Alexandre Roulet

Autonomous rotor heat engine

The triumph of heat engines is their ability to convert the disordered energy of thermal sources into useful mechanical motion. In recent years, much effort has been devoted to generalizing thermodynamic notions to the quantum regime, partly motivated by the promise of surpassing classical heat engines. Here, we instead adopt a bottom-up approach: we propose a realistic autonomous heat engine that can serve as a test bed for quantum effects in the context of thermodynamics. Our model draws inspiration from actual piston engines and is built from closed-system Hamiltonians and weak bath coupling terms. We analytically derive the performance of the engine in the classical regime via a set of nonlinear Langevin equations. In the quantum case, we perform numerical simulations of the master equation. Finally, we perform a dynamic and thermodynamic analysis of the engines behavior for several parameter regimes in both the classical and quantum case and find that the latter exhibits a consistently lower efficiency due to additional noise.

Solving the scattering of N photons on a two-level atom without computation

We propose a novel approach for solving the scattering of light onto a two-level atom coupled to a one-dimensional waveguide. We first express the physical quantity of interest in terms of Feynman diagrams and treat the atom as a non-saturable linear beamsplitter. By using the atomic response to our advantage, a relevant substitution is then made that captures the nonlinearity of the atom, and the final result is obtained in terms of simple integrals over the initial incoming wavepackets. The procedure is not limited to post-scattering quantities and allows for instance to derive the atomic excitation during the scattering event.

Quantum absorption refrigerator with trapped ions

A remarkable progress has recently been achieved in studies of thermodynamics and heat machines, with experiments probing down to micro and nano-scale systems such as the single Brownian particle [1], as well as the single atom [2]. However, despite several theoretical proposals [3, 4], implementation of heat machines in the fully quantum regime remains a challenge. We report on an experimental realization of a quantum absorption refrigerator in a system of the three trapped 171Yb+ ions. The normal modes of motion are coupled by a trilinear Hamiltonian ab†c† and represent “hot”, “work” and “cold” bodies of the refrigerator (Figure 1). We investigate the equilibrium properties of the refrigerator, and demonstrate the absorption refrigeration effect with the modes being prepared in thermal states.

Rabi oscillation in a quantum cavity: Markovian and non-Markovian dynamics

We investigate the Rabi oscillation of an atom placed inside a quantum cavity where each mirror is formed by a chain of atoms trapped near a one-dimensional waveguide. This proposal was studied previously with the use of Markov approximation, where the delay due to the finite travel time of light between the two cavity mirrors is neglected. We show that Rabi oscillation analogous to that obtained with high-finesse classical cavities is achieved only when this travel time is much larger than the time scale that characterizes the superradiant response of the mirrors. Therefore, the delay must be taken into account and the dynamics of the problem is inherently non-Markovian. Parameters of interest such as the Rabi frequency and the cavity loss rate due to photon leakage through the mirrors are obtained.

Rectification of light in the quantum regime

One of the missing elements for realising an integrated optical circuit is a rectifying device playing the role of an optical diode. A proposal based on a pair of two-level atoms strongly coupled to a one-dimensional waveguide showed a promising behavior based on a semiclassical study [Fratini et al., Phys. Rev. Lett. 113, 243601 (2014)]. Our study in the full quantum regime shows that, in such a device, rectification is a purely multiphoton effect. For an input field in a coherent state, rectification reaches up to

An autonomous single-piston engine with a quantum rotor

70%

Quantum and classical dynamics of a three-mode absorption refrigerator

for the range of power in which one of the two atoms is excited, but not both.

Two photons on an atomic beam splitter: Nonlinear scattering and induced correlations

Pistons are elementary components of a wide variety of thermal engines, allowing to convert input fuel into rotational motion. Here, we propose a single-piston engine where the rotational degree of freedom is effectively realized by the flux of a Josephson loop -- a quantum rotor -- while the working volume corresponds to the effective length of a superconducting resonator. Our autonomous design implements a Carnot cycle, relies solely on standard thermal baths and can be implemented with circuit quantum electrodynamics. We demonstrate how the engine is able to extract a net positive work via its built-in synchronicity using a filter cavity as an effective valve, eliminating the need for external control.

Proposal for monitoring and heralding position states of atoms in a one-dimensional waveguide

We study the quantum and classical evolution of a system of three harmonic modes interacting via a trilinear Hamiltonian. With the modes prepared in thermal states of different temperatures, this model describes the working principle of an absorption refrigerator that transfers energy from a cold to a hot environment at the expense of free energy provided by a high-temperature work reservoir. Inspired by a recent experimental realization with trapped ions, we elucidate key features of the coupling Hamiltonian that are relevant for the refrigerator performance. The coherent system dynamics exhibits rapid effective equilibration of the mode energies and correlations, as well as a transient enhancement of the cooling performance at short times. We find that these features can be fully reproduced in a classical framework.

Rabi oscillation in a quantum cavity

Optical emitters strongly coupled to photons propagating in one-dimensional waveguides are a promising platform for optical quantum information processing. Here, we present a theoretical study of the scattering of two indistinguishable photons on a single two-level atom in a Hong-Ou-Mandel set-up. By computing the dynamics, we can describe the system at any time of the scattering event. This allows us to highlight the one-to-one correspondence between the saturation of the atom and the effective interaction induced between the photons. Furthermore, we discuss the integrability of the atomic beamsplitter and provide an intuitive picture for the correlations observed between the outgoing photons.