In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlO(x)/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlO(x)/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.

We present an efficient method for the characterization of two coupled discrete quantum systems, one of which can be controlled and measured. For two systems with transition frequencies ωq, ωr, and coupling strength g we show how to obtain estimates of g and ωr whose error decreases exponentially in the number of measurement shots rather than as a power law expected in simple approaches. Our algorithm can thereby identify g and ωr simultaneously with high precision in a few hundred measurement shots. This is achieved by adapting measurement settings upon data as it is collected. We also introduce a method to eliminate erroneous estimates with small overhead. Our algorithm is robust against the presence of relaxation and typical noise. Our results are applicable to many candidate technologies for quantum computation, in particular, for the characterization of spurious two-level systems in superconducting qubits or stripline resonators. Parameter estimation in many microscopic and some macroscopic systems inevitably involves quantum measurements. This implies that parameters cannot be identified with a single measurement shot since the outcome of such a measurement is generally random. Instead, the standard approach is to determine ensemble averages for many experiments and fit the parameters of certain quantitative models to those averages. The most common example for this is spectroscopy: it involves direct measurement of the energy splittings between quantum states in the form of resonances to incoming radiation. Typically, a large ensemble average is produced by gathering data from a large number of independent trials, either simultaneously on an ensemble of molecules (in nuclear magnetic resonance [1]) or from many repetitions of a specific experiment (in optical spectroscopy of single molecules, quantum dots, or superconducting qubits [2]).While being reliable in many contexts, this approach is often too resource intensive. Specifically, the error in the estimate of a single expectation value at a fixed measurement setting decreases in proportion to M − 1 2 r after M r measurement shots. Moreover, many choices of measurement settings are usually required for complex measurement tasks. Such slowness of parameter estimation can also turn into imprecision in the estimate if the parameters of interest drift as a function of time, broadening spectroscopic signatures.Imprecision in system characterization is particularly problematic for quantum information processing applications. These require extremely precise logic operations, usually implemented as pulses. The pulse parameters such as length, amplitude, and carrier frequency, depend on the system parameters. In manufactured solid state qubits, this is rather central as they are subject to fabrication uncertainty.In this Letter, we demonstrate that advanced spectroscopy can be performed far more efficiently. Our re-(b) (a) Figure 1: (color online) (a) Theoretically obtained swap spectrum in frequency-waiting time plane.Here, δ = (ωq − ωr,0)/(2g0), with ωq the q...

We propose a model for the pair of electromagnon excitations observed in the class of multiferroic materials RMnO 3 ͑R is a rare-earth ion͒. The model is based on a harmonic cycloid ground state interacting with a zone-edge magnon and its twin excitation separated in momentum space by two times the cycloid wave vector. The pair of electromagnons is activated by cross coupling between magnetostriction and spin-orbit interactions. Remarkably, the spectral weight of the twin electromagnon is directly related to the presence of a magnetically induced oscillatory polarization in the ground state. This leads to the surprising prediction that TbMnO 3 has an oscillatory polarization with amplitude 50 times larger than its uniform polarization.

Adaptive data collection and analysis, where data are being fed back to update the measurement settings, can greatly increase speed, precision, and reliability of the characterization of quantum systems. However, decoherence tends to make adaptive characterization difficult. As an example, we consider two coupled discrete quantum systems. When one of the systems can be controlled and measured, the standard method to characterize another, with an unknown frequency ωr, is swap spectroscopy. Here, adapting measurements can provide estimates whose error decreases exponentially in the number of measurement shots rather than as a power law in conventional swap spectroscopy. However, when the decoherence time is so short that an excitation oscillating between the two systems can only undergo less than a few tens of vacuum Rabi oscillations, this approach can be marred by a severe limit on accuracy unless carefully designed. We adopt machine learning techniques to search for efficient policies for the characterization of decohering quantum systems. We find, for instance, that when the system undergoes more than 2 Rabi oscillations during its relaxation time T1, O(10 3 ) measurement shots are sufficient to reduce the squared error of the Bayesian initial prior of the unknown frequency ωr by a factor O(10 4 ) or larger. We also develop policies optimized for extreme initial parameter uncertainty and for the presence of imperfections in the readout.

We study differential shot noise in mesoscopic diffusive normal-superconducting (NS) heterostructures at finite voltages where nonlinear effects due to the superconducting proximity effect arise. A numerical scattering-matrix approach is adopted. Through an NS contact, we observe that the shot noise shows a reentrant dependence on voltage due to the superconducting proximity effect but the differential Fano factor stays approximately constant. Furthermore, we consider differential shot noise in the structures where an insulating barrier is formed between normal and superconducting regions and calculate the differential Fano factor as a function of barrier height. PACS numbers: 74.40.+k, 74.50.+r Shot noise is fluctuation of the current that is due to the discrete nature of the charge carriers. It is the only source of noise at zero temperature. Current noise contains information on the physics of transport phenomenon not contained in the conductance. A classical value S = 2e|I| = S Poisson for the noise, observed, e.g., in a vacuum diode, is obtained in the tunneling limit, i.e., when the transmission probabilities for open scattering channels are small and there are no correlations among the charge carriers. However, two effects may reduce the noise below its Poissonian value: inelastic scattering (not considered in this paper) and reduced noise in channels with finite transmission amplitudes. 1 In a phase-coherent conductor, all the transfer coefficients are not necessarely small. Instead, e.g., in a diffusive phase-coherent metallic wire the distribution function of the transmission coefficients has a bimodal form 2 such that almost closed and almost open channels are preferred. This results in a shot noise that is one-third of the Poissonian noise independently of the sample-specific properties such as the number of channels or the degree of disorder. 1,3 During the last decade the noise properties of mesoscopic conductors have been under intense study (for a review, see Ref. 4). There has also been increasing interest to comprehend the interplay of phase coherence and superconducting proximity effect in mesoscopic physics. Recently the doubling of the shot noise in normalsuperconducting heterojunctions, predicted in the linear regime in Ref. 5, has been verified experimentally. 6 While the theory of classical shot noise and the quantum mechanical results in the linear regime have been discussed before, little attention has been paid to mesoscopic noise at finite voltages when the presence of superconductivity induces nonlinear behavior in the transport coefficients. In Ref. 7 current correlations in very small hybrid NS structures at finite voltages were discussed. Eliminating the effects due to the finite size of the structure and fully taking into account the effects arising in a diffusive phase coherent sample, however, requires larger structures to be studied. In Ref. 8 a counting-field approach to the Keldysh Green's-function method was adopted to calculate numerically the statistics of current in a...