An atom in open space can be detected by means of resonant absorption and reemission of electromagnetic waves, known as resonance fluorescence, which is a fundamental phenomenon of quantum optics. We report on the observation of scattering of propagating waves by a single artificial atom. The behavior of the artificial atom, a superconducting macroscopic two-level system, is in a quantitative agreement with the predictions of quantum optics for a pointlike scatterer interacting with the electromagnetic field in one-dimensional open space. The strong atom-field interaction as revealed in a high degree of extinction of propagating waves will allow applications of controllable artificial atoms in quantum optics and photonics.A single atom interacting with electromagnetic modes of free space is a fundamental example of an open quantum system (Fig. 1A) [1]. The interaction between the atom (or molecule, quantum dot, et cetera) and a resonant electromagnetic field is particularly important for quantum electronics and quantum information processing. In three-dimensional (3D) space, however, although perfect coupling (with 100% extinction of transmitted power) is theoretically feasible [2], experimentally achieved extinction has not exceeded 12% [3][4][5][6][7] because of spatial mode mismatch between incident and scattered waves. This problem can be avoided by an efficient coupling of the atom to the continuum of electromagnetic modes confined in a 1D transmission line (Fig. 1B) as proposed in [8,9]. Here we demonstrate extinction of 94% on an artificial atom coupled to the open 1D transmission line. The situation with the atom interacting with freely propagating waves is qualitatively different from that of the atom interacting with a single cavity mode; the latter has been used to demonstrate a series of cavity quantum electrodynamics (QED) phenomena [10][11][12][13][14][15][16][17][18]. Moreover in open space, the atom directly reveals such phenomena known from quantum optics as anomalous dispersion and strongly nonlinear behavior in elastic (Rayleigh) scattering near the atomic resonance[1]. Furthermore, spectrum of inelastically scattered radiation is observed and exhibits the resonance fluorescence triplet (the Mollow triplet) [19][20][21][22][23] under a strong drive.Our artificial atom is a macroscopic superconducting loop, interrupted by Josephson junctions (Fig. 1B) [identical to a flux qubit [24]] and threaded by a bias flux Φ b close to a half flux quantum Φ 0 /2, and shares a segment with the transmission line [25], which results in a loop-line mutual inductance M mainly due to kinetic * On leave from Physical-Technical Institute, Tashkent 100012, Uzbekistan † On leave from Lebedev Physical Institute, Moscow 119991, Russia inductance of the shared segment [26]. The two lowest eigenstates of the atom are naturally expressed via superpositions of two states with persistent current, I p , flowing clockwise or counterclockwise. In energy eigenbasis the lowest two levels |g and |e are described by the truncated...

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.

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, ∼ 20 GHz) modulates the flux threading the dc SQUID, and, thereby, the resonant frequency of the cavity field (the signal, ∼ 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.Degenerate parametric amplifiers are phase sensitive amplifiers, which can in principle amplify one of the two quadratures of a signal without introducing extra noise. 1,2 Parametric amplifiers based on the nonlinear inductance of a Josephson junction have been studied for a long time, 3 including one which demonstrated vacuum noise squeezing. 4 Recently, there has been a renewed interest in parametric amplifiers 5,6,7 due in part to the increasing need for quantum-limited amplification in the field of quantum information processing using superconducting circuits. 8,9 In the present work, we design a Josephson parametric amplifier, comprising a superconducting transmissionline resonator terminated by a dc SQUID (superconducting quantum interference device). Contrary to the previous works, the pump is not used to directly modulate a current through the Josephson junction, but is instead used to modulate a flux through the dc SQUID. 10 The resonant frequency of the resonator, namely, the band center of the signal, is widely controllable by a dc flux also applied to the SQUID (see also Ref. [7]). Moreover, as the pump and the signal are applied to different ports and their frequencies are twice different (see below), it is straightforward to separate the output signal from the pump. This is a unique property of the fluxpumping scheme; it arises because the finite dc flux bias allows linear coupling of the pump even in the absence of a dc current bias across the SQUID. 11 We operated such a flux-driven parametric amplifier and characterized its basic properties. Figure 1a shows a schematic diagram of the flux-driven parametric amplifier. The primary component of the amplifier is a transmission-line resonator defined by its coupling capacitance C c and a dc SQUID termination. The magnetic flux Φ penetrating the SQUID loop changes the boundary condition of the resonator at the right end (by the change of the Josephson inductance), and hence enables us to control the resonant frequency. 12,13 The resonant frequency f 0 for the first mode (λ/4 ≥ d, where λ is the wavelength and d is the cavity length) is schematically drawn as a function of Φ/Φ 0 in Fig. 1b, where Φ 0 is a flux quantum (see also Fig. 2a). We now assume the cavity resonance is set to a particular value, f 0dc , by applying a dc flux Φ dc (open circle in the figure). We then apply microwaves at a frequency 2f 0dc to the pump line wh...

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.

The performance of spintronics depends on the spin polarization of the current. In this study half-metallic Co-based full-Heusler alloys and a spin filtering device (SFD) using a ferromagnetic barrier have been investigated as highly spin-polarized current sources. The multilayers were prepared by magnetron sputtering in an ultrahigh vacuum and microfabricated using photolithography and Ar ion etching. We investigated two systems of Co-based full-Heusler alloys, Co 2 Cr 1−x Fe x Al(CCFA(x)) and Co 2 FeSi 1−x Al x (CFSA(x)) and revealed the structure and magnetic and transport properties. We demonstrated giant tunnel magnetoresistance (TMR) of up to 220% at room temperature and 390% at 5 K for the magnetic tunnel junctions (MTJs) using Co 2 FeSi 0.5 Al 0.5 (CFSA(0.5)) Heusler alloy electrodes. The 390% TMR corresponds to 0.81 spin polarization for CFSA(0.5) at 5 K. We also investigated the crystalline structure and local structure around Co atoms by x-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analyses, respectively, for CFSA films sputtered on a Cr-buffered MgO (001) substrate followed by post-annealing at various temperatures in an ultrahigh vacuum. The disordered structures in CFSA films were clarified by NMR measurements and the relationship between TMR and the disordered structure was discussed. We clarified that the TMR of the MTJs with CFSA(0.5) electrodes depends on the structure, and is significantly higher for L2 1 than B2 in the crystalline structure. The second part of this paper is devoted to a SFD using a ferromagnetic barrier. The Co ferrite is investigated as a ferromagnetic barrier because of its high Curie temperature and high resistivity. We demonstrate the strong spin filtering effect through an ultrathin insulating ferrimagnetic Co-ferrite barrier at a low temperature. The barrier was prepared by the surface plasma oxidization of a CoFe 2 film deposited on a MgO (001) single crystal substrate, wherein the spinel structure of CoFe 2 O 4 (CFO) and an epitaxial relationship of MgO(001)[100]/CoFe 2 (001)]110]/CFO(001)[100] were induced. A SFD consisting of CoFe 2 /CFO/Ta on a MgO (001) substrate exhibits the inverse TMR of −124% at 10 K when the configuration of the magnetizations of CFO and CoFe 2 changes from parallel to antiparallel. The inverse TMR suggests the negative spin polarization of CFO, which is consistent with the band structure of CFO obtained by first principle calculation. The −124% TMR corresponds to the spin filtering efficiency of 77% by the CFO barrier.