## Welcome to the Quantum Thermodynamics Group

The theory of thermodynamics was a driving force in the industrial revolution. By enabling the development of devices such as steam engines and refrigerators, it had a tremendous impact. At the nanoscale, where systems experience fluctuations and quantum effects, our thermodynamic understanding is still being expanded. Our group is a part of this exciting development which promises to produce important contributions to emerging nano- and quantum-technologies.

## New Publications and Preprints

#### Optical coherent feedback control of a mechanical oscillator

Feedback is a powerful and ubiquitous technique both in classical and quantum system control. In its standard implementation it relies on measuring the state of a system, classically processing and feeding back the extracted information. In quantum physics, however, measurements not only read out the state of the system, but also modify it irreversibly. A different kind of feedback which coherently processes and feeds back quantum signals without actually measuring the system is possible. This is known as coherent feedback. Here, we report on the realization of an optical coherent feedback platform to control the motional state of a nanomechanical membrane in an optical cavity. The coherent feedback loop consists of a light field interacting twice with the same mechanical mode through different cavity modes, without any measurement taking place. Our theoretical analysis provides the optimal cooling conditions, showing that this new technique enables ground-state cooling. Experimentally, we show that we can cool the membrane to a state with *n*=4.89±0.14 phonons (480*μ*K) in a 20K environment.

#### Quantum Fokker-Planck Master Equation for Continuous Feedback Control

Phys. Rev. Lett. **129**, 050401 (2022)

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.

#### Full counting statistics of the photocurrent through a double quantum dot embedded in a driven microwave resonator

Phys. Rev. B 106, 205135 (2022)

Detection of single, itinerant microwave photons is an important functionality for emerging quantum technology applications as well as of fundamental interest in quantum thermodynamics experiments. Here we theoretically investigate the fluctuations of the photocurrent in a photodetector consisting of a double quantum dot coupled to a microwave resonator. We find that for ideal, unity efficiency detection, the fluctuations of the charge current reproduce the Poisson statistics of the incoming photons. Additionall, the finite-frequency noise gives insight into the short-time behavior of the detector. Our results give novel insight into microwave photon-electron interactions in hybrid dot-resonator systems and provide guidance for further experiments on continuous detection of single microwave photons.

#### Probabilistically violating the first law of thermodynamics in a quantum heat engine

Fluctuations of thermodynamic observables, such as heat and work, contain relevant information on the underlying physical process. These fluctuations are however not taken into account in the traditional laws of thermodynamics. While the second law is extended to fluctuating systems by the celebrated fluctuation theorems, the first law is generally believed to hold even in the presence of fluctuations. Here we show that in the presence of quantum fluctuations, also the first law of thermodynamics may break down. To illustrate our results, we provide a detailed case-study of work and heat fluctuations in a quantum heat engine based on a circuit QED architecture.

#### A thermodynamically consistent Markovian master equation beyond the secular approximation

New J. Phys. **23**, 123013 (2021)

Markovian master equations provide a versatile tool for describing open quantum systems when memory effects of the environment may be neglected. As these equations are of an approximate nature, they often do not respect the laws of thermodynamics when no secular approximation is performed in their derivation. Here we introduce a Markovian master equation that is thermodynamically consistent and provides an accurate description whenever memory effects can be neglected. Our results enable a thermodynamically consistent description of a variety of systems where the secular approximation breaks down.

#### Efficient and continuous microwave photoconversion in hybrid cavity-semiconductor nanowire double quantum dot diodes

Converting incoming photons to electrical current is the key operation principle of optical photodetectors and it enables a host of emerging quantum information technologies. Here we 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 our photodiode device, an absorbed photon gives rise to a single electron tunneling through the double dot, with a conversion efficiency reaching 6%.

#### Violating the thermodynamic uncertainty relation in the three-level maser

Phys. Rev. E **104**, L012103 (2021)

The Thermodynamic Uncertainty Relation (TUR), a trade-off between power, efficiency, and low fluctuations, can be violated in the prototypical Scovil and Schulz-duBois maser. Comparing this maser to a classical analogue sheds light onto the relation between TUR violations and quantum coherence. Our results indicate that the coherent nature of the dynamics responsible for TUR violations is not encoded in the off-diagonal elements of the steady state density matrix.

## News

##### 03 October 2022

Joël Aschwanden started to work on his master thesis in our group. Welcome!

##### 6 September 2022

Patrick is co-organizing the Young Faculty Meeting for the Swiss Academy of Sciences, Platform Mathematics, Astronomy and Physics.

##### 22 July 2022

Follow us on our new Twitter account: @QTD_Basel.

##### 01 July 2022

Saulo Moreira from Lund University presented a joint work with Patrick on the thermodynamics of a quantum dot coupled to a finite sized reservoir at the Quantum Thermodynamics Conference 2022. Find the video here.

##### 15 March 2022

Aaron Daniel started to work on his master thesis in our group. Welcome!

##### 16 February 2022

Patrick Potts was awarded the IOP Trusted Reviewer Status.

##### 31 January - 04 February 2022

Patrick Potts presented at the workshop Openness as a resource: Accessing new quantum states with dissipative mechanisms.

##### 07 December 2021

Patrick Potts is featured in an SNI Insight article.

##### 15 October 2021

Matteo Brunelli started a Postdoc in the Quantum Thermodynamics Group. Welcome!

##### 15 September 2021

Kacper Prech started his PhD in the Quantum Thermodynamics Group. Welcome!

##### 23 - 27 August 2021

Patrick Potts lectured on *Thermodynamics in Superconducting Circuits* at the Quantum Thermodynamics summer school 2021. See the lecture here: part 1, part 2.

##### 01 August 2021

Marcelo Janovitch started his PhD in the Quantum Thermodynamics Group. Welcome!

##### 27 May 2021

Patrick Potts talked about Quantum Thermodynamics at a QSIT Seminar.

##### 01 May 2021

The Quantum Thermodynamics Group was started.