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EDP Sciences eBooks

10 Bohr’s legacy in Cavity QED

S. Haroche, J. M. Raimond

Published Nov 4, 2020• 0 citations
Publisher

Abstract

Bohr played a central role in the interpretation of quantum mechanics.He based many discussions of its strange consequences on thought experiments.He imagined moving slit interferometers, able to record which-path information, he played with photon boxes, storing photons for such long times that they could be weighed.The technological progress provided by quantum physics itself now make it possible to realize some of these thought experiments.We describe in this paper a few experiments based on microwave Cavity Quantum Electrodynamics (CQED) techniques.We trap photons in boxes and weigh them with scales at the atomic level.We are able to count the number of photons in the box without absorbing them in an ideal Quantum Non Demolition measurement of the field intensity.We can record which-path information in an atomic interferometer and directly illustrate complementarity.We can also prepare mesoscopic quantum superpositions reminiscent of the famous Schrödinger cat.We can get insight into the decoherence of these states and into ways to protect them, for instance by quantum feedback strategies.

DOI
10.1051/978-2-7598-2265-2.c015

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Bohr played a central role in the interpretation of quantum mechanics.He based many discussions of its strange consequences on thought experiments.He imagined moving slit interferometers, able to record which-path information, he played with photon boxes, storing photons for such long times that they could be weighed.The technological progress provided by quantum physics itself now make it possible to realize some of these thought experiments.We describe in this paper a few experiments based on microwave Cavity Quantum Electrodynamics (CQED) techniques.We trap photons in boxes and weigh them with scales at the atomic level.We are able to count the number of photons in the box without absorbing them in an ideal Quantum Non Demolition measurement of the field intensity.We can record which-path information in an atomic interferometer and directly illustrate complementarity.We can also prepare mesoscopic quantum superpositions reminiscent of the famous Schrödinger cat.We can get insight into the decoherence of these states and into ways to protect them, for instance by quantum feedback strategies.
The founders of quantum physics used to analyse thought experiments, to discuss state superposition, complementarity and entanglement. They imagined manipulating single particles, such as electrons, atoms or photons; they would be surprised to see that experiments with isolated atoms, molecules or photons are now carried on in many laboratories throughout the world. In our Cavity Quantum Electrodynamics (CQED) studies, we stabilize and count photon number states in the cavity. We also prepare and reconstruct photonic superposition states suspended between different 'classical realities', generating a laboratory version of Schrodinger famous 'Cat', which he had imagined to be simultaneously dead and alive. We have also investigated the decoherence process, which explains the transition between the quantum and the classical worlds. CQED physics has recently been extended to artificial 'atoms' made from superconductors, in a new domain of mesoscopic physics called Circuit QED. This development opens the way to applications in quantum information science.
Microwave photons trapped in a superconducting cavity constitute an ideal system to realize some of the thought experiments imagined by the founding fathers of quantum physics. The interaction of these trapped photons with Rydberg atoms crossing the cavity illustrates fundamental aspects of measurement theory. The experiments performed with this "photon box" at the Ecole Normale Supérieure (ENS) belong to the domain of quantum optics called "Cavity Quantum Electrodynamics." We have realized the non-destructive counting of photons, the recording of field quantum jumps, the preparation and reconstruction of "Schrödinger cat" states of radiation and the study of their decoherence, which provides a striking illustration of the transition from the quantum to the classical world. These experiments have also led to the demonstration of basic steps in quantum information processing, including the deterministic entanglement of atoms and the realization of quantum gates using atoms and photons as quantum bits. This lecture starts with an introduction stressing the connection between the ENS photon box and the ion trap experiments of David Wineland, whose accompanying lecture recalls his own contribution to the field of single particle control. I give then a personal account of the early days of Cavity Quantum Electrodynamics before describing the main experiments performed at ENS during the last twenty years and concluding with a discussion comparing our work to other research dealing with the control of single quantum particles.
Microwave photons trapped in a superconducting cavity constitute an ideal system to realize some of the thought experiments imagined by the founding fathers of quantum physics. The interaction of these trapped photons with Rydberg atoms crossing the cavity illustrates fundamental aspects of measurement theory. The experiments performed with this at the Ecole Normale Superieure (ENS) belong to the domain of quantum optics called Quantum Electrodynamics. We have realized the non-destructive counting of photons, the recording of field quantum jumps, the preparation and reconstruction of Schrodinger cat states of radiation and the study of their decoherence, which provides a striking illustration of the transition from the quantum to the classical world. These experiments have also led to the demonstration of basic steps in quantum information processing, including the deterministic entanglement of atoms and the realization of quantum gates using atoms and photons as quantum bits. This lecture starts with an introduction stressing the connection between the ENS photon box and the ion trap experiments of David Wineland, whose accompanying lecture recalls his own contribution to the field of single particle control. I give then a personal account of the early days of Cavity Quantum Electrodynamics before describing the main experiments performed at ENS during the last twenty years and concluding with a discussion comparing our work to other research dealing with the control of single quantum particles.
Microwave photons trapped in a superconducting cavity constitute an ideal system to realize some of the thought experiments imagined by the founding fathers of quantum physics. The interaction of these trapped photons with Rydberg atoms crossing the cavity illustrates fundamental aspects of measurement theory. The experiments performed with this "photon box" at Ecole Normale Supérieure (ENS) belong to the domain of quantum optics called "Cavity Quantum Electrodynamics". We have realized the non-destructive counting of photons, the recording of field quantum jumps, the preparation and reconstruction of "Schrödinger cat" states of radiation and the study of their decoherence, which provides a striking illustration of the transition from the quantum to the classical world. These experiments have also led to the demonstration of basic steps in quantum information processing, including the deterministic entanglement of atoms and the realization of quantum gates using atoms and photons as quantum bits. This lecture starts by an introduction stressing the connection between the ENS photon box and the ion trap experiments of David Wineland, whose accompanying lecture recalls his own contribution to the field of single particle control. I give then a personal account of the early days of Cavity Quantum Electrodynamics before describing the main experiments performed at ENS during the last twenty years and concluding by a discussion comparing our work to other researches dealing with the control of single quantum particles.
Microwave photons trapped in a superconducting cavity constitute an ideal system to realize some of the thought experiments imagined by the founding fathers of quantum physics. The interaction of these trapped photons with Rydberg atoms crossing the cavity illustrates fundamental aspects of measurement theory. The experiments performed with this ``photon box'' at Ecole Normale Sup\'erieure (ENS) belong to the domain of quantum optics called ``cavity quantum electrodynamics.'' We have realized the nondestructive counting of photons, the recording of field quantum jumps, the preparation and reconstruction of ``Schr\"odinger cat'' states of radiation and the study of their decoherence, which provides a striking illustration of the transition from the quantum to the classical world. These experiments have also led to the demonstration of basic steps in quantum information processing, including the deterministic entanglement of atoms and the realization of quantum gates using atoms and photons as quantum bits. This lecture starts by an introduction stressing the connection between the ENS photon box and the ion-trap experiments of David Wineland, whose accompanying lecture recalls his own contribution to the field of single particle control. I give then a personal account of the early days of cavity quantum electrodynamics before describing the main experiments performed at ENS during the last 20 years and concluding by a discussion comparing our work to other researches dealing with the control of single quantum particles.
This chapter describes two cavity quantum electrodynamics (CQED) studies guided by the spirit of thought experiments. The first one is a direct implementation of the recoiling slit interferometer imagined by Bohr. Section 6.1 discusses a closely related experiment, in which a Ramsey interferometer is used instead of the unrealistic Young's one proposed by Bohr. The second experiment, described in Section 6.2, realizes another dream of early quantum mechanics by achieving a non-destructive detection of a single photon. Section 6.3 shows that the quantum non-demolition (QND) measurement of a single photon by a Rydberg atom corresponds to the operation of a control-not quantum gate in which the photon is the control bit and the atom the target. Section 6.4 discusses the extension of the atom-field manipulations to larger fields, based on the dispersive atom-cavity interaction. Section 6.5 describes an alternative statistical method, also based on the atom-field dispersive interaction. The method relies on the preliminary translation of the field in phase space by controlled complex amplitudes.