Recent progress in solid state quantum information processing has stimulated the search for ultralow-noise amplifiers and frequency converters in the microwave frequency range, which could attain the ultimate limit imposed by quantum mechanics. In this article, we report the first realization of an intrinsically phase-preserving, non-degenerate superconducting parametric amplifier, a so far missing component. It is based on the Josephson ring modulator, which consists of four junctions in a Wheatstone bridge configuration. The device symmetry greatly enhances the purity of the amplification process and simplifies both its operation and analysis. The measured characteristics of the amplifier in terms of gain and bandwidth are in good agreement with analytical predictions.Using a newly developed noise source, we also show that our device operates within a factor of three of the quantum limit. This development opens new applications in the area of quantum analog signal processing.In this article, we focus on parametric amplifiers which are powered by an ac source with frequency f p also known as the "pump". Such amplifiers operate with a minimal number of degrees of freedom and are the natural candidates for ultra low noise operation [1,2]. A single spatial and temporal mode of the electromagnetic field with carrier frequency f can be decomposed into its in-phase A cos 2πf t arXiv:0912.3407v1 [cond-mat.mes-hall]

We present the first measurements of the third moment of the voltage fluctuations in a conductor. This technique can provide new and complementary information on the electronic transport in conducting systems. The measurement was performed on nonsuperconducting tunnel junctions as a function of voltage bias, for various temperatures and bandwidths up to 1 GHz. The data demonstrate the significant effect of the electromagnetic environment of the sample.

We report measurements of nonequilibrium noise in a diffusive normal metal-superconductor (N-S) junction in the presence of both dc bias and high-frequency ac excitation. We find that the shot noise of a diffusive N-S junction is doubled compared to a normal diffusive conductor. Under ac excitation of frequency nu the shot noise develops features at bias voltages |V| = hnu/(2e), which bear all the hallmarks of a photon-assisted process. Observation of these features provides clear evidence that the effective charge of the current carriers is 2e, due to Andreev reflection.

We observe a novel signature of coherent transport in the nonequilibrium current fluctuations of a diffusive metallic conductor shorter than the electron phase-breaking length. Under illumination with monochromatic microwaves, the shot noise develops features at voltages corresponding to the photon energies, V nhn͞e, which are oscillatory functions of the microwave power, while the conductance and the I-V curve are unaffected. The observed effect bears a strong resemblance to photon-assisted tunneling, although this marks the first demonstration in a linear system, and in a system without an explicit tunnel barrier. [S0031-9007(98)05538-0] PACS numbers: 73.50. Td, 73.23.Ps, 73.50.Mx, 73.50.Pz Studies of the nonequilibrium current fluctuations have recently emerged as a probe of various mesoscopic systems [1]. The nature of the shot noise can reveal information about the transport which is not evident merely by examining the conductance or the time-averaged transport. For example, correlations due to the Fermi statistics of the electrons can lead to a partial suppression of the shot noise [2,3]. It has been demonstrated [4,5] that even a metallic wire will produce shot noise, provided that its length is short compared to the electron-electron inelastic length, L ee . Time-dependent phenomena in nanostructured systems have also become an active area of research lately [6], and it is interesting to ask what the effect of a time-dependent potential on the current fluctuations might be. Indeed, Lesovik and Levitov [7] recently proposed a new two-particle interference phenomenon, the "nonstationary Aharonov-Bohm effect," which should be observable in the shot noise of a phase-coherent conductor under a high-frequency excitation. As we describe below, this phenomenon is formally identical to the process of "photon-assisted tunneling" (PAT) [8], but applied to shot noise of the system rather than its current-voltage characteristic.In this Letter, we present measurements of the nonequilibrium noise of a diffusive, phase-coherent metallic conductor in the presence of both a high-frequency (2-40 GHz) ac excitation and a dc bias. The noise displays features generated by the time-dependent excitation that bear all the hallmarks of photon-assisted tunneling. Because of the linear nature of the conductance in this mesoscopic system, however, the photon-assisted signatures appear only in the noise, not in the conductance.The theory of photon-assisted tunneling [8] was developed some time ago to explain the features observed [9] in superconducting tunnel junctions when they were illuminated with microwave radiation. It was extended to derive the quantum-limited performance [10] of heterodyne detectors based on these devices. More recently, PAT has been demonstrated in a variety of systems, including semiconductor quantum dots [11], metallic singleelectron transistors [12], semiconductor superlattices [13], and other dual-barrier semiconductor devices [14]. In all of these systems, the conductance displays a strong nonlinearity...

We report resistance oscillations as a function of magnetic field for individual aluminum and silver thin-film rings, 1-2 /u,m in diameter, between 1.2 and 10 K. This is the first observation in single rings of oscillations with a flux period of h/2e predicted by Al'tshuler et al Oscillations periodic in hi e are also seen in the l-/xm Ag rings at higher fields. These electron interference effects in metal rings are the solid-state analog of the Aharonov-Bohm effect for electrons in vacuum. PACS numbers: 71.55.Jv, 72.15.Lh, 73.60.Dt In recent years there has been major interest, and a dramatic advance, in the understanding of electron transport in random systems, especially those of reduced dimensionality. 1 The early studies of one-and two-dimensional systems verified the picture, originally proposed by Thouless, of localization effects which reduce the conductivity at T-• 0. It is now clear that localization phenomena result from electron interference: Coherent backscattering adds quantum corrections to the classical electron-diffusion results. 1 Localization experiments confirm the striking result that the length over which electrons can retain phase memory and interfere is the inelastic diffusion length //, which can be > 1 ftm at low temperatures. (1 /xm= 10 4 A.) In contrast, the mean o free path / in typical metal films is of order 10 to 100 A.Recently there have been predictions 2 " 5 that explicit, oscillatory electron interference effects should be seen in the macroscopic electrical properties of small metal rings and cylinders in a perpendicular (axial) magnetic field B ( = VxA). The vector potential A adds to the quantum phase along a path from a to b by an amount (in mks units) A = 27r(e//?) f A-rfl. J aFor electrons in vacuum this leads to the well-known Aharonov-Bohm interference effect 6 for the electron intensity, where the electron travels on either of two paths which enclose a magnetic flux <& [see Fig. 1(a)]. The flux is given by = B x area = 0 A • d\. The flux periodicity of the resulting intensity is h/e. Aharonov-Bohm resistance oscillations have been reported in very pure single-crystal cylinders with long mean free paths, 3 with a period of h/e. Observation of such interference effects in a ring of disordered metal (with significant elastic scattering) would demonstrate elegantly the connection between macroscopic transport and the essentially quantum-mechanical behavior at the microscopic level.The first prediction of interference effects in disordered normal-metal rings and cylinders was by Al'tshuler et al 2 They predicted that the low-temperature resistance would be a periodic function of the enclosed flux, with a period of h/2e. This periodicity results from the interference paths shown in Fig. Kb). 7 Different theoretical arguments, which start with a model of a closed chain of atoms, also predict a flux periodicity of h/2e for various physical quantities. 5