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We have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on Si substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation (resulting in fewer two-level fluctuators), feasibility for epitaxial tunnel barriers (further reducing energy relaxation and dephasing), and a larger superconducting gap of ~ 5.2 meV for NbN compared to ~ 0.3 meV for Al (suppressing the excitation of quasiparticles). Replacing conventional MgO by a Si substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time \({T}_{1}=16.3 {\mu }\text{s}\) and a spin-echo dephasing time \({T}_{2}=21.5 {\mu }\text{s}\). These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to previously reported NbN-based qubits with MgO substrates (\({T}_{1}\approx {T}_{2}\approx 0.5 {\mu }\text{s}\)). These results are an important step towards constructing a new platform for superconducting quantum hardware.

Description, Experimental microwave teleportation brings quantum local area networks within reach by exceeding the no-cloning limit., The field of quantum communication promises to provide efficient and unconditionally secure ways to exchange information, particularly, in the form of quantum states. Meanwhile, recent breakthroughs in quantum computation with superconducting circuits trigger a demand for quantum communication channels between spatially separated superconducting processors operating at microwave frequencies. In pursuit of this goal, we demonstrate the unconditional quantum teleportation of propagating coherent microwave states by exploiting two-mode squeezing and analog feedforward over a macroscopic distance of d = 0.42 m. We achieve a teleportation fidelity of F = 0.689 ± 0.004, exceeding the asymptotic no-cloning threshold. Thus, the quantum nature of the teleported states is preserved, opening the avenue toward unconditional security in microwave quantum communication.

Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. In particular, remote state preparation aims at the creation of a desired and known quantum state at a remote location using classical communication and quantum entanglement. We present an experimental realization of deterministic continuous-variable remote state preparation in the microwave regime over a distance of 35 cm. By employing propagating two-mode squeezed microwave states and feedforward, we achieve the remote preparation of squeezed states with up to 1.6 dB of squeezing below the vacuum level. Finally, security of remote state preparation is investigated by using the concept of the one-time pad and measuring the von Neumann entropies. We find nearly identical values for the entropy of the remotely prepared state and the respective conditional entropy given the classically communicated information and, thus, demonstrate close-to-perfect security., Continuous-variable remote state preparation in the microwave domain would allow to leverage the superconducting technology for quantum networks applications. Here, the authors show how to deterministically prepare squeezed Gaussian states across 35 cm using previously shared entanglement.

Single photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photon extremely challenging. Here, we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an "impedance-matched" artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. We attain a single-photon detection efficiency of 0.66±0.06 with a reset time of ~400 ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing.

Single-photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photons extremely challenging. Here we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an impedance-matched artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. Each signal photon deterministically induces a Raman transition in the Λ system and excites the qubit. The subsequent dispersive readout of the qubit produces a discrete ‘click’. We attain a high single-photon-detection efficiency of 0.66±0.06 with a low dark-count probability of 0.014±0.001 and a reset time of ∼400 ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing., Japan Society for the Promotion of Science (KAKENHI Grants 25400417, 26220601 and 15K17731), Japan. Science and Technology Agency. ImPACT Program of Council for Science, National Institute of Information and Communications Technology (Japan)

Two-mode squeezing is a fascinating example of quantum entanglement manifested in cross-correlations of non-commuting observables between two subsystems. At the same time, these subsystems themselves may contain no quantum signatures in their self-correlations. These properties make two-mode squeezed (TMS) states an ideal resource for applications in quantum communication. Here, we generate propagating microwave TMS states by a beam splitter distributing single mode squeezing emitted from distinct Josephson parametric amplifiers along two output paths. We experimentally study the fundamental dephasing process of quantum cross-correlations in continuous-variable propagating TMS microwave states and accurately describe it with a theory model. In this way, we gain the insight into finite-time entanglement limits and predict high fidelities for benchmark quantum communication protocols such as remote state preparation and quantum teleportation. We acknowledge support by the German Research Foundation through FE 1564/1-1, Spanish MINECO/FEDER FIS2015-69983-P, UPV/EHU PhD grant, Basque Government Grant IT986-16, European Project AQuS (Project No. 640800), Elite Network of Bavaria through the program ExQM, the projects JST ERATO (Grant No. JPMJER1601) and JSPS KAKENHI (Grant No. 26220601 and Grant No. 15K17731). E.S. acknowledges support from a TUM August-Wilhelm Scheer Visiting Professorship and hospitality of Walther-Meissner-Institut and TUM Institute for Advanced Study. We would like to thank K. Kusuyama for assistance with part of the JPA fabrication.

Superconductivity provides a canonical example of a quantum phase of matter. When superconducting islands are connected by Josephson junctions in a lattice, the low temperature state of the system can map to the celebrated XY model and its associated universality classes. This has been used to experimentally implement realizations of Mott insulator and Berezinskii--Kosterlitz--Thouless (BKT) transitions to vortex dynamics analogous to those in type-II superconductors. When an external magnetic field is added, the effective spins of the XY model become frustrated, leading to the formation of topological defects (vortices). Here we observe the many-body dynamics of such an array, including frustration, via its coupling to a superconducting microwave cavity. We take the design of the transmon qubit, but replace the single junction between two antenna pads with the complete array. This allows us to probe the system at 10 mK with minimal self-heating by using weak coherent states at the single (microwave) photon level to probe the resonance frequency of the cavity. We observe signatures of ordered vortex lattice at rational flux fillings of the array., Main text (5 pages, 3 figures) and Supplementary Material (5 pages, 2 figures)

For quantum information processing with superconducting circuits, efficient amplification of weak microwave signals is often required. For this purpose, Josephson parametric amplifiers (JPAs) are commonly used, due to their quantum-limited noise performance. The authors show that flux-driven JPAs exhibit hysteresis, even for negligible screening parameters, and that their amplification properties depend strongly on the resonator characteristics. These phenomena are important for a full understanding of the magnetic field response and parametric amplification in JPAs, and thus their application.

We report the experimental observation of high-order sideband transitions at the single-photon level in a quantum circuit system of a flux qubit ultrastrongly coupled to a coplanar waveguide resonator. With the coupling strength reaching 10% of the resonator's fundamental frequency, we obtain clear signatures of higher-order red and first-order blue-sideband transitions, which are mainly due to the ultrastrong Rabi coupling. Our observation advances the understanding of ultrastrongly-coupled systems and paves the way to study high-order processes in the quantum Rabi model at the single-photon level., Comment: Accepted in Physical Review A. 12 pages, 6 figures

Hybrid quantum networks of stationary and ``flying'' qubits are essential for distributed quantum information processing. In superconducting quantum computation, two-qubit gates are currently realized by the interaction between neighboring qubits. The authors propose a gate comprising a superconducting ``atom'' and a microwave photon, in which gate operation is completed $d\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}y$ (not probabilistically) upon reflection of the photon. This gate's type can be continuously varied $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$, enabling $r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e$ entanglement of many ``atoms'' via a single photon, or creation of a quantum domino effect.

We propose a superconducting circuit that shows a super-radiant phase transition (SRPT) in the thermal equilibrium. The existence of the SRPT is confirmed analytically in the limit of an infinite number of artificial atoms. We also perform numerical diagonalization of the Hamiltonian with a finite number of atoms and observe an asymptotic behavior approaching the infinite limit as the number of atoms increases. The SRPT can also be interpreted intuitively in a classical analysis., 8 pages, 3 figures

Displacement of propagating quantum states of light is a fundamental operation for quantum communication. It enables fundamental studies on macroscopic quantum coherence and plays an important role in quantum teleportation protocols with continuous variables. In our experiments, we have successfully implemented this operation for propagating squeezed microwave states. We demonstrate that, even for strong displacement amplitudes, there is no degradation of the squeezing level in the reconstructed quantum states. Furthermore, we confirm that path entanglement generated by using displaced squeezed states remains constant over a wide range of the displacement power.

The utilization of a high-Tc superconductor for implementing a superconducting qubit is to be expected. Recent researches on the quantum property of Josephson junctions in high-Tc superconductors indicate that the low energy quasiparticle excitation is weak enough to observe the macroscopic quantum tunneling. Therefore, a detailed study on the quantum property of high-Tc Josephson junctions becomes more important for applications. We show our experimental results of the macroscopic tunneling of current biased intrinsic Josephson junctions in Bi-2212 and its resonant activation in the presence of microwave radiation.

We report an experimental observation of the macroscopic quantum tunnelling (MQT) in a d-wave high-temperature superconductor (HTSC) Bi2Sr2CaCu2O8+δ intrinsic Josephson junction (IJJs). It is well known that the current-biased Josephson tunnel junction provides an ideal stage for studying a variety of macroscopic quantum phenomena such as MQT. They have been studied and observed in low-temperature superconductor (LTSC) Josephson junctions since the early 1980s and have been established well by now. On the other hand, in the case of HTSC the d-wave pairing symmetry allows for dissipative quasiparticle excitations within the superconducting energy gap, which is expected to severely interfere with an observation of MQT. Contrary to this naive conception, our experimental results have explained that the effects of the nodal quasiparticles are not strong enough to obscure the observation of the MQT. Furthermore, we found that the MQT in IJJs was observed at approximately 1 K, which was ten times higher than that of LTSC Josephson junctions. This higher classical-to-quantum crossover temperature is due to their high plasma frequency. We also discuss the resonant activation of IJJs in the presence of microwave radiation.

By properly driving a qubit-resonator system in the strong dispersive regime, we implement an "impedance-matched" $\Lambda$ system in the dressed states, where a resonant single photon deterministically induces a Raman transition and excites the qubit. Combining this effect and a fast dispersive readout of the qubit, we realize a detector of itinerant microwave photons. We theoretically analyze the single-photon response of the $\Lambda$ system and evaluate its performance as a detector. We achieve a high detection efficiency close to unity without relying on precise temporal control of the input pulse shape and under a conservative estimate of the system parameters. The detector can also be reset quickly by applying microwave pulses, which allows a short dead time and a high repetition rate., Comment: 11 pages, 9 figures

We propose a scheme for continuous detection of itinerant microwave photons in circuit quantum electrodynamics. In the proposed device, a superconducting qubit is coupled dispersively to two resonators: one is used to form an impedance-matched $\Lambda$ system that deterministically captures incoming photons, and the other is used for continuous monitoring of the event. The present scheme enables efficient photon detection: for realistic system parameters, the detection efficiency reaches 0.9 with a bandwidth of about ten megahertz., Comment: 7 pages, 5 figures

We report a current dependence of flux–flow resistance (FFR) and transport properties in intrinsic Jospehson junctions (IJJs) under magnetic fields parallel to an ab- plane. In Bi 2 Sr 2 CaCu 2 O 8+ d IJJs with the ab- plane dimensions of 1.8×10.5 μm 2 , the oscillations of FFR have been observed with two apparent periods of 0.382 T in low fields and 0.765 T in high fields. The dominant period H p =0.765 T is decided by a sample width and corresponds to the field for adding one flux quantum per layer. Under certain conditions, we also observed the mergence of two peaks on the oscillating FFR with half period 1/2 H p into one peak with the period H p in low fields and the inversions between bottoms and peaks in high fields. We found that this current-dependent FFR implying information of vortex lattice correlates with the transport properties such as current steps on current–voltage curves.

Josephson vortex lattice in narrow Bi-2212 intrinsic Josephson junctions have been studied. Formation of rectangular Josephson vortex lattice is expected in narrow width due to the edge effect. Current–voltage (I–V) characteristics were measured as a function of the magnetic field applied parallel to their a–b planes. Maximum zero bias current (IC,branch) and flux-flow voltage were measured. We observed that the IC,branch monotonically decreased with magnetic field up to 0.6 T. Above 0.6 T, IC,branch showed an enhancement, accompanied by an enhancement of flux-flow voltage thus flux-flow speed, up to 1.0 T. This result indicates a change in the configuration of vortex lattices from triangular to other structure (possibly rectangular) by the edge effect in which configuration both the flux-flow voltage and IC,branch are enhanced due to an increase in the maximum velocity of the vortex lattice.

After a brief description of the double-sided fabrication process, we give an example to show that a quite large array of intrinsic Josephson junctions (IJJs) can be made by superconductively connecting 256 stacks of junctions together. Each of these stacks contains only 40 junctions which prevents problems causing by heating. We present a detailed discussion on a new structure we have developed in which an extra electrode can be fabricated in the middle of an array by a slight modification of the double-sided process. Using this middle electrode, we can determine further information on the physical processes inside the stack than is possible without such an electrode. Finally we shall discuss terahertz generation in IJJs when an external magnetic field is applied perpendicular to the width of the junctions.

Oscillations of Josephson vortex-flow resistance have been studied in narrow (L/spl sim/2-4/spl lambda//sub J/) Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// intrinsic Josephson junctions as a function of magnetic field applied parallel to junction edges. Lengths of junctions fabricated are /spl sim/1 /spl mu/m in order to enhance pinning effects of Josephson vortex lattice to the junction edges and thus to enhance formation of rectangular vortex lattice which would lead an in-phase mode of the junctions as a necessary condition for the THz generator application. The observed Josephson vortex flow resistance showed a periodic oscillation with a period (H/sub p/) corresponding to one flux quantum enters per junction, namely, corresponding to the rectangular vortex lattice. Observed strong oscillation was a result of collective behavior of the entry and escape of vortices in a form of rectangular vortex lattice. Peaks in the oscillations were found at the fields H=nH/sub p/, here n shows an integer number. Here, Josephson vortex lattice flow speed shows a local maximum. With this magnetic field, outermost vortex rows geometrically match to the edges of junctions. Contrary to this, a minimum of flow speed, namely pinned state of the vortex lattice, was observed at H=(n+1/2)H/sub p/.

The control of the critical current density ( J C ) and the junction resistance ( R N ) along the c axis of intrinsic Josephson junctions (IJJs) on a high- T C superconductor is very important for applying the IJJs to electronic devices. For controlling these junction parameters, we have clarified the relationship of J C , R N , and the carrier concentration in Bi 2 Sr 2 CaCu 2 O 8+ δ whiskers by changing the carrier concentration with an annealing process. We determined the electrical transport characteristics of the IJJs. As a result, the J C increased, and the R N decreased systematically when the carrier concentration increased. The values of J C and R N could be controlled by a change in the carrier concentration.

We report on serial dc superconducting quantum interference devices (dc SQUIDs) arrays of two-stacked intrinsic Josephson junctions (IJJs) on a Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) whisker with a submicron hole fabricated by a 3-D focused ion beam (FIB) etching method. The junction area, the number of elementary junctions, and the loop area of the SQUIDs are 3, 20, and 0.42 μm 2 , respectively. The SQUIDs show typical I – V characteristics of IJJs with a critical current I C of 12 A at 4.2 K. The modulation parameter β L (=2 I C L / Φ 0 ) is 6.4, and the inductance L of the SQUID is 1120 pH. That includes the inductances of the elementary junction around a SQUID loop. In a strong magnetic field, the Fraunhofer pattern is obtained and well fitted to the calculated value from the junction size; Δ B = Φ 0 / tW ; t is the interlayer distance; and W is the junction width. In a low magnetic field, modulation depth of approximately 10% of the I C corresponding to the value of β L is observed. The modulation period of 80 G approximately agrees with the value calculated from the SQUID.

Cross-whisker intrinsic Josephson junction has been developed as a probe of symmetry of the superconducting order parameter. Two B2212 single crystal whiskers were connected on a MgO substrate at their c -plane with various crossing angles. Angular dependence of the critical current densities showed d-wave-like 4-fold symmetry. However, the angular dependence was much stronger than that of the conventional d x 2 − y 2 -wave. The critical current density of the cross-whisker junction with cross-angle α =45° was not completely zero. Shapiro steps and Fraunhofer pattern were clearly observed in 45° twisted cross-whisker junctions. It is suggested that the superconducting gap exists even at nodal direction.

The superconducting transition temperature ( T c ), the junction resistance ( R N ) and the critical current density along the c -axis ( J c ) of Bi-2212 whiskers with different oxygen content have been measured. T c showed a systematical change for the c -axis length. Since the change of c -axis length corresponds to the change of oxygen content, it can be considered that this behavior explains the clear change in carrier density on the whisker from overdoped to underdoped region. J c of intrinsic Josephson junctions fabricated on the whisker exponentially decreases with decreasing the carrier density on whiskers. It indicates that the transport along the c -axis is due to a tunneling process through an insulating barrier.

By driving a dispersively coupled qubit-resonator system, we realize an "impedance-matched" $\Lambda$ system that has two identical radiative decay rates from the top level and interacts with a semi-infinite waveguide. It has been predicted that a photon input from the waveguide deterministically induces a Raman transition in the system and switches its electronic state. We confirm this through microwave response to a continuous probe field, observing near-perfect ($99.7\%$) extinction of the reflection and highly efficient ($74\%$) frequency down-conversion. These proof-of-principle results lead to deterministic quantum gates between material qubits and microwave photons and open the possibility for scalable quantum networks interconnected with waveguide photons., Comment: 10 pages, 8 figures

We report the characteristics of stacked junctions fabricated by the focused ion beam etching method on a Bi-2212 whisker with low carrier concentration. In order to achieve low carrier concentration, the whisker edges were etched and the sample was annealed in 1 atm of flowing Ar at 200 °C for 1 h. A drop of critical current density (Jc) occurred when junction area (S) was decreased to the submicron range, and then the charging energy of the stack (Ec) was comparable to Josephson coupling energy (EJ). The Ec/EJ–Jc curves of the samples were similar to the theoretical curves which explains a fall of Jc due to Coulomb blockade for low TC superconductor. Also, the annealed sample showed high junction resistance and low EJ, owing to a low carrier concentration by annealing.

For development of a spin-filtering device with a ferromagnetic barrier, very thin Co-ferrite layers are prepared by the plasma oxidization of the CoFe2 surface deposited on MgO(001) single crystal substrates and postannealing process. Spin-filtering junctions consisting of CoFe2/(MgO/)Co-ferrite/Ta are fabricated and exhibit nonlinear J-V curves. By the insertion of a MgO layer, clear independent rotation of two magnetic moments is observed

Single-crystal whiskers of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub x/ with a length of about 4 mm were grown from an Sb-doped precursor. The optimum nominal composition of precursors in this Sb-doping method is Y/sub 2/Ba/sub 2.75/Cu/sub 3/Sb/sub 0.5/O/sub x/. Sb is not detected in the whiskers by energy-dispersive x-ray spectroscopy (EDS). The standard 4-probe transport measurements for the as-grown whiskers show a critical temperature T/sub C/ of about 90 K and a c-axis critical current density J/sub C/ of 9.51/spl times/10/sup 4/ A/cm/sup 2/ at 89 K.

In one-dimensional optical setups, light-matter interaction is drastically enhanced by the interference between the incident and scattered fields. Particularly, in the impedance-matched \Lambda-type three-level systems, a single photon deterministically induces the Raman transition and switches the electronic state of the system. Here we show that such a \Lambda system can be implemented by using dressed states of a driven superconducting qubit and a resonator. The input microwave photons are perfectly absorbed and are down-converted into other frequency modes in the same waveguide. The proposed setup is applicable to single-photon detection in the microwave domain., Comment: 6 pages, 5 figures

We have investigated the microwave response of a transmon qubit coupled directly to a transmission line. In a transmon qubit, owing to its weak anharmonicity, a single driving field may generate dressed states involving more than two bare states. We confirmed the formation of three-state dressed states by observing all of the six associated Rabi sidebands, which appear as either amplification or attenuation of the probe field. The experimental results are reproduced with good precision by a theoretical model incorporating the radiative coupling between the qubit and the microwave., 7 pages, 5 figures

A driven qubit-resonator system functions as an impedance-matched Δ-system under certain conditions. We analyze the microwave response of such a system and reveal the possibility of deterministic down-conversion of microwave photons upon a single reflection.

The control of the critical current density (Jc) and the junction resistance (RN) along the c-axis of intrinsic Josephson junctions (IJJs) on a high-Tc superconductor is very important for applying the IJJs to electronic devices. For controlling these junction parameters, we have clarified the relationship of Jc, RN and the carrier density in Bi2Sr2CaCu2O8+δ whiskers by changing the carrier density with an annealing process. We determined the electrical transport characteristics of the IJJs. As a result, the Jc increased, and the RN decreased systematically when the carrier density increased. The values of Jc and RN could be controlled by a change in the carrier density.

We demonstrate enhancement of the dispersive frequency shift in a coplanar waveguide resonator induced by a capacitively-coupled superconducting flux qubit in the straddling regime. The magnitude of the observed shift, 80 MHz for the qubit-resonator detuning of 5 GHz, is quantitatively explained by the generalized Jaynes-Cummings model which takes into account the contribution of the qubit higher energy levels. By applying the enhanced dispersive shift to the qubit readout, we achieved 90% contrast of the Rabi oscillations which is mainly limited by the energy relaxation of the qubit., 10 pages, 8 figures

Path entanglement constitutes an essential resource in quantum information and communication protocols. Here, we demonstrate frequency-degenerate entanglement between continuous-variable quantum microwaves propagating along two spatially separated paths. We combine a squeezed and a vacuum state using a microwave beam splitter. Via correlation measurements, we detect and quantify the path entanglement contained in the beam splitter output state. Our experiments open the avenue to quantum teleportation, quantum communication, or quantum radar with continuous variables at microwave frequencies., Comment: 16 pages, 14 figures

We have studied Nb lambda/2 coplanar-waveguide (CPW) resonators whose resonant frequencies are 10-11 GHz. The resonators have different film thicknesses, t=0.05, 0.1, 0.2, and 0.3 um. We measured at low temperatures, T=0.02-5 K, one of the scattering-matrix element, S_21, which is the transmission coefficient from one port to the other. At the base temperatures, T=0.02-0.03 K, the resonators are overcoupled to the input/output microwave lines, and the loaded quality factors are on the order of 10^3. The resonant frequency has a considerably larger film-thickness dependence compared to the predictions by circuit simulators which calculate the inductance of CPW taking into account L_g only, where L_g is the usual magnetic inductance determined by the CPW geometry. By fitting a theoretical S_21 vs. frequency curve to the experimental data, we determined for each film thickness, the phase velocity of the CPW with an accuracy better than 0.1%. The large film-thickness dependence must be due to the kinetic inductance L_k of the CPW center conductor. We also measured S_21 as a function of temperature up to T=4-5 K, and confirmed that both thickness and temperature dependence are consistent with the theoretical prediction for L_k., Comment: 7 pages, 8 figures

We have developed a Josephson parametric amplifier, comprising a superconducting coplanar waveguide resonator terminated by a dc SQUID (superconducting quantum interference device). An external field (the pump, $\sim 20$ GHz) modulates the flux threading the dc SQUID, and, thereby, the resonant frequency of the cavity field (the signal, $\sim 10$ GHz), which leads to parametric signal amplification. We operated the amplifier at different band centers, and observed amplification (17 dB at maximum) and deamplification depending on the relative phase between the pump and the signal. The noise temperature is estimated to be less than 0.87 K., 4 pages, 4 figures

Solid-state superconducting circuits are versatile systems in which quantum states can be engineered and controlled. Recent progress in this area has opened up exciting possibilities for exploring fundamental physics as well as applications in quantum information technology; in a series of experiments it was shown that such circuits can be exploited to generate quantum optical phenomena, by designing superconducting elements as artificial atoms that are coupled coherently to the photon field of a resonator. Here we demonstrate a lasing effect with a single artificial atom - a Josephson-junction charge qubit - embedded in a superconducting resonator. We make use of one of the properties of solid-state artificial atoms, namely that they are strongly and controllably coupled to the resonator modes. The device is essentially different from existing lasers and masers; one and the same artificial atom excited by current injection produces many photons., 13 pages, 3 figures

While Josephson-junction-like structures intrinsic to the layered cuprate high temperature superconductors offer an attractive stage for exploiting possible applications to new quantum technologies, the low energy quasiparticle excitations characteristically present in these d-wave superconductors may easily destroy the coherence required. Here we demonstrate for the first time the feasibility of macroscopic quantum tunneling in the intrinsic Josephson junctions of a high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8 + delta), and find it to be characterized by a high classic-to-quantum crossover temperature and a relatively weak quasiparticle dissipation.

Spin transfer magnetization switching is studied in nano-pillars with a synthetic antiferromagnetic free layer (SyAF), CoFe/Ru/CoFe, under an external magnetic field. The resistance change for the conventional type nano-pillar (CoFe) and SyAF type sample is almost the same, demonstrating full reversal of the magnetization. On the other hand, the critical current as a function of magnetic field for the SyAF type is found to behave in the opposite way with that of the conventional type.

Annealing temperature dependence of the tunnel magnetoresistance (TMR) ratio for magnetic tunnel junctions with CoFeO/sub x/ inserted in the pinned layer was investigated. A junction with a CoFe/CoFeO/sub x//CoFe pinned layer exhibited TMR ratios of 46.5% and 42.5% after annealing at 350/spl deg/C and 375/spl deg/C, respectively. The reason for the improvement of the thermal stability is related to oxygen diffusion from the CoFeO/sub x/ layer, and there is a possibility that CoFeO/sub x/ plays the role of a Mn (in MnIr exchange layer) diffusion barrier.

We study the system where a superconducting flux qubit is capacitively coupled to an LC resonator. In three devices with different coupling capacitance, the magnitude of the dispersive shift is enhanced by the third level of the qubit and quantitatively agrees with the theory. We show by numerical calculation that the capacitive coupling plays an essential role for the enhancement in the dispersive shift. We investigate the coherence properties in two of these devices, which are in the strong-dispersive regime, and show that the qubit energy relaxation is currently not limited by the coupling. We also observe the discrete ac-Stark effect, a hallmark of the strong-dispersive regime, in accordance with the theory.