Superconducting Spin Qubits

We propose and theoretically investigate spin superconducting qubits. Spin superconducting qubit consists of a single spin confined in a Josephson junction. We show that owing to spin-orbit interaction, superconducting difference across the junction can polarize this spin. We demonstrate that this enables single qubit operations and more complicated quantum gates, where spins of different qubits interact via a mutual inductance of superconducting loop where the junctions are embedded. Recent experimental realizations of Josephson junctions made of semiconductor quantum dots in contact with superconducting leads have shown that the number of electrons in the quantum dot can be tuned by a gate voltage. Spin superconducting qubit is realized when the number of electrons is odd. We discuss the qubit properties at phenomenological level. We present a microscopic theory that enables us to make accurate estimations of the qubit parameters by evaluating the spin-dependent Josephson energy in the framework of fourth-order perturbation theory.Comment: 11 pages, 8 figure

Statistics of Transmission Eigenvalues for a Disordered Quantum Point Contact

We study the distribution of transmission eigenvalues of a quantum point contact with nearby impurities. In the semi-classical case (the chemical potential lies at the conductance plateau) we find that the transmission properties of this system are obtained from the ensemble of Gaussian random reflection matrices. The distribution only depends on the number of open transport channels and the average reflection eigenvalue and crosses over from the Poissonian for one open channel to the form predicted by the circuit theory in the limit of large number of open channels.Comment: 8 pages, 3 figure

Coulomb blockade in quantum dots under AC pumping

We study conductance through a quantum dot under Coulomb blockade conditions in the presence of an external periodic perturbation. The stationary state is determined by the balance between the heating of the dot electrons by the perturbation and cooling. We analyze two cooling mechanisms: electron exchange with the cold contacts and emission of phonons. Together with the usual linear Ohmic heating of the dot electrons we consider possible effects of dynamic localization. The combination of the abovementioned factors may result in a drastic change of the shape of the Coulomb blockade peak with respect to the usual equilibrium one.Comment: 12 pages, 8 figure

Resonant Tunneling in a Dissipative Environment

We measure tunneling through a single quantum level in a carbon nanotube quantum dot connected to resistive metal leads. For the electrons tunneling to/from the nanotube, the leads serve as a dissipative environment, which suppresses the tunneling rate. In the regime of sequential tunneling, the height of the single-electron conductance peaks increases as the temperature is lowered, although it scales more weekly than the conventional 1/T. In the resonant tunneling regime (temperature smaller than the level width), the peak width approaches saturation, while the peak height starts to decrease. Overall, the peak height shows a non-monotonic temperature dependence. We associate this unusual behavior with the transition from the sequential to the resonant tunneling through a single quantum level in a dissipative environment.Comment: 5 pages, 5 figure

Topological Gauge Structure and Phase Diagram for Weakly Doped Antiferromagnets

We show that the topological gauge structure in the phase string theory of the {\rm t-J} model gives rise to a global phase diagram of antiferromagnetic (AF) and superconducting (SC) phases in a weakly doped regime. Dual confinement and deconfinement of holons and spinons play essential roles here, with a quantum critical point at a doping concentration $x_c\simeq 0.043$. The complex experimental phase diagram at low doping is well described within such a framework.Comment: 4 pages, 2 figures, modified version, to appear in Phys. Rev. Let

0-pi transitions in Josephson junctions with antiferromagnetic interlayers

We show that the dc Josephson current through superconductor-antiferromagnet-superconductor (S/AF/S) junctions manifests a remarkable atomic scale dependence on the interlayer thickness. At low temperatures the junction is either a 0- or pi-junction depending on whether the AF interlayer consists of an even or odd number of atomic layers. This is associated with different symmetries of the AF interlayers in the two cases. In the junction with odd AF interlayers an additional pi-0 transition can take place as a function of temperature. This originates from the interplay of spin-split Andreev bound states. Experimental implications of these theoretical findings are discussed.Comment: 4 pages, 2 figure

Modulated Floquet Topological Insulators

Floquet topological insulators are topological phases of matter generated by the application of time-periodic perturbations on otherwise conventional insulators. We demonstrate that spatial variations in the time-periodic potential lead to localized quasi-stationary states in two-dimensional systems. These states include one-dimensional interface modes at the nodes of the external potential, and fractionalized excitations at vortices of the external potential. We also propose a setup by which light can induce currents in these systems. We explain these results by showing a close analogy to px+ipy superconductors

Influence of e-e scattering on the temperature dependence of the resistance of a classical ballistic point contact in a two-dimensional electron system

We experimentally investigate the temperature (T) dependence of the resistance of a classical ballistic point contact (PC) in a two-dimensional electron system (2DES). The split-gate PC is realized in a high-quality AlGaAs/GaAs heterostructure. The PC resistance is found to drop by more than 10% as T is raised from 0.5 K to 4.2 K. In the absence of a magnetic field, the T dependence is roughly linear below 2 K and tends to saturate at higher T. Perpendicular magnetic fields on the order of a few 10 mT suppress the T-dependent contribution dR. This effect is more pronounced at lower temperatures, causing a crossover to a nearly parabolic T dependence in a magnetic field. The normalized magnetic field dependencies dR(B) permit an empiric single parameter scaling in a wide range of PC gate voltages. These observations give strong evidence for the influence of electron-electron (e-e) scattering on the resistance of ballistic PCs. Our results are in qualitative agreement with a recent theory of the e-e scattering based T dependence of the conductance of classical ballistic PCs [ Phys. Rev. Lett. 101 216807 (2008) and Phys. Rev. B 81 125316 (2010)].Comment: as publishe

Noise in a Quantum Point Contact due to a Fluctuating Impurity Configuration

We propose a theoretical model for the low-frequency noise observed in a quantum point contact (QPC) electrostatically defined in the 2D electron gas at a GaAs-AlGaAs interface. In such contacts electron scattering by soft impurity- or boundary potentials coherently splits an incoming wave function between different transverse modes. Interference between these modes have been suggested to explain observed non-linearities in the QPC-conductance. In this study we invoke the same mechanism and the time-dependent current due to soft dynamical impurity scattering in order to analyze the low-frequency (telegraph-like) noise which has been observed along with a nonlinear conductance. For the simplified case of a channel with two extended (current carrying) modes, a simple analytical formula for the noise intensity is derived. Generally we have found qualitative similarities between the noise and the square of the transconductance. Nevertheless, incidentally there may be situations when noise is suppressed but transconductance enhanced.Comment: 9 revte

Interplay of electromagnetic noise and Kondo effect in quantum dots

We investigate the influence of an electromagnetic environment, characterized by a finite impedance $Z(\omega)$, on the Kondo effect in quantum dots. The circuit voltage fluctuations couple to charge fluctuations in the dot and influence the spin exchange processes transferring charge between the electrodes. We discuss how the low-energy properties of a Kondo quantum dot subject to dynamical Coulomb blockade resemble those of Kondo impurities in Luttinger liquids. Using previous knowledge based on the bosonization of quantum impurity models, we show that low-voltage conductance anomalies appear at zero temperature. The conductance can vanish at low temperatures even in presence of a screened impurity spin. Moreover, the quantitative determination of the corresponding Kondo temperature depends on the full frequency-dependent impedance of the circuit. This is demonstrated by a weak-coupling calculation in the Kondo interaction, taking into account the full distribution $P(E)$ of excited environmental modes.Comment: 10 pages, 4 figures, revised version, new titl