28 research outputs found
Non-geometric pumping effects on the performance of interacting quantum-dot heat engines
Periodically driven quantum dots can act as counterparts of cyclic thermal
machines at the nanoscale. In the slow-driving regime of geometric pumping,
such machines have been shown to operate in analogy to a Carnot cycle. For
larger driving frequencies, which are required in order to increase the cooling
power, the efficiency of the operation decreases. Up to which frequency a
close-to-optimal performance is still possible depends on the magnitude and
sign of on-site electron-electron interaction. Extending our previous detailed
study on cyclic quantum-dot refrigerators [Phys. Rev. B 106, 035405 (2022)], we
here find that the optimal cooling power remains constant up to weak
interaction strength compared to the cold-bath temperature. By contrast, the
work cost depends on the interaction via the dot's charge relaxation rate, as
the latter sets the typical driving frequency for the onset of non-geometric
pumping contributions.Comment: 6 pages, 3 figures, Proceedings of the FQMT'22 conferenc
An autonomous quantum machine to measure the thermodynamic arrow of time
According to the Second Law of thermodynamics, the evolution of physical
systems has a preferred direction, that is characterized by some positive
entropy production. Here we propose a direct way to measure the stochastic
entropy produced while driving a quantum open system out of thermal
equilibrium. The driving work is provided by a quantum battery, the system and
the battery forming an autonomous machine. We show that the battery's energy
fluctuations equal work fluctuations and check Jarzynski's equality. Since
these energy fluctuations are measurable, the battery behaves as an embedded
quantum work meter and the machine verifies a generalized fluctuation theorem
involving the information encoded in the battery. Our proposal can be
implemented with state-of-the-art opto-mechanical systems. It paves the way
towards the experimental demonstration of fluctuation theorems in quantum open
systems.Comment: 9 pages, 5 figure
Geometric energy transport and refrigeration with driven quantum dots
We study geometric energy transport in a slowly driven single-level quantum dot weakly coupled to electronic contacts and with strong on-site interaction, which can be either repulsive or attractive. Exploiting a recently discovered fermionic duality for the evolution operator of the master equation, we provide compact and insightful analytic expressions of energy pumping curvatures for any pair of driving parameters. This enables us to systematically identify and explain the pumping mechanisms for different driving schemes, thereby also comparing energy and charge pumping. We determine the concrete impact of many-body interactions and show how particle-hole symmetry and fermionic duality manifest, both individually and in combination, as system-parameter symmetries of the energy pumping curvatures. Building on this transport analysis, we study the driven dot acting as a heat pump or refrigerator, where we find that the sign of the on-site interaction plays a crucial role in the performance of these thermal machines
Charge, spin, and heat shot noises in the absence of average currents: Conditions on bounds at zero and finite frequencies
Nonequilibrium situations where selected currents are suppressed are of
interest in fields like thermoelectrics and spintronics, raising the question
of how the related noises behave. We study such zero-current charge, spin, and
heat noises in a two-terminal mesoscopic conductor. In the presence of voltage,
spin and temperature biases, the nonequilibrium (shot) noises of charge, spin,
and heat can be arbitrarily large, even if their average currents vanish.
However, as soon as a temperature bias is present, additional equilibrium
(thermal-like) noise necessarily occurs. We show that this equilibrium noise
sets an upper bound on the zero-current charge and spin shot noises, even if
additional voltage or spin biases are present. We demonstrate that these bounds
can be overcome for heat transport by breaking the spin and electron-hole
symmetries, respectively. By contrast, we show that the bound on the charge
noise for strictly two-terminal conductors even extends into the
finite-frequency regime.Comment: 17 pages, 5 figure
Charge, spin, and heat shot noises in the absence of average currents: Conditions on bounds at zero and finite frequencies
Nonequilibrium situations where selected currents are suppressed are of interest in fields like thermoelectrics and spintronics, raising the question of how the related noises behave. We study such zero-current charge, spin, and heat noises in a two-terminal mesoscopic conductor. In the presence of voltage, spin, and temperature biases, the nonequilibrium (shot) noises of charge, spin, and heat can be arbitrarily large, even if their average currents vanish. However, as soon as a temperature bias is present, additional equilibrium (thermal-like) noise necessarily occurs. We show that this equilibrium noise sets an upper bound on the zero-current charge and spin shot noises, even if additional voltage or spin biases are present. We demonstrate that these bounds can be overcome for heat transport by breaking the spin and electron-hole symmetries, respectively. By contrast, we show that the bound on the charge noise for strictly two-terminal conductors even extends into the finite-frequency regime
Integrated microcavity optomechanics with a suspended photonic crystal mirror above a distributed Bragg reflector
Increasing the interaction between light and mechanical resonators is an
ongoing endeavor in the field of cavity optomechanics. Optical microcavities
allow for boosting the interaction strength through their strong spatial
confinement of the optical field. In this work, we follow this approach by
realizing a sub-wavelength-long, free-space optomechanical microcavity on-chip
fabricated from an (Al,Ga)As heterostructure. A suspended GaAs photonic crystal
mirror is acting as a highly reflective mechanical resonator, which together
with a distributed Bragg reflector forms an optomechanical microcavity. We
demonstrate precise control over the microcavity resonance by change of the
photonic crystal parameters. The interplay between the microcavity mode and a
guided resonance of the photonic crystal modifies the cavity response and
results in a stronger dynamical backaction on the mechanical resonator compared
to conventional optomechanical dynamics.Comment: 11 pages, 6 figures + Supplementary Material with 10 pages, 12
figures, 2 table
Optomechanical cooling with coherent and squeezed light: the thermodynamic cost of opening the heat valve
Ground-state cooling of mechanical motion by coupling to a driven optical
cavity has been demonstrated in various optomechanical systems. In our work, we
provide a so far missing thermodynamic performance analysis of optomechanical
sideband cooling in terms of a heat valve. As performance quantifiers, we
examine not only the lowest reachable effective temperature (phonon number) but
also the evacuated-heat flow as an equivalent to the cooling power of a
standard refrigerator, as well as appropriate thermodynamic efficiencies, which
all can be experimentally inferred from measurements of the cavity output light
field. Importantly, in addition to the standard optomechanical setup fed by
coherent light, we investigate two recent alternative setups for achieving
ground-state cooling: replacing the coherent laser drive by squeezed light or
using a cavity with a frequency-dependent (Fano) mirror. We study the dynamics
of these setups within and beyond the weak-coupling limit and give concrete
examples based on parameters of existing experimental systems. By applying our
thermodynamic framework, we gain detailed insights into these three different
optomechanical cooling setups, allowing a comprehensive understanding of the
thermodynamic mechanisms at play.Comment: 28 pages, 14 figures, 2 tables Small revision of the main text,
corrected typos in the appendices, added study of the stability of the
systems and comparison with absorption refrigerators in appendi
Ultrastrong coupling between electron tunneling and mechanical motion
The ultrastrong coupling of single-electron tunneling and nanomechanical
motion opens exciting opportunities to explore fundamental questions and
develop new platforms for quantum technologies. We have measured and modelled
this electromechanical coupling in a fully-suspended carbon nanotube device and
report a ratio of , where ~MHz is the
coupling strength and ~MHz is the mechanical resonance
frequency. This is well within the ultrastrong coupling regime and the highest
among current electromechanical platforms. Even higher ratios could be achieved
with improvement on device design
Stability of long-sustained oscillations induced by electron tunneling
Self-oscillations are the result of an efficient mechanism generating
periodic motion from a constant power source. In quantum devices, these
oscillations may arise due to the interaction between single electron dynamics
and mechanical motion. Due to the complexity of this mechanism, these
self-oscillations may irrupt, vanish, or exhibit a bistable behavior causing
hysteresis cycles. We observe these hysteresis cycles and characterize the
stability of different regimes in single and double quantum dot configurations.
In particular cases, we find these oscillations stable for over 20 seconds,
many orders of magnitude above electronic and mechanical characteristic
timescales, revealing the robustness of the mechanism at play. The experimental
results are reproduced by our theoretical model that provides a complete
understanding of bistability in nanoelectromechanical devices.Comment: 11 pages, 10 figures, includes the complete paper and the
Supplemental Materia
Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.
RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)