56 research outputs found
Universal Noise in Continuous Transport Measurements of Interacting Fermions
We propose and analyze continuous measurements of atom number and atomic
currents using dispersive probing in an optical cavity. For an atom-number
measurement in a closed system, we relate both the detection noise and the
heating rate due to measurement back-action to Tan's contact, and identify an
emergent universal quantum non-demolition (QND) regime in the good-cavity
limit. We then show that such a continuous QND measurement of atom number
serves as a quantum-limited current transducer in a two-terminal setup. We
derive a universal bound on the precision of current measurement, which results
from a tradeoff between detection noise and back-action of the atomic current
measurement. Our results apply regardless of the strength of interaction or the
state of matter and set fundamental bounds on future precision measurements of
transport properties in cold-atom quantum simulators.Comment: 13 pages, 6 figures; Accepted for publication in Phys. Rev.
Bosonic superfluid transport in a quantum point contact
We present a microscopic theory of heat and particle transport of an
interacting, low temperature Bose-Einstein condensate in a quantum point
contact. We show that, in contrast to charged, fermionic superconductors,
bosonic systems feature tunneling processes of condensate elements, leading to
the presence of odd-order harmonics in the AC Josephson current. A crucial role
is played by an anomalous tunneling process where condensate elements are
coherently converted into phonon excitations, leading to even-order harmonics
in the AC currents as well as a DC contribution. At low bias, we find
dissipative components obeying Ohm's law, and bias-independent nondissipative
components, in sharp contrast to fermionic superconductors. Analyzing the DC
contribution, we find zero thermopower and Lorenz number at zero temperature, a
breakdown of the bosonic Wiedemann-Franz law. These results highlight
importance of the anomalous tunneling process inherent to charge neutral
superfluids. The consequences could readily be observed in existing cold-atom
transport setups.Comment: 11 pages, 5 figure
A quantum trampoline for ultra-cold atoms
We have observed the interferometric suspension of a free-falling
Bose-Einstein condensate periodically submitted to multiple-order diffraction
by a vertical 1D standing wave. The various diffracted matter waves recombine
coherently, resulting in high contrast interference in the number of atoms
detected at constant height. For long suspension times, multiple-wave
interference is revealed through a sharpening of the fringes. We use this
scheme to measure the acceleration of gravity
Strongly correlated Fermions strongly coupled to light
Strong quantum correlations in matter are responsible for some of the most
extraordinary properties of material, from magnetism to high-temperature
superconductivity, but their integration in quantum devices requires a strong,
coherent coupling with photons, which still represents a formidable technical
challenge in solid state systems. In cavity quantum electrodynamics, quantum
gases such as Bose-Einstein condensates or lattice gases have been strongly
coupled with light. However, neither Fermionic quantum matter, comparable to
electrons in solids, nor atomic systems with controlled interactions, have thus
far been strongly coupled with photons. Here we report on the strong coupling
of a quantum-degenerate unitary Fermi gas with light in a high finesse cavity.
We map out the spectrum of the coupled system and observe well resolved dressed
states, resulting from the strong coupling of cavity photons with each spin
component of the gas. We investigate spin-balanced and spin-polarized gases and
find quantitative agreement with ab-initio calculation describing light-matter
interaction. Our system offers complete and simultaneous control of atom-atom
and atom-photon interactions in the quantum degenerate regime, opening a wide
range of perspectives for quantum simulation.Comment: Updated reference
Connecting strongly correlated superfluids by a quantum point contact
Point contacts provide simple connections between macroscopic particle
reservoirs. In electric circuits, strong links between metals, semiconductors
or superconductors have applications for fundamental condensed-matter physics
as well as quantum information processing. However for complex, strongly
correlated materials, links have been largely restricted to weak tunnel
junctions. Here we study resonantly interacting Fermi gases connected by a
tunable, ballistic quantum point contact, finding a non-linear current-bias
relation. At low temperature, our observations agree quantitatively with a
theoretical model in which the current originates from multiple Andreev
reflections. In a wide contact geometry, the competition between superfluidity
and thermally activated transport leads to a conductance minimum. Our system
offers a controllable platform for the study of mesoscopic devices based on
strongly interacting matter.Comment: 5 pages, 4 figures, 7 pages supplementar
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