tone spectroscopy. This allows us to directly observe the modification of the qubit's energy levels by the dynamical Stark shift caused by the driving signal. A theoretical treatment including corrections from both, rotating and counter-rotating frame, allowed us to completely explain the observed experimental results and to reconstruct the influence of the strong driving to the dissipative dynamics as well as to the coupling constants of the qubit. As one potential application, the tunability of the minimal energy-level splitting of a superconducting qubit by a microwave induced dynamical Stark shift can help to overcome the parameter spread induced by the micro fabrication of superconducting artificial quantum circuits., Comment: 23 pages, 4 figures

We have constructed a microwave detector based on the voltage switching of an underdamped Josephson junction, that is positioned at a current antinode of a {\lambda}/4 coplanar waveguide resonator. By measuring the switching current and the transmission through a waveguide capacitively coupled to the resonator at different drive frequencies and temperatures we are able to fully characterize the system and assess its detection efficiency and sensitivity. Testing the detector by applying a classical microwave field with the strength of a single photon yielded a sensitivity parameter of 0.5 in qualitative agreement with theoretical calculations., Comment: 5 pages, 5 figures

We demonstrate amplification (and attenuation) of a probe signal by a driven two-level quantum system in the Landau-Zener-St\"{u}ckelberg-Majorana regime by means of an experiment, in which a superconducting qubit was strongly coupled to a microwave cavity, in a conventional arrangement of circuit quantum electrodynamics. Two different types of flux qubit, specifically a conventional Josephson junctions qubit and a phase-slip qubit, show similar results, namely, lasing at the working points where amplification takes place. The experimental data are explained by the interaction of the probe signal with Rabi-like oscillations. The latter are created by constructive interference of Landau-Zener-St\"{u}ckelberg-Majorana (LZSM) transitions during the driving period of the qubit. A detailed description of the occurrence of these oscillations and a comparison of obtained data with both analytic and numerical calculations are given.

The Josephson effect in ballistic point contacts between single-band and multi-band superconductors was investigated. It was found that in the case of Josephson junctions formed by a single-band and an $s_\pm$-wave two-band superconductor as well as by a single-band and a three-band superconductor the junctions become frustrated, demonstrating the $\phi$-contact properties. Depending on the ground state of a three-band superconductor with broken time-reversal symmetry (BTRS), the Josephson junction can have from one to three energy minima, some of which can be locally stable. We also study the behavior of a dc SQUID based on the Josephson junctions between single-band and multi-band superconductors. Some features on the dependences of the critical current and the total magnetic flux on the applied flux of a dc SQUID based on the Josephson point contacts between a single-band superconductor and an $s_\pm$-wave superconductor, three-band superconductor with BTRS and three-band superconductor without BTRS as compared to the conventional dc SQUIDs based on single-band superconductors were found. The results can be used as an experimental tool to detect the existence of multi-band structure and BTRS., Comment: 23 pages, 11 figures

We present a design for the experimental integration of ion trapping and superconducting qubit systems as a step towards the realization of a quantum hybrid system. The scheme addresses two key difficulties in realizing such a system; a combined microfabricated ion trap and superconducting qubit architecture, and the experimental infrastructure to facilitate both technologies. Developing upon work by Kielpinski et al. [1] we describe the design, simulation and fabrication process for a microfabricated ion trap capable of coupling an ion to a superconducting microwave LC circuit with a coupling strength in the tens of kHz. We also describe existing difficulties in combining the experimental infrastructure of an ion trapping setup into a dilution fridge with superconducting qubits and present solutions that can be immediately implemented using current technology.

The behavior of a dc SQUID, based on a dirty point contacts between a single-band and three-band superconductor with broken time-reversal symmetry is investigated. Using earlier obtained results for Josephson effects in such systems new features in characteristics of a dc SQUID are revealed. It is shown that in the case of a BTRS (broken time-reversal symmetry) three-band superconductor for the applied external magnetic flux, which is divisible by the half-integer flux, strong degeneracy of ground states of a dc SQUID is taken place. This can lead to the appearance of possible multi-hysteresis loops on a dependence of a total flux in the dc SQUID from the externally applied flux. The number of these loops depends on the position of ground states of a three-band superconductor. Also it is found that dependencies of a critical current on applied magnetic flux can have complicated multi-periodic forms, which are differ from strictly periodic characteristics for conventional dc SQUIDs and Fraunhofer patterns for Josephson contacts in the external magnetic field., Comment: 11 pages, 6 figures

We study the response of a magnetic-field-driven superconducting qubit strongly coupled to a superconducting coplanar waveguide resonator. We observed a strong amplification/damping of a probing signal at different resonance points corresponding to a one and two-photon emission/absorption. The sign of the detuning between the qubit frequency and the probe determines whether amplification or damping is observed. The larger blue detuned driving leads to two-photon lasing while the larger red detuning cools the resonator. Our experimental results are in good agreement with the theoretical model of qubit lasing and cooling at the Rabi frequency.

Two parallel stacks of coupled Josephson junctions are investigated to clarify the physics of transitions between the rotating and oscillating states and their effect on the IV-characteristics of the system. The detailed study of phase dynamics and bias dependence of the superconducting and diffusion currents allows one to explain all features of simulated IV-characteristics and demonstrate the correspondence in their behavior. The coupling between JJ in the stacks leads to the branching of IV-characteristics and a decrease in the hysteretic region. The crucial role of the diffusion current in the formation of the IV-characteristic of the parallel stacks of coupled Josephson junctions is demonstrated. We discuss the effect of symmetry in a number of junctions in the stacks and show a decrease of the branching in the symmetrical stacks. The observed effects might be useful for development of superconducting electronic devices based on intrinsic Josephson junctions.

We investigate the complex conductivity of a highly disordered MoC superconducting film with $k_Fl\approx 1$, where $k_F$ is the Fermi wavenumber and $l$ is the mean free path, derived from experimental transmission characteristics of coplanar waveguide resonators in a wide temperature range below the superconducting transition temperature $T_c$. We find that the original Mattis-Bardeen model with a finite quasiparticle lifetime, $\tau$, offers a perfect description of the experimentally observed complex conductivity. We show that $\tau$ is appreciably reduced by scattering effects. Characteristics of the scattering centers are independently found by the scanning tunneling spectroscopy and agree with those determined from the complex conductivity., Comment: 8 pages, 10 figures

We analyze a system composed of a superconducting flux qubit coupled to a transmission-line resonator driven by two signals with frequencies close to the resonator's harmonics. The first strong signal is used for exciting the system to a high energetic state while a second weak signal is applied for probing effective eigenstates of the system. In the framework of doubly dressed states we showed the possibility of amplification and attenuation of the probe signal by direct transitions at the Rabi frequency. We present a brief review of theoretical and experimental works where a direct resonance at Rabi frequency have been investigated in superconducting flux qubits. The interaction of the qubit with photons of two harmonics has prospects to be used as a quantum amplifier (microwave laser) or an attenuator., Comment: This paper is the extended version of the talk given by one of the authors at the Conference On Nuclei And Mesoscopic Physics, 5-9 May 2014, Michigan State University, East Lansing, USA

We study the microwave absorption of a driven three-level quantum system, which is realized by a superconducting flux quantum circuit (SFQC), with a magnetic driving field applied to the two upper levels. The interaction between the three-level system and its environment is studied within the Born-Markov approximation, and we take into account the effects of the driving field on the damping rates of the three-level system. We study the linear response of the driven three-level SFQC to a weak probe field. The linear magnetic susceptibility of the SFQC can be changed by both the driving field and the bias magnetic flux. When the bias magnetic flux is at the optimal point, the transition from the ground state to the second excited state is forbidden and the three-level SFQC has a ladder-type transition. Thus, the SFQC responds to the probe field like natural atoms with ladder-type transitions. However, when the bias magnetic flux deviates from the optimal point, the three-level SFQC has a cyclic transition, thus it responds to the probe field like a combination of natural atoms with ladder-type transitions and natural atoms with $\Lambda$-type transitions. In particular, we provide detailed discussions on the conditions for realizing electromagnetically induced transparency and Autler-Townes splitting in three-level SFQCs., Comment: 20 pages, 8 figures

We analyse a system composed of a qubit coupled to electromagnetic fields of two high quality quantum oscillators. Particular realization of such a system is the superconducting qubit coupled to a transmission-line resonator driven by two signals with frequencies close to the resonator's harmonics. One strong signal is used for exciting the system to a high energetic state while the second weak signal is applied for probing effective eigenstates of the system. We demonstrate that a description of the system dynamics as doubly dressed qubit is applicable. Experiments show that in the case of high quality resonators the energy levels and the resonance conditions can be probed even for high driving amplitudes. The interaction of the qubit with photons of two harmonics has prospects to be used as a quantum amplifier or an attenuator., Comment: 10 pages, 3 figures

Based on the numerical solution non-stationary Ginzburg-Landau equations, we investigated the evolution of the order parameter of superconducting channels with different lengths under applied voltage (so-called voltage-driven regime). We calculated current-voltage characteristics for channels of different lengths and found out the origin of theirs characteristic disorder oscillations. For very long channels in the certain voltage interval we reveal the chaotic dynamic of the order parameter. Collected data allowed us to plot the most complete and detailed phase diagram of the resistive state of the superconducting channel in the voltage-driven regime., Comment: 13 pages, 7 figures

The ultimate goal and the theoretical limit of weak signal detection is the ability to detect a single photon against a noisy background. [...] In this paper we show, that a combination of a quantum metamaterial (QMM)-based sensor matrix and quantum non-demolition (QND) readout of its quantum state allows, in principle, to detect a single photon in several points, i.e., to observe its wave front. Actually, there are a few possible ways of doing this, with at least one within the reach of current experimental techniques for the microwave range. The ability to resolve the quantum-limited signal from a remote source against a much stronger local noise would bring significant advantages to such diverse fields of activity as, e.g., microwave astronomy and missile defence. The key components of the proposed method are 1) the entangling interaction of the incoming photon with the QMM sensor array, which produces the spatially correlated quantum state of the latter, and 2) the QND readout of the collective observable (e.g., total magnetic moment), which characterizes this quantum state. The effects of local noise (e.g., fluctuations affecting the elements of the matrix) will be suppressed relative to the signal from the spatially coherent field of (even) a single photon., Comment: 13 pages, 4 figures

We demonstrate amplification of a microwave signal by a strongly driven two-level system in a coplanar waveguide resonator. The effect known from optics as dressed-state lasing is observed with a single quantum system formed by a persistent current (flux) qubit. The transmission through the resonator is enhanced when the Rabi frequency of the driven qubit is tuned into resonance with one of the resonator modes. Amplification as well as linewidth narrowing of a weak probe signal has been observed. The laser emission at the resonator's fundamental mode has been studied by measuring the emission spectrum. We analyzed our system and found an excellent agreement between the experimental results and the theoretical predictions obtained in the dressed-state model., Comment: 5 pages, 4 figures

Two basic physical models, a two-level system and a harmonic oscillator, are realized on the mesoscopic scale as coupled qubit and resonator. The realistic system includes moreover the electronics for controlling the distance between the qubit energy levels and their populations and to read out the resonator's state, as well as the unavoidable dissipative environment. Such rich system is interesting both for the study of fundamental quantum phenomena on the mesoscopic scale and as a promising system for future electronic devices. We present recent results for the driven superconducting qubit-resonator system, where the resonator can be realized as an LC circuit or a nanomechanical resonator. Most of the results can be described by the semiclassical theory, where a qubit is treated as a quantum two-level system coupled to the classical driving field and the classical resonator. Application of this theory allows to describe many phenomena for the single and two coupled superconducting qubits, among which are the following: the equilibrium-state and weak-driving spectroscopy, Sisyphus damping and amplification, Landau-Zener-St\"uckelberg interferometry, the multiphoton transitions of both direct and ladder- type character, and creation of the inverse population for lasing., Comment: 20 pages, 15 figures

In parametric systems, squeezed states of radiation can be generated via extra work done by external sources. This eventually increases the entropy of the system despite the fact that squeezing is reversible. We investigate the entropy increase due to squeezing and show that it is quadratic in the squeezing rate and may become important in the repeated operation of tunable oscillators (quantum buses) used to connect qubits in various proposed schemes for quantum computing., Comment: 11 pages, 1 figure

Frequency-selective readout for superconducting qubits opens the way towards scaling qubit circuits up without increasing the number of measurement lines. Here we demonstrate the readout of an array of 7 flux qubits located on the same chip. Each qubit is placed near an individual lambda/4 resonator which, in turn, is coupled to a common microwave transmission line. We performed spectroscopy of all qubits and determined their parameters in a single measurement run., Comment: 3 pages, 3 figures

We study a flux qubit in a coplanar waveguide resonator by measuring transmission through the system. In our system with the flux qubit decoupled galvanically from the resonator, the intermediate coupling regime is achieved. In this regime dispersive readout is possible with weak backaction on the qubit. The detailed theoretical analysis and simulations give a good agreement with the experimental data and allow to make the qubit characterization., Comment: 4 pages, 3 figures, to be published in Phys. Rev. B

We study the loss rate for a set of lambda/2 coplanar waveguide resonators at millikelvin temperatures (20 mK - 900mK) and different applied powers (3E-19 W - 1E-12 W). The loss rate becomes power independent below a critical power. For a fixed power, the loss rate increases significantly with decreasing temperature. We show that this behavior can be caused by two-level systems in the surrounding dielectric materials. Interestingly, the influence of the two-level systems is of the same order of magnitude for the different material combinations. That leads to the assumption that the nature of these two-level systems is material independent., Comment: 3 pages, 5 figures, Submitted to Applied Physics Letters

We study spectroscopy of artificial solid-state four-level quantum system. This system is formed by two coupled superconducting flux qubits. When multiple driving frequency of the applied microwaves matches the energy difference between any two levels, the transition to the upper level is induced. We demonstrate two types of the multi-photon transitions: direct transitions between two levels and ladder-type transitions via an intermediate level. For the latter transitions, in particular, the inverse population of the excited state with respect to the ground one is realized. These processes can be useful for the control of the level population for the multilevel scalable quantum systems., Comment: 4 pages, 2 figures; v.2: minor changes

We analyze the back-action influence of nuclear spins on the motion of the cantilever of a magnetic force resonance microscope. We calculate the contribution of nuclear spins to the damping and frequency shift of the cantilever. We show that, at the Rabi frequency, the energy exchange between the cantilever and the spin system cools or heats the cantilever depending on the sign of the high-frequency detuning. We also show that the spin noise leads to a significant damping of the cantilever motion., Comment: 15 pqges, 11 figures. The last part of Section IV.A and Section IV B were rewritten. We added three new graphs: Figs. 5, 7, 9 and all graphs in this subsection were recalculated for T=600 mK as in the experiment. Several new references were added

We demonstrate theoretically the noise-stimulated enhancement of quantum coherence in a superconducting flux qubit. First, an external classical noise can increase the off-diagonal components of the qubit density matrix. Second, in the presence of noise, the Rabi oscillations survive for times significantly longer than the Rabi decay time in a noiseless system. These Rabi oscillations appear as a modulation of the forced response of the qubit to the ac driving field. These effects can be considered as a manifestation of quantum stochastic resonance and are relevant to experimental techniques, such as Rabi spectroscopy., Comment: Figures replaced. References added. Text improved

We compare the results of ground state and spectroscopic measurements carried out on superconducting flux qubits which are effective two-level quantum systems. For a single qubit and for two coupled qubits we show excellent agreement between the parameters of the pseudospin Hamiltonian found using both methods. We argue, that by making use of the ground state measurements the Hamiltonian of N coupled flux qubits can be reconstructed as well at temperatures smaller than the energy level separation. Such a reconstruction of a many-qubit Hamiltonian can be useful for future quantum information processing devices., Comment: 4 pages, 4 figures

The interaction of flux qubits with a low frequency tank circuit is studied. It is shown that changes in the state of the interacting qubits influence the effective impedance of the circuit, which is the essence of the so-called impedance measurement technique. The multiphoton resonant excitations in both single flux qubits and pairs of coupled flux qubits are investigated. In particular, we compare our theoretical results with recent spectroscopy measurements, Landau-Zener interferometry, and multiphoton fringes., Comment: 11 pages, 11 figures; v.2: minor changes

High-quality superconducting oscillators have been successfully used for quantum control and readout devices in conjunction with superconducting qubits. Also, it is well known that squeezed states can improve the accuracy of measurements to subquantum, or at least subthermal, levels. Here we show theoretically how to produce squeezed states of microwave radiation in a superconducting oscillator with tunable parameters. The circuit impedance, and thus the resonance frequency, can be changed by controlling the state of an RF SQUID inductively coupled to the oscillator. By repeatedly shifting the resonance frequency between any two values, it is possible to produce squeezed and subthermal states of the electromagnetic field in the (0.1--10) GHz range, even when the relative frequency change is small. We propose experimental protocols for the verification of squeezed state generation, and for their use to improve the readout fidelity when such oscillators serve as quantum transducers.

We analyze the influence of noise on magnetic properties of a su- perconducting loop which contains three Josephson junctions. This circuit is a classical analog of a persistent current (flux) qubit. A loop supercurrent induced by external magnetic field in the presence of thermal fluctuations is calculated. In order to get connection with experiment we calculate the impedance of the low-frequency tank cir- cuit which is inductively coupled with a loop of interest. We compare obtained results with the results in quantum mode - when the three junction loop exhibits quantum tunneling of the magnetic flux. We demonstrate that the tank-loop impedance in the classical and quan- tum modes have different temperature dependence and can be easily distinguished experimentally., Comment: 19 pages 9 figures

We use the density matrix formalism to analyze the interaction of interferometer-type superconducting qubits with a high quality tank circuit, which frequency is well below the gap frequency of a qubit. We start with the ground state characterization of the superconducting flux and charge qubits. Then, by making use of a dressed state approach we describe the qubits' spectroscopy when the qubit is irradiated by a microwave field which is tuned to the gap frequency. The last section of the paper is devoted to continuous monitoring of qubit states by using a DC SQUID in the inductive mode., Comment: 11 pages, 5 figures; the title and abstract are slightly changed; several typos are corrected; in order to make our argumentation more clear we added some comments in the introduction and other sections

Laser cooling of the atomic motion paved the way for remarkable achievements in the fields of quantum optics and atomic physics, including Bose-Einstein condensation and the trapping of atoms in optical lattices. More recently superconducting qubits were shown to act as artificial two-level atoms, displaying Rabi oscillations, Ramsey fringes, and further quantum effects. Coupling such qubits to resonators brought the superconducting circuits into the realm of quantum electrodynamics (circuit QED). It opened the perspective to use superconducting qubits as micro-coolers or to create a population inversion in the qubit to induce lasing behavior of the resonator. Furthering these analogies between quantum optical and superconducting systems we demonstrate here Sisyphus cooling of a low frequency LC oscillator coupled to a near-resonantly driven superconducting qubit. In the quantum optics setup the mechanical degrees of freedom of an atom are cooled by laser driving the atom's electronic degrees of freedom. Here the roles of the two degrees of freedom are played by the LC circuit and the qubit's levels, respectively. We also demonstrate the counterpart of the Sisyphus cooling, namely Sisyphus amplification. Parallel to the experimental demonstration we analyze the system theoretically and find quantitative agreement, which supports the interpretation and allows us to estimate system parameters., Comment: 7 pages, 4 figures

Nonlinear effects in mesoscopic devices can have both quantum and classical origins. We show that a three-Josephson-junction (3JJ) flux qubit in the _classical_ regime can produce low-frequency oscillations in the presence of an external field in resonance with the (high-frequency) harmonic mode of the system, $\omega$. Like in the case of_quantum_ Rabi oscillations, the frequency of these pseudo-Rabi oscillations is much smaller than $\omega$ and scales approximately linearly with the amplitude of the external field. This classical effect can be reliably distinguished from its quantum counterpart because it can be produced by the external perturbation not only at the resonance frequency $\omega$ and its subharmonics ($\omega/n$), but also at its overtones, $n\omega$.

Controllable adiabatic evolution of a multi-qubit system can be used for adiabatic quantum computation (AQC). This evolution ends at a configuration where the Hamiltonian of the system encodes the solution of the problem to be solved. As a first steps towards realization of AQC we have investigated two, three and four flux qubit systems. These systems were characterized by making use of a radio-frequency method. We designed two-qubit systems with coupling energies up to several kelvins. For the three-flux-qubit systems we determined the complete ground-state flux diagram in the three dimensional flux space around the qubits common degeneracy point. We show that the system`s Hamiltonian can be completely reconstructed from our measurements. Our concept for the implementation of AQC, by making use of flux qubits, is discussed., Comment: 6 pages, 7 figures, Applied Superconductivity Conference 2006

We propose to use the quantum properties of a superconducting flux qubit in the construction of a magnetometer with quantum limited sensitivity. The main advantage of a flux qubit is that its noise is rather low, and its transfer functions relative to the measured flux can be made to be about 10mV/$\Phi_0$, which is an order of magnitude more than the best value for a conventional SQUID magnetometer. We analyze here the voltage-to-flux, the phase-to-flux transfer functions and the main noise sources. We show that the experimental characteristics of a flux qubit, obtained in recent experiments, allow the use of a flux qubit as magnetometer with energy resolution close to the Planck constant., Comment: 3 pages, 6 figures

We implement a classical counterpart of a scalable design for adiabatic quantum computation. The key element of this design is a coupler providing controllable coupling between two bistable elements (in our case superconducting rings with a single Josephson junction playing the role of a classical counterpart of superconducting flux qubits) The coupler is also a superconducting ring with a single Josephson junction that operates in the non-hysteretic mode. The flux coupling between two bistable rings can be controlled by changing the magnetic flux through the coupler. Thereby, the coupling can be tuned from ferromagnetic trough zero to to anti-ferromagnetic., Comment: 3 pages, 3 figures v2: extended discussion experimental results

We have realized controllable coupling between two three-junction flux qubits by inserting an additional coupler loop between them, containing three Josephson junctions. Two of these are shared with the qubit loops, providing strong qubit--coupler interaction. The third junction gives the coupler a nontrivial current--flux relation; its derivative (i.e., the susceptibility) determines the coupling strength J, which thus is tunable in situ via the coupler's flux bias. In the qubit regime, J was varied from ~45 (antiferromagnetic) to ~ -55 mK (ferromagnetic); in particular, J vanishes for an intermediate coupler bias. Measurements on a second sample illuminate the relation between two-qubit tunable coupling and three-qubit behavior., Comment: REVTeX4, 4pp., 6 figs, 10 EPS figure files; N.B.: "Alec" is my first, and "Maassen van den Brink" my family name. v2: included discussion of a 2nd sample with sign-tunability v3:rewritten for publication

The ground state susceptibility of a system consisting of three flux-qubits was measured in the complete three dimensional flux space around the common degeneracy point of the qubits. The system's Hamiltonian could be completely reconstructed from measurements made far away from the common degeneracy point. The subsequent measurements made around this point show complete agreement with the theoretical predictions which follow from this Hamiltonian. The ground state anti-crossings of the system could be read-out directly from these measurements. This allows one to determine the ground-state flux diagram, which provides the solution for the non-polynomial optimization problem MAXCUT encoded in the Hamiltonian of the three-flux-qubit system. Our results show that adiabatic quantum computation can be demonstrated with this system provided that the energy gap and/or the speed of the read-out is increased., Comment: accepted for publication by Europhysics Letters