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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Quantum jumps of light recording the birth and death of a photon in a cavity

Stefan Kuhr, S. Gleyzes, Christine Guerlin, J. Bernu, S. Deléglise, Ulrich B. Hoff, M. Brune, J. M. Raimond, S. Haroche

Published Jun 1, 2007• 52 citations
Publisher

Abstract

Summary form only given. A microscopic system under continuous observation exhibits at random times sudden jumps between its states. Quantum jumps of trapped massive particles (electrons, ions or molecules) have already been observed, which is not the case of the jumps of light quanta. Here we report on the first observation of photon number quantum jumps. Microwave photons are stored in a superconducting cavity for times in the second range. They are repeatedly probed by a stream of non-absorbing atoms. An atom interferometer measures the atomic dipole phase shift induced by the non-resonant cavity field, so that the final atom state reveals directly the presence of a single photon in the cavity. Sequences of hundreds of atoms highly correlated in the same state, are interrupted by sudden state-switchings. These telegraphic signals record, for the first time, the birth, life and death of individual photons. Applying a similar QND procedure to mesoscopic fields with tens of photons opens new perspectives for the exploration of the quantum to classical boundary.

DOI
10.1109/cleoe-iqec.2007.4386894

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Europhysics news•Jan 1, 1993•0 citations
Summary form only given. A microscopic system under continuous observation exhibits at random times sudden jumps between its states. Quantum jumps of trapped massive particles (electrons, ions or molecules) have already been observed, which is not the case of the jumps of light quanta. Here we report on the first observation of photon number quantum jumps. Microwave photons are stored in a superconducting cavity for times in the second range. They are repeatedly probed by a stream of non-absorbing atoms. An atom interferometer measures the atomic dipole phase shift induced by the non-resonant cavity field, so that the final atom state reveals directly the presence of a single photon in the cavity. Sequences of hundreds of atoms highly correlated in the same state, are interrupted by sudden state-switchings. These telegraphic signals record, for the first time, the birth, life and death of individual photons. Applying a similar QND procedure to mesoscopic fields with tens of photons opens new perspectives for the exploration of the quantum to classical boundary.
A microscopic system under continuous observation exhibits at random times sudden jumps between its states. The detection of this essential quantum feature requires a quantum non-demolition (QND) measurement1, 2, 3 repeated many times during the system evolution. Quantum jumps of trapped massive particles (electrons, ions or molecules4,5,6,7,8) have been observed, which is not the case of the jumps of light quanta. Usual photodetectors absorb light and are thus unable to detect the same photon twice. They must be replaced by a transparent counter 'seeing' photons without destroying them3. Moreover, the light has to be stored over a duration much longer than the QND detection time. We have fulfilled these challenging conditions and observed photon number quantum jumps. Microwave photons are stored in a superconducting cavity for times in the second range. They are repeatedly probed by a stream of non-absorbing atoms. An atom interferometer measures the atomic dipole phase shift induced by the non-resonant cavity field, so that the final atom state reveals directly the presence of a single photon in the cavity. Sequences of hundreds of atoms highly correlated in the same state, are interrupted by sudden state-switchings. These telegraphic signals record, for the first time, the birth, life and death of individual photons. Applying a similar QND procedure to mesoscopic fields with tens of photons opens new perspectives for the exploration of the quantum to classical boundary9, 10.
The photon number parity is a binary observable playing an important role in the description of the non-classical features of light. In microwave experiments, this observable can be measured by using a single Rydberg atom evolving in a two dimension space and interacting dispersively with a field stored in one or two cavities. The procedure, based on atomic interferometry, maps the field parity (+ 1 or − 1) onto the states of the two-level atom. By performing repeated measurements of the field parity, it is possible to observe field quantum jumps, to reconstruct the complete Wigner function of an arbitrary field and to study how it evolves due to decoherence.
Starting with early experiments about lifetimes of excited atoms near metallic boundaries, I review the evolution of Cavity Quantum Electrodynamics in the microwave and in the optical domains. The coherent coupling of single atoms with photons in high-Q cavities has led to the demonstration of fundamental quantum effects in light-matter interaction processes and to the generation of micromasers and microlasers emitting radiation with non-classical properties. It has made possible the development of single photon light sources and efficient detectors of single atoms as well as the implementation of quantum information and communication procedures in which atoms and photons play the roles of quantum bits. Mesoscopic superpositions of field states have been generated by entangling single atoms to fields made of many photons and the decoherence of these states has been studied, shedding new light on the quantum to classical boundary. Cavity Quantum Electrodynamics has also led recently to the quantum non-demolition counting of photons in a cavity, opening new perspectives for studying non-classical states of radiation. These atomic physics experiments are now extended - with promising applications in perspective - to the study of solid-state devices with microcavities built on substrates and coupled to artificial atoms such as quantum dots or superconducting junctions.
The experimental observation of quantum jumps is an example of single open quantum systems that, when monitored, evolve in terms of stochastic trajectories conditioned on measurements results. Here we present a proposal that allows the experimental observation of a much larger class of quantum trajectories in cavity QED systems. In the microwave regime, three-level atoms probe the cavity field while in the optical case the field probes atomic states. In the first situation the cavity field evolves stochastically between highly non-classical states while the second case allows for the monitoring of engineered thermal baths that are crucial for recent proposals for probing entanglement decay and also for entanglement protection.
Experiments involving so-called elastic collisions between an atom and a cavity should allow the non-destructive detection of very small numbers of photons and the direct measurement of cavity field quantum jumps.An atom and a small cavity with highly reflecting walls can couple and correlate strongly to each other, resulting in pheno mena which may lead to the development of new kinds of field sensors to detect photons without destroying them; the same pheno mena also illustrate basic concepts of quan tum mechanics.The study of these effects, at first theoretical and now entering an acti ve experimental phase, forms a part of cav ity quantum electrodynamics [1], a growing field spanning atomic physics and quantum optics with many potential applications. The Atom-Cavity "Molecule"Consider an atom in a cavity whose size matches exactly the wavelength of a photon the atom can emit in a transition between two of its energy levels.If the atom Is initially excited and the resonator empty, a photon will be radiated within a short period of time in the cavity.Since this photon is trapped in the atom's vicinity by the reflecting cavity walls, it will be absorbed at a later time, the atom jumping back to the upper level, com mencing an ideally endless cycle of photon emission and absorption processes.Re markably, the exchanged photon also me diates a small force between the atom and the cavity walls [2], in the same way as charged particles exert a force on each other by exchanging a photon, or atoms attract each other in a molecule by trading electrons.The molecule analogy can help us under stand some basic features of the atom-cavi ty system.Two atoms with chemical affinity exchange an electron at a rate which de pends on their distance apart.As a result, they experience a force, which either ties them together in a molecular bound state, or repels them from each other in an "antibind ing" state.The same physics goes on in the atom-cavity system, albeit at a quite diffe rent energy scale.This system has two energy eigenstates, namely a binding one in which the excitation is shared symmetrically between the atom and the cavity (sum of the excited atom with no photon and the deexcited atom with one photon), and an anti-