Abstract

It is found that Andreev reflection provides a deterministic teleportation process at an ideal normal-superconductor interface, making it behave like an information mirror. However, it is challenging to control the Andreev reflection in a spatially-separated junction due to the mode mixing at the interface. We theoretically propose the laser-induced Andreev reflection between two-component Fermi superfluid and normal states without mode mixing via spatially-uniform Rabi couplings. By analyzing the tunneling current up to the fourth order, we find that the Andreev current exhibits unconventional non-Ohmic transport at zero temperature. The Andreev current gives the only contribution in the synthetic junction system at zero detunings regardless of the ratio of the chemical potential bias to the superfluid gap, which is in sharp contrast to that in conventional junctions. Our result may give a potential impact on theoretical and experimental study of quantum many-body phenomena, and also pave a way for understanding the black hole information paradox through the Andreev reflection as a quantum-information mirror.

Quantum transport can be simulated with ultracold atoms by employing spin
superpositions of fermions interacting with spin-dependent potentials. Here we
first extend this scheme to an arbitrary number of spin components so as to
simulate transport through a multiterminal quantum dot, deriving a current
formula in terms of potential phase shifts, and then show that a Fano resonance
manifests itself in measuring a linear conductance at low temperature. We also
study consequences of a weak interparticle interaction on quantum transport in
one dimension, finding that the conductance vanishes for an attractive
interaction due to a bulk spin gap, while it for a repulsive interaction is
enhanced by a power law with lowering the temperature or the chemical potential
difference.

We present a microscopic theory of heat and particle transport of a weakly<br />
interacting, low temperature Bose-Einstein condensate in a quantum point<br />
contact. We show that the presence of gapless phonon modes in the condensate<br />
yields a contact resistance at zero temperature and a corresponding nonzero DC<br />
conductance. This originates from the tunneling process that condensate<br />
elements are coherently converted into phonon excitations and vice versa,<br />
yielding a linear current-bias relation. As a consequence, we predict zero<br />
thermopower and Lorenz number at zero temperature, a breakdown of the bosonic<br />
Wiedemann-Franz law. These effects are found to dominate the transport<br />
properties up to temperatures of the order of the chemical potential, with a<br />
leading power law temperature dependence behavior for heat transport<br />
coefficients. The consequences on heat and particle transport measurements in<br />
bosonic two-terminal setups should be readily observable in existing<br />
experiments.

We introduce a Floquet spinor Bose-Einstein condensate induced by a<br />
periodically driven quadratic Zeeman coupling whose frequency is larger than<br />
any other energy scales. By examining a spin-1 system available in ultracold<br />
atomic gases, we demonstrate that such an external driving field has great<br />
effect on the condensate through emergence of a unique spin-exchange<br />
interaction. We uncover that the ferromagnetic condensate has several<br />
unconventional stationary states and thus exhibits rich continuous phase<br />
transitions. On the other hand, the antiferromagnetic condensate is found to<br />
possess a nontrivial metastable region, which supports unusual elementary<br />
excitations and hysteresis phenomena.

We investigate thermal evolution of radio-frequency (RF) spectra of a<br />
spin-imbalanced Fermi gas near a Feshbach resonance in which degenerate<br />
Fermi-polaron and classical Boltzmann-gas regimes emerge in the low-temperature<br />
and high-temperature limits, respectively. By using a self-consistent framework<br />
of strong-coupling diagrammatic approaches, both of the ejection and reserve RF<br />
spectra available in cold-atom experiments are analyzed. We find a variety of<br />
transfers from Fermi polarons to Boltzmann gas such that a thermal crossover<br />
expected in the weak-coupling regime is shifted to a sharp transition near<br />
unitarity and to double-peak coexistence of attractive and repulsive branches<br />
in the strong-coupling regime. Our theory provides semiquantitative<br />
descriptions for a recent experiment on the ejection RF spectroscopy at<br />
unitarity [Z. Yan {\it et al}., arXiv:1811.00481v1] and demonstrate that the RF<br />
spectroscopy is an essential probe to examine effects of strong correlations<br />
and finite temperature.

We propose and analyze continuous measurements of atom number and atomic<br />
currents using dispersive measurements in an optical cavity. For an atom-number<br />
measurement in a closed system, we relate both the detection noise and the<br />
heating rate due to measurement back-action to Tan&#039;s contact, and demonstrate<br />
an emergent universal quantum non-demolition (QND) regime in the good cavity<br />
limit. We then show that such a continuous QND measurement of atom number<br />
serves as a quantum-limited current transducer in the two-terminal setup. We<br />
derive a universal bound on precision of current measurement, which results<br />
from a tradeoff between detection noise and back-action of the atomic current<br />
measurement. Our results apply regardless of interaction strength or phases of<br />
matter and set fundamental bounds on future precision measurements of transport<br />
properties in cold-atom quantum simulators.

An extremely polarized mixture of an ultracold Fermi gas is expected to<br />
reduce to a Fermi polaron system, which consists of a single impurity immersed<br />
in the Fermi sea of majority atoms. By developing a many-body T -matrix theory,<br />
we investigate spectral properties of the polarized mixture in experimentally<br />
relevant regimes in which the system of finite impurity concentration at<br />
nonzero temperature is concerned. We explicitly demonstrate presence of polaron<br />
physics in the polarized limit and discuss effects of many polarons in an<br />
intermediate regime in a self-consistent manner. By analyzing the spectral<br />
function at finite impurity concentration, we extract the attractive and<br />
repulsive polaron energies. We find that a renormalization of majority atoms<br />
via an interaction with minority atoms and a thermal depletion of the impurity<br />
chemical potential are of significance to depict the many-polaron regime.

We have discussed the phase structure and low-energy excitations in spinor BECs, which involve the nontrivial properties compared to a condensate without spin degrees of freedom. Since a spinor BEC is an ideal system to study condensates with internal degrees of freedom, it is expected that it sheds light on physics of superfluid ^3He, high-T_c superconductor, and color superconductor in dense quantum chromodynamics.