Single-qubit reaped quantum state tomography.

Quantum state tomography is the experimental procedure of determining an unknown state. It is not only essential for the verification of resources and processors of quantum information but is also important in its own right with regard to the foundation of quantum mechanics. Standard methods have been elusive for large systems because of the enormous number of observables to be measured and the exponential complexity of data post-processing.

Scaling Law for Irreversible Entropy Production in Critical Systems.

We examine the Jarzynski equality for a quenching process across the critical point of second-order phase transitions, where absolute irreversibility and the effect of finite-sampling of the initial equilibrium distribution arise in a single setup with equal significance. We consider the Ising model as a prototypical example for spontaneous symmetry breaking and take into account the finite sampling issue by introducing a tolerance parameter. The initially ordered spins become disordered by quenching the ferromagnetic coupling constant.

Recurrent Delocalization and Quasiequilibration of Photons in Coupled Systems in Circuit Quantum Electrodynamics.

We explore the photon population dynamics in two coupled circuit QED systems. For a sufficiently weak intercavity photon hopping, as the photon-cavity coupling increases, the dynamics undergoes double transitions first from a delocalized to a localized phase and then from the localized to another delocalized phase. The latter delocalized phase is distinguished from the former one; instead of oscillating between the two cavities, the photons rapidly quasiequilibrate over the two cavities.

Polarity-tunable magnetic tunnel junctions based on ferromagnetism at oxide heterointerfaces.

Complex oxide systems have attracted considerable attention because of their fascinating properties, including the magnetic ordering at the conducting interface between two band insulators, such as LaAlO3 and SrTiO3. However, the manipulation of the spin degree of freedom at the LaAlO3/SrTiO3 heterointerface has remained elusive. Here, we have fabricated hybrid magnetic tunnel junctions consisting of Co and LaAlO3/SrTiO3 ferromagnets with the insertion of a Ti layer in between, which clearly exhibit magnetic switching and the tunnelling magnetoresistance effect below 10 K.

Negative tunneling magneto-resistance in quantum wires with strong spin-orbit coupling.

We consider a two-dimensional magnetic tunnel junction of the FM/I/QW(FM+SO)/I/N structure, where FM, I and QW(FM+SO) stand for a ferromagnet, an insulator and a quantum wire with both magnetic ordering and Rashba spin-orbit (SOC), respectively. The tunneling magneto-resistance (TMR) exhibits strong anisotropy and switches sign as the polarization direction varies relative to the quantum-wire axis, due to interplay among the one-dimensionality, the magnetic ordering, and the strong SOC of the quantum wire.

Quantum resistor-capacitor circuit with Majorana fermion modes in a chiral topological superconductor.

We investigate the mesoscopic resistor-capacitor circuit consisting of a quantum dot coupled to spatially separated Majorana fermion modes in a chiral topological superconductor. We find substantially enhanced relaxation resistance due to the nature of Majorana fermions, which are their own antiparticles and are composed of particle and hole excitations in the same abundance. Further, if only a single Majorana mode is involved, the zero-frequency relaxation resistance is completely suppressed due to a destructive interference.

Transport measurement of Andreev bound states in a Kondo-correlated quantum dot.

We report nonequilibrium transport measurements of gate-tunable Andreev bound states in a carbon nanotube quantum dot coupled to two superconducting leads. In particular, we observe clear features of two types of Kondo ridges, which can be understood in terms of the interplay between the Kondo effect and superconductivity. In the first type (type I), the coupling is strong and the Kondo effect is dominant.

Interferometric and noise signatures of Majorana fermion edge states in transport experiments.

Domain walls between superconducting and magnetic regions placed on top of a topological insulator support transport channels for Majorana fermions. We propose to study noise correlations in a Hanbury Brown-Twiss type interferometer and find three signatures of the Majorana nature of the channels. First, the average charge current in the outgoing leads vanishes.

Superconducting qubits coupled to torsional resonators.

We propose a scheme of strong and tunable coupling between a superconducting phase qubit and a nanomechanical torsional resonator. In our scheme the torsional resonator directly modulates the largest energy scale (the Josephson coupling energy) of the phase qubit, and the coupling strength is very large. We analyze the quantum correlation effects in the torsional resonator as a result of the strong coupling to the phase qubit.

Superconducting junction of a single-crystalline au nanowire for an ideal Josephson device.

We report on the fabrication and measurements of a superconducting junction of a single-crystalline Au nanowire, connected to Al electrodes. The current-voltage characteristic curve shows a clear supercurrent branch below the superconducting transition temperature of Al and quantized voltage plateaus on application of microwave radiation, as expected from Josephson relations. Highly transparent (0.

Josephson current in strongly correlated double quantum dots.

We study the Josephson current through a serial double quantum dot and the associated 0-π transitions which result from the subtle interplay between the superconductivity, the Kondo physics, and the interdot superexchange interaction. The competition between them is examined by tuning the relative strength Δ/T(K) of the superconducting gap and the Kondo temperature, for different strengths of the superexchange coupling determined by the interdot tunneling t relative to the level broadening Γ. We find strong renormalization of t, a significant role of the superexchange coupling J, and a rich phase diagram of the 0 and π-junction regimes.

Quantum interference in radial heterostructure nanowires.

Core/shell heterostructure nanowires are one of the most interesting mesoscopic systems potentially suitable for the study of quantum interference phenomena. Here, we report on experimental observations of both the Aharonov-Bohm (h/e) and the Altshuler-Aronov-Spivak (h/2e) oscillations in radial core/shell (In2O3/InOx) heterostructure nanowires. For a long channel device with a length-to-width ratio of about 33, the magnetoresistance curves at low temperatures exhibited a crossover from low-field h/2e oscillation to high-field h/ e oscillation.

Quantum key distribution using quantum Faraday rotators.

We propose a new quantum key distribution (QKD) protocol based on the fully quantum mechanical states of Faraday rotators. The protocol is unconditionally secure against collective attacks for a multi-photon source of up to two photons on a noisy environment. It is also robust against impersonation attacks.

Eightfold shell filling in a double-wall carbon nanotube quantum dot.

We fabricated a quantum-dot device consisting of an individual double-wall carbon nanotube and studied its electrical transport properties at low temperatures. In the negative bias region, the gate modulation curve exhibited quasiperiodic current oscillations, attributed to the Coulomb blockade of single-electron tunneling. We observed both four- and eightfold shell filling in the Coulomb diamond structures.

Decoherence-driven quantum transport.

We propose a new mechanism to generate a dc current of particles at zero bias based on a noble interplay between coherence and decoherence. We show that a dc current arises if the transport process in one direction is maintained coherent while the process in the opposite direction is incoherent. We provide possible implementations of the idea using an atomic Michelson interferometer and a ring interferometer.

Shot noise enhancement from non-equilibrium plasmons in Luttinger liquid junctions.

We consider a quantum wire double junction system with each wire segment described by a spinless Luttinger model, and study theoretically shot noise in this system in the sequential tunnelling regime. We find that the non-equilibrium plasmonic excitations in the central wire segment give rise to qualitatively different behaviour compared to the case with equilibrium plasmons. In particular, shot noise is greatly enhanced by them, and exceeds the Poisson limit.

Kondo effect in a quantum dot coupled to ferromagnetic leads: a numerical renormalization group analysis.

We investigate the effects of spin-polarized leads on the Kondo physics of a quantum dot using the numerical renormalization group method. Our study demonstrates in an unambiguous way that the Kondo effect is not necessarily suppressed by the lead polarization: While the Kondo effect is quenched for the asymmetric Anderson model, it survives even for finite polarizations in the regime where charge fluctuations are negligible. We propose the linear tunneling magnetoresistance as an experimental signature of these behaviors.

Effect of quantum fluctuations in an Ising system on small-world networks.

We study quantum Ising spins placed on small-world networks. A simple model is considered in which the coupling between any given pair of spins is a nonzero constant if they are linked in the small-world network, and zero otherwise. By applying a transverse magnetic field, we have investigated the effect of quantum fluctuations.