The fastest possible collective response of a quantum many-body system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such “ultrafast” processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic many-body system, revealing in real time the formation dynamics of quasi-particles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast time-domain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions.

https://science.sciencemag.org/content/sci/354/6308/96.full.pdf

Ultrafast many-body interferometry of impurities coupled to a Fermi sea

Quantum tomography of individual electrons, which in principle yields complete knowledge of their quantum states, is demonstrated by initially preparing them in a well-controlled quantum state called a leviton.

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Quantum tomography of an electron

The edge channels of the quantum Hall effect provide one dimensional chiral and ballistic wires along which electrons can be guided in an optics-like setup. Electronic propagation can then be analyzed using concepts and tools derived from optics. After a brief review of electron optics experiments performed using stationary current sources which continuously emit electrons in the conductor, this paper focuses on triggered sources, which can generate on-demand a single particle state. It first outlines the electron optics formalism and its analogies and differences with photon optics and then turns to the presentation of single electron emitters and their characterization through the measurements of the average electrical current and its correlations. This is followed by a discussion of electron quantum optics experiments in the Hanbury-Brown and Twiss geometry where two-particle interferences occur. Finally, Coulomb interactions effects and their influence on single electron states are considered.

/pdf/electron-quantum-optics-in-ballistic-chiral-conductors-4nq3pcibun.pdf

Electron quantum optics in ballistic chiral conductors

Ultracold atoms confined by engineered magnetic or optical potentials are ideal to study phenomena otherwise difficult to realize or probe in the solid state, thanks to the ability to control the atomic interaction strength, number of species, density and geometry. Here, we review quantum transport phenomena in atomic gases that mirror and can either better elucidate or show fundamental differences with respect to those observed in mesoscopic and nanoscopic systems. We discuss the significant progress in transport experiments in atomic gases, the similarities and differences between transport in cold atoms and in condensed matter systems, and survey theoretical predictions that are difficult to verify in conventional set-ups. Ultracold-atom experiments enable more flexibility in the study of quantum transport phenomena that are otherwise difficult to probe in solid-state systems. A survey of recent advances highlights the challenges and opportunities of this approach.

Quantum transport in ultracold atoms

Two-particle interference is a fundamental feature of quantum mechanics, and is even less intuitive than wave–particle duality for a single particle. In this duality, classical concepts—wave or particle—are still referred to, and interference happens in ordinary space-time. On the other hand, two-particle interference takes place in a mathematical space that has no classical counterpart. Entanglement lies at the heart of this interference, as it does in the fundamental tests of quantum mechanics involving the violation of Bell's inequalities1,2,3,4. The Hong, Ou and Mandel experiment5 is a conceptually simpler situation, in which the interference between two-photon amplitudes also leads to behaviour impossible to describe using a simple classical model. Here we report the realization of the Hong, Ou and Mandel experiment using atoms instead of photons. We create a source that emits pairs of atoms, and cause one atom of each pair to enter one of the two input channels of a beam-splitter, and the other atom to enter the other input channel. When the atoms are spatially overlapped so that the two inputs are indistinguishable, the atoms always emerge together in one of the output channels. This result opens the way to testing Bell's inequalities involving mechanical observables of massive particles, such as momentum, using methods inspired by quantum optics6,7, and to testing theories of the quantum-to-classical transition8,9,10,11. Our work also demonstrates a new way to benchmark non-classical atom sources12,13 that may be of interest for quantum information processing14 and quantum simulation15. The Hong–Ou–Mandel effect—in which two indistinguishable photons that enter a 50:50 beam-splitter are found only as a pair at one of the two outputs, leading to a dip in the coincidence rate of the detectors—is now realized with 4He atoms instead of photons; this opens the way to performing basic quantum-physics experiments with mechanical observables of massive particles. The Hong-Ou-Mandel effect, in which two indistinguishable photons entering a 50:50 beam splitter lead to a dip in the coincidence rate of the detectors, demonstrates basic features of the theory of quantum mechanics and has no classical analogue. It is the basis of many other experiments in quantum information and quantum optics. Here Marc Cheneau and colleagues realize the Hong-Ou-Mandel experiments with helium-4 atoms instead of photons, building on previous developments on atom pair production. Since atoms, as opposed to photons, are massive particles, this experiment opens possibilities to perform basic quantum physics experiments, like Bell tests, with observables of massive particles. In the long run, this could contribute to research on the effect of mass and gravity in quantum mechanics, especially in relation to the quantum-to-classical transition.

Atomic Hong–Ou–Mandel experiment

/pdf/minimal-excitation-states-for-electron-quantum-optics-using-2uuw8gzbvx.pdf

Thibaut Jullien

Papers

Minimal-excitation states for electron quantum optics using levitons