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In the microscopic world, multipartite entanglement has been achieved with various types of nanometer sized two-level systems such as trapped ions, atoms and photons. On the macroscopic scale ranging from micrometers to millimeters, recent experiments have demonstrated bipartite and tripartite entanglement for electronic quantum circuits with superconducting Josephson junctions. It remains challenging to bridge these largely different length scales by constructing hybrid quantum systems. Doing this may allow for manipulating the entanglement of individual microscopic objects separated by macroscopically large distances in a quantum circuit. Here we report on the experimental demonstration of induced coherent interaction between two intrinsic two-level states (TLSs) formed by atomic-scale defects in a solid via a superconducting phase qubit. The tunable superconducting circuit serves as a shuttle communicating quantum information between the two microscopic TLSs. We present a detailed comparison between experiment and theory and find excellent agreement over a wide range of parameters. We then use the theoretical model to study the creation and movement of entanglement between the three components of the quantum system., Comment: 11 pages, 5 figures

We use high-precision spectroscopy and detailed theoretical modelling to determine the form of the coupling between a superconducting phase qubit and a two-level defect. Fitting the experimental data with our theoretical model allows us to determine all relevant system parameters. A strong qubit-defect coupling is observed, with a nearly vanishing longitudinal component. Using these estimates, we quantitatively compare several existing theoretical models for the microscopic origin of two-level defects., Comment: 3 pages, 2 figures. Supplementary material, lclimits_supp.pdf

We present experimental results on a dispersive scheme for reading out a Josephson phase qubit. A capacitively shunted dc-SQUID is used as a nonlinear resonator which is inductively coupled to the qubit. We detect the flux state of the qubit by measuring the amplitude and phase of a microwave pulse reflected from the SQUID resonator. By this low-dissipative method, we reduce the qubit state measurement time down to 25 microseconds, which is much faster than using the conventional readout performed by switching the SQUID to its non-zero dc voltage state. The demonstrated readout scheme allows for reading out multiple qubits using a single microwave line by employing frequency-division multiplexing., Comment: 4 pages, 4 figures

We have developed a combined photolithography and electron-beam lithography fabrication process for sub-\mum to \mum-size Nb/Al-AlOx/Nb Josephson junctions. In order to define the junction size and protect its top electrode during anodic oxidation, we developed and used the new concept of an aluminum hard mask. Josephson junctions of sizes down to 0.5 \mum2 have been fabricated and thoroughly characterized. We found that they have a very high quality, which is witnessed by the IV curves with quality parameters Vm > 50 mV and Vgap = 2.8 mV at 4.2 K, as well as IcRN products of 1.75-1.93 mV obtained at lower temperatures. In order to test the usability of our fabrication process for superconducting quantum bits, we have also designed, fabricated and experimentally investigated phase qubits made of these junctions. We found a relaxation time of T1 = 26 ns and a dephasing time of T2 = 21 ns.

Controlled manipulation of quantum states is central to studying natural and artificial quantum systems. If a quantum system consists of interacting sub-units, the nature of the coupling may lead to quantum levels with degenerate energy differences. This degeneracy makes frequency-selective quantum operations impossible. For the prominent group of transversely coupled two-level systems, i.e. qubits, we introduce a method to selectively suppress one transition of a degenerate pair while coherently exciting the other, effectively creating artificial selection rules. It requires driving two qubits simultaneously with the same frequency and specified relative amplitude and phase. We demonstrate our method on a pair of superconducting flux qubits. It can directly be applied to the other superconducting qubits, and to any other qubit type that allows for individual driving. Our results provide a single-pulse controlled-NOT gate for the class of transversely coupled qubits.

We present experimental results on the crosstalk between two AC-operated dispersive bifurcation detectors, implemented in a circuit for high-fidelity readout of two strongly coupled flux qubits. Both phase-dependent and phase-independent contributions to the crosstalk are analyzed. For proper tuning of the phase the measured crosstalk is 0.1 % and the correlation between the measurement outcomes is less than 0.05 %. These results show that bifurcative readout provides a reliable and generic approach for multi-partite correlation experiments., Comment: Copyright 2010 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters and may be found at http://link.aip.org/link/?apl/96/123508

We demonstrate a new method to directly manipulate the state of individual two-level systems (TLS) in phase qubits. It allows one to characterize the coherence properties of TLS using standard microwave pulse sequences, while the qubit is used only for state readout. We apply this method to measure the temperature dependence of TLS coherence for the first time. The energy relaxation time $T_1$ is found to decrease quadratically with temperature for the two TLS studied in this work, while their dephasing time measured in Ramsey and spin-echo experiments is found to be $T_1$ limited at all temperatures., Comment: 4 pages, 5 figures

We measure the dispersive energy-level shift of an $LC$ resonator magnetically coupled to a superconducting qubit, which clearly shows that our system operates in the ultrastrong coupling regime. The large mutual kinetic inductance provides a coupling energy of $\approx0.82$~GHz, requiring the addition of counter-rotating-wave terms in the description of the Jaynes-Cummings model. We find a 50~MHz Bloch-Siegert shift when the qubit is in its symmetry point, fully consistent with our analytical model., Comment: Published version (4 pages, 4 figures), including supplementary material (2 pages, 4 figures)

The difference between the phases of superconducting order parameter plays in superconducting circuits the role similar to that played by the electrostatic potential difference required to drive a current in conventional circuits. This fundamental property can be altered by inserting in a superconducting circuit a particular type of weak link, the so-called Josephson $\pi$-junction having inverted current-phase relation and enabling a shift of the phase by $\pi$. We demonstrate the operation of three superconducting circuits -- two of them are classical and one quantum -- which all utilize such $\pi$-phase shifters realized using superconductor-ferromagnet-superconductor sandwich technology. The classical circuits are based on single-flux-quantum cells, which are shown to be scalable and compatible with conventional niobium-based superconducting electronics. The quantum circuit is a $\pi$-phase biased qubit, for which we observe coherent Rabi oscillations and compare the measured coherence time with that of conventional superconducting phase qubits., Comment: 14 pages, 4 figures

We report on two different manipulation procedures of a tunable rf SQUID. First, we operate this system as a flux qubit, where the coherent evolution between the two flux states is induced by a rapid change of the energy potential, turning it from a double well into a single well. The measured coherent Larmor-like oscillation of the retrapping probability in one of the wells has a frequency ranging from 6 to 20 GHz, with a theoretically expected upper limit of 40 GHz. Furthermore, here we also report a manipulation of the same device as a phase qubit. In the phase regime, the manipulation of the energy states is realized by applying a resonant microwave drive. In spite of the conceptual difference between these two manipulation procedures, the measured decay times of Larmor oscillation and microwave-driven Rabi oscillation are rather similar. Due to the higher frequency of the Larmor oscillations, the microwave-free qubit manipulation allows for much faster coherent operations., Comment: Proceedings of Nobel Symposium "Qubits for future quantum computers", Goeteborg, Sweden, May 25-28, 2009; to appear in Physica Scripta

We present experimental studies of high quality underdamped 0, pi, and 0-pi ferromagnetic Josephson tunnel junctions of intermediate length L (lambda_J < L < 5 lambda_J, where lambda_J is the Josephson penetration depth). The junctions are fabricated as Nb/Al_2O_3/Cu_40Ni_60/Nb Superconductor-Insulator-Ferromagnet-Superconductor heterostructures. Using microwave spectroscopy, we have investigated the eigenfrequencies of 0, pi, and 0-pi Josephson junctions in the temperature range 1.9K...320mK. Harmonic, subharmonic and superharmonic pumping is observed in experiment, and the experimental data are compared with numerical simulations. Escape rate measurements without applied microwaves at temperatures T down to 20mK show that the width of the switching current histogram decreases with temperature and saturates below T=150mK. We analyze our data in the framework of the short junction model. The differences between experimental data and theoretical predictions are discussed., Comment: submitted to RPB

We experimentally demonstrate the coherent oscillations of a tunable superconducting flux qubit by manipulating its energy potential with a nanosecond-long pulse of magnetic flux. The occupation probabilities of two persistent current states oscillate at a frequency ranging from 6 GHz to 21 GHz, tunable via the amplitude of the flux pulse. The demonstrated operation mode allows to realize quantum gates which take less than 100 ps time and are thus much faster compared to other superconducting qubits. An other advantage of this type of qubit is its insensitivity to both thermal and magnetic field fluctuations., Comment: 5 pages, 5 figures

We present experimental studies of static and dynamic properties of 0, pi and 0-pi superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson junctions of small and intermediate length. In the underdamped limit these junctions exhibit a rich dynamical behavior such as resonant steps on the current-voltage characteristics. Varying the experimental conditions, zero field steps, Fiske steps and Shapiro steps are observed with a high resolution. A strong signature of the 0-pi Josephson junction is demonstrated by measuring the critical current as a function of two components (B_x, B_y) of an in-plane magnetic field. The experimental observation of a half-integer zero field step in 0-pi SIFS junctions is presented., Comment: 9 pages, 7 figures, 1 table

We experimentally investigate the temperature dependence of Rabi oscillations and Ramsey fringes in superconducting phase qubits driven by microwave pulses. In a wide range of temperatures, we find that both the decay time and the amplitude of these coherent oscillations remain nearly unaffected by thermal fluctuations. The oscillations are observed well above the crossover temperature from thermally activated escape to quantum tunneling for undriven qubits. In the two-level limit, coherent qubit response rapidly vanishes as soon as the energy of thermal fluctuations kT becomes larger than the energy level spacing of the qubit. Our observations shed new light on the origin of decoherence in superconducting qubits. The experimental data suggest that, without degrading already achieved coherence times, phase qubits can be operated at temperatures much higher than those reported till now., Comment: 4 pages, 4 figures

We have investigated macroscopic quantum tunneling in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ intrinsic Josephson junctions at millikelvin temperatures using microwave irradiation. Measurements show that the escape rate for uniformly switching stacks of N junctions is about $N^2$ times higher than that of a single junction having the same plasma frequency. We argue that this gigantic enhancement of macroscopic quantum tunneling rate in stacks is boosted by current fluctuations which occur in the series array of junctions loaded by the impedance of the environment., Comment: 4 pages and 5 figures