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1.
Phys Rev Lett ; 129(5): 050401, 2022 Jul 29.
Artigo em Inglês | MEDLINE | ID: mdl-35960579

RESUMO

Measurement and feedback control are essential features of quantum science, with applications ranging from quantum technology protocols to information-to-work conversion in quantum thermodynamics. Theoretical descriptions of feedback control are typically given in terms of stochastic equations requiring numerical solutions, or are limited to linear feedback protocols. Here we present a formalism for continuous quantum measurement and feedback, both linear and nonlinear. Our main result is a quantum Fokker-Planck master equation describing the joint dynamics of a quantum system and a detector with finite bandwidth. For fast measurements, we derive a Markovian master equation for the system alone, amenable to analytical treatment. We illustrate our formalism by investigating two basic information engines, one quantum and one classical.

2.
Phys Rev Lett ; 127(24): 246802, 2021 Dec 10.
Artigo em Inglês | MEDLINE | ID: mdl-34951773

RESUMO

In voltage- and temperature-biased coherent conductors quantum screening effects occur if the conductor's transmission is energy dependent. Here, we show that an additional ac-driven terminal can act as a probe for a direct readout of such effects, hitherto unexplored. We find that screening of charges induced by the static biases impacts already their standard linear thermoelectric response coefficients due to nonlinear effects when accounting for the frequency of the time-dependent driving. Those effects should be observable under realistic experimental conditions and can literally be switched on and off with the ac driving.

3.
Nat Commun ; 12(1): 5130, 2021 Aug 26.
Artigo em Inglês | MEDLINE | ID: mdl-34446735

RESUMO

Converting incoming photons to electrical current is the key operation principle of optical photodetectors and it enables a host of emerging quantum information technologies. The leading approach for continuous and efficient detection in the optical domain builds on semiconductor photodiodes. However, there is a paucity of efficient and continuous photon detectors in the microwave regime, because photon energies are four to five orders of magnitude lower therein and conventional photodiodes do not have that sensitivity. Here we tackle this gap and demonstrate how microwave photons can be efficiently and continuously converted to electrical current in a high-quality, semiconducting nanowire double quantum dot resonantly coupled to a cavity. In particular, in our photodiode device, an absorbed photon gives rise to a single electron tunneling through the double dot, with a conversion efficiency reaching 6%.

4.
Nat Commun ; 11(1): 367, 2020 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-31953442

RESUMO

Quantum calorimetry, the thermal measurement of quanta, is a method of choice for ultrasensitive radiation detection ranging from microwaves to gamma rays. The fundamental temperature fluctuations of the calorimeter, dictated by the coupling of it to the heat bath, set the ultimate lower bound of its energy resolution. Here we reach this limit of fundamental equilibrium fluctuations of temperature in a nanoscale electron calorimeter, exchanging energy with the phonon bath at very low temperatures. The approach allows noninvasive measurement of energy transport in superconducting quantum circuits in the microwave regime with high efficiency, opening the way, for instance, to observe quantum jumps, detecting their energy to tackle central questions in quantum thermodynamics.

5.
J Crit Care ; 56: 324, 2020 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-31706803

RESUMO

This study aimed to explore what patients rate as being of the greatest importance and less important, when being cared for in the intensive care unit (ICU). The aim was also to examine the extent to which these topics are met. In the Patient Empowerment Questionnaire (PEQ-ICU), patients were first asked to rate the importance of 28 items, and then how often those topics were met during their stay in the ICU. Having trust/confidence in staff, Receive visits from next of kin, Staff being positive to visitors, Receive pain relief, Staff showing human warmth, and Staff trying to strengthen my life spirit were the items that most patients evaluated as being of the greatest importance. The items Staff being positive to visitors, Receiving pain relief and Receive visits from next of kin, were the items most frequently c onsidered as "always met", while the items Have influence and Receive help to look forward were less often met. It was found that there is a potential for improvement in helping the ICU patients to maintain contact with reality, remind them about their importance to someone or something, and what they could look forward to when becoming healthier and returning to ordinary life.

6.
Phys Rev E ; 100(5-1): 052137, 2019 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-31869995

RESUMO

Thermodynamic uncertainty relations quantify how the signal-to-noise ratio of a given observable is constrained by dissipation. Fluctuation relations generalize the second law of thermodynamics to stochastic processes. We show that any fluctuation relation directly implies a thermodynamic uncertainty relation, considerably increasing their range of applicability. In particular, we extend thermodynamic uncertainty relations to scenarios which include measurement and feedback. Since feedback generally breaks time-reversal invariance, the uncertainty relations involve quantities averaged over the forward and the backward experiment defined by the associated fluctuation relation. This implies that the signal-to-noise ratio of a given experiment can in principle become arbitrarily large as long as the corresponding backward experiment compensates, e.g., by being sufficiently noisy. We illustrate our results with the Szilard engine as well as work extraction by free energy reduction in a quantum dot.

7.
Entropy (Basel) ; 21(8)2019 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-33267490

RESUMO

The trade-off between large power output, high efficiency and small fluctuations in the operation of heat engines has recently received interest in the context of thermodynamic uncertainty relations (TURs). Here we provide a concrete illustration of this trade-off by theoretically investigating the operation of a quantum point contact (QPC) with an energy-dependent transmission function as a steady-state thermoelectric heat engine. As a starting point, we review and extend previous analysis of the power production and efficiency. Thereafter the power fluctuations and the bound jointly imposed on the power, efficiency, and fluctuations by the TURs are analyzed as additional performance quantifiers. We allow for arbitrary smoothness of the transmission probability of the QPC, which exhibits a close to step-like dependence in energy, and consider both the linear and the non-linear regime of operation. It is found that for a broad range of parameters, the power production reaches nearly its theoretical maximum value, with efficiencies more than half of the Carnot efficiency and at the same time with rather small fluctuations. Moreover, we show that by demanding a non-zero power production, in the linear regime a stronger TUR can be formulated in terms of the thermoelectric figure of merit. Interestingly, this bound holds also in a wide parameter regime beyond linear response for our QPC device.

8.
Phys Rev Lett ; 121(21): 210603, 2018 Nov 23.
Artigo em Inglês | MEDLINE | ID: mdl-30517817

RESUMO

Fluctuation relations are powerful equalities that hold far from equilibrium. However, the standard approach to include measurement and feedback schemes may become inapplicable in certain situations, including continuous measurements, precise measurements of continuous variables, and feedback induced irreversibility. Here we overcome these shortcomings by providing a recipe for producing detailed fluctuation relations. Based on this recipe, we derive a fluctuation relation which holds for arbitrary measurement and feedback control. The key insight is that fluctuations inferable from the measurement outcomes may be suppressed by postselection. Our detailed fluctuation relation results in a stringent and experimentally accessible inequality on the extractable work, which is saturated when the full entropy production is inferable from the data.

9.
Phys Rev Lett ; 118(25): 256801, 2017 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-28696742

RESUMO

We show theoretically that a thermoelectric heat engine, operating exclusively due to quantum-mechanical interference, can reach optimal linear-response performance. A chiral edge state implementation of a close-to-optimal heat engine is proposed in an electronic Mach-Zehnder interferometer with a mesoscopic capacitor coupled to one arm. We demonstrate that the maximum power and corresponding efficiency can reach 90% and 83%, respectively, of the theoretical maximum. The proposed heat engine can be realized with existing experimental techniques and has a performance robust against moderate dephasing.

11.
Phys Rev Lett ; 112(7): 076803, 2014 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-24579624

RESUMO

We propose and analyze the use of hybrid microwave cavities as quantum heat engines. A possible realization consists of two macroscopically separated quantum-dot conductors coupled capacitively to the fundamental mode of a microwave cavity. We demonstrate that an electrical current can be induced in one conductor through cavity-mediated processes by heating up the other conductor. The heat engine can reach Carnot efficiency with optimal conversion of heat to work. When the system delivers the maximum power, the efficiency can be a large fraction of the Carnot efficiency. The heat engine functions even with moderate electronic relaxation and dephasing in the quantum dots. We provide detailed estimates for the electrical current and output power using realistic parameters.

13.
Anesthesiology ; 106(1): 26-32, 2007 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-17197842

RESUMO

BACKGROUND: Awareness during general anesthesia can cause late psychological symptoms. Selection bias may have affected the results in previous retrospective studies. The authors used prospective consecutive collection to recruit patients with previous awareness. METHODS: In a cohort of 2,681 consecutive patients scheduled to undergo general anesthesia, 98 considered themselves to have been aware during previous surgery. Six patients died before inclusion, and another 13 were excluded (4 cases of stroke or dementia, 7 declined to participate, and 2 could not be located). Seventy-nine patients were interviewed by telephone, and medical records were checked in uncertain cases. The interview followed a structured protocol, including seven late symptoms (anxiety, chronic fear, nightmares, flashbacks, indifference, loneliness, and lack of confidence in future life). Three persons independently assessed the interviews for classification, to determine whether awareness had occurred. RESULTS: Four cases were performed using regional anesthesia, and another 29 were not considered as awareness by the assessors. Therefore, the final analyses included 46 patients. Twenty (43%) had experienced pain, and 30 (65%) described acute emotional reactions during the awareness episode. Fifteen (33%) patients had experienced late psychological symptoms afterward. In 6 of those cases, the symptoms lasted for more than 2 months, and 1 patient had a diagnosis of post-traumatic stress disorder. Acute emotional reactions were significantly related to late psychological symptoms (P<0.05). CONCLUSION: The method for recruiting awareness cases in studies on late psychological symptoms may affect the result. The authors found fewer and milder problems, despite a similar degree of initial problems as in previous studies.


Assuntos
Anestesia Geral/psicologia , Conscientização , Transtornos Mentais/etiologia , Complicações Pós-Operatórias/etiologia , Adolescente , Adulto , Idoso , Idoso de 80 Anos ou mais , Criança , Cognição , Emoções , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Estudos Prospectivos
14.
Phys Rev Lett ; 91(18): 187002, 2003 Oct 31.
Artigo em Inglês | MEDLINE | ID: mdl-14611307

RESUMO

We derive the full counting statistics of charge transfer through a voltage biased superconducting junction. We find that, for measurement times much longer than the inverse Josephson frequency, the counting statistics describes a correlated transfer of quanta of multiple electron charges, each quantum associated with the transfer of a single quasiparticle. An expression for the counting statistics in terms of the quasiparticle scattering amplitudes is derived.

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