Summary form only given.Decoherence, i.e., the rapid destruction of the phase relation between two quantum states of a system caused by the entanglement of these two states with two different states of the environment, is the main limiting factor for quantum information processing. For this reason, decoherence control is now becoming a rapidly expanding field of investigation. Decoherence control would be particularly useful in high-Q electromagnetic cavities. In fact, one of the few experimental realizations of a quantum gate is the Caltech's quantum phase gate, demonstrating conditional quantum dynamics between polarized modes in a high-finesse optical cavity. Moreover, the progressive decoherence of a mesoscopic Schrodinger cat state has been observed for the first time in the experiment of Brune et al. (1996) where the linear superposition of two coherent states of a field in a microwave cavity, with classically distinct phases, has been generated and detected. Here we propose a photodetection-mediated feedback scheme based on a very simple idea: whenever a photon leaks out from the cavity and is detected by a photodetector, a feedback loop supplies the cavity mode with another photon, using a single ion trapped in the cavity and appropriately shaped laser pulses.Summary form only given.Decoherence, i.e., the rapid destruction of the phase relation between two quantum states of a system caused by the entanglement of these two states with two different states of the environment, is the main limiting factor for quantum information processing. For this reason, decoherence control is now becoming a rapidly expanding field of investigation. Decoherence control would be particularly useful in high-Q electromagnetic cavities. In fact, one of the few experimental realizations of a quantum gate is the Caltech's quantum phase gate, demonstrating conditional quantum dynamics between polarized modes in a high-finesse optical cavity. Moreover, the progressive decoherence of a mesoscopic Schrodinger cat state has been observed for the first time in the experiment of Brune et al. (1996) where the linear superposition of two coherent states of a field in a microwave cavity, with classically distinct phases, has been generated and detected. Here we propose a photodetection-mediated feedback scheme based on a very simple idea: whenever a photon leaks out from the cavity and is detected by a photodetector, a feedback loop supplies the cavity mode with another photon, using a single ion trapped in the cavity and appropriately shaped laser pulses.

Conference on Lasers and Electro-OpticsNov 27, 20021 citation

Autofeedback Scheme for Schrödinger Cat Preservation in Microwave Cavities

Mauro Fortunato, P. Tombesi, David Vitali +1 more

We propose a feedback scheme for limiting in a significant way the decoherence of a linear superposition of two coherent states with opposite phases in a high-Q microwave cavity. It is based on the injection of appropriately prepared “probe” and “feedback” Rydberg atoms, and greatly improves the model presented in [D. Vitali et al., Phys. Rev. Lett. 79, 2442 (1997)]. In the present scheme, the information transmission between the probe and the feedback atom is directly mediated by a second auxiliary cavity, making the detection efficiency for the probe atom no longer a critical parameter. The decoherence time of the superposition state can be significantly increased using presently available technology.We propose a feedback scheme for limiting in a significant way the decoherence of a linear superposition of two coherent states with opposite phases in a high-Q microwave cavity. It is based on the injection of appropriately prepared “probe” and “feedback” Rydberg atoms, and greatly improves the model presented in [D. Vitali et al., Phys. Rev. Lett. 79, 2442 (1997)]. In the present scheme, the information transmission between the probe and the feedback atom is directly mediated by a second auxiliary cavity, making the detection efficiency for the probe atom no longer a critical parameter. The decoherence time of the superposition state can be significantly increased using presently available technology.

Fortschritte der PhysikMay 1, 20002 citations

Mesoscopic entanglement in cavity QED experiments

M. Brune, J. M. Raimond, S. Haroche

A single circular Rydberg atom is used to prepare in a high quality superconducting cavity a “Schrödinger cat” state, quantum superposition of two coherent fields with different classical phases. A second atom probes the cavity state after a tunable delay. The decay of the quantum correlations between the two atoms reveals the evolution of the field state from an initial quantum superposition into a mere statistical mixture. The time scale of this decay decreases when the separation between the two components increases. This is the first experimental observation of the dynamics of decoherence, lying at the heart of the quantum measurement process.A single circular Rydberg atom is used to prepare in a high quality superconducting cavity a “Schrödinger cat” state, quantum superposition of two coherent fields with different classical phases. A second atom probes the cavity state after a tunable delay. The decay of the quantum correlations between the two atoms reveals the evolution of the field state from an initial quantum superposition into a mere statistical mixture. The time scale of this decay decreases when the separation between the two components increases. This is the first experimental observation of the dynamics of decoherence, lying at the heart of the quantum measurement process.

Mysteries, Puzzles, and Paradoxes in Quantum MechanicsJan 1, 19990 citations

Entanglement, Mesoscopic Superpositions and Decoherence Studies with Atoms and Photons in a Cavity

S. Haroche

Experiments involving Rydberg atoms crossing one at a time a superconducting cavity lead to new tests of quantum theory, such as the generation of entangled pairs of atoms of the Einstein-Podolsky-Rosen type, the preparation of mesoscopic superpositions of field states in the cavity ("Schrödinger cats") and the observation of the progressive decoherence of these states. These experiments illustrate basic aspects of the quantum-classical boundary and demonstrate steps of quantum logic operations essential for quantum information processing.Experiments involving Rydberg atoms crossing one at a time a superconducting cavity lead to new tests of quantum theory, such as the generation of entangled pairs of atoms of the Einstein-Podolsky-Rosen type, the preparation of mesoscopic superpositions of field states in the cavity ("Schrödinger cats") and the observation of the progressive decoherence of these states. These experiments illustrate basic aspects of the quantum-classical boundary and demonstrate steps of quantum logic operations essential for quantum information processing.

Physica ScriptaJan 1, 199830 citations

Observing the Progressive Decoherence of the “Meter” in a Quantum Measurement

M. Brune, E. W. Hagley, J. Dreyer +5 more

A mesoscopic superposition of quantum states involving radiation fields with classically distinct phases was created and its progressive decoherence observed. The experiment involved Rydberg atoms interacting one at a time with a few photon coherent field trapped in a high $Q$ microwave cavity. The mesoscopic superposition was the equivalent of an `` $\mathrm{atom}+\mathrm{measuring}\mathrm{apparatus}$ '' system in which the ``meter'' was pointing simultaneously towards two different directions---a ``Schr\"odinger cat.'' The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement.A mesoscopic superposition of quantum states involving radiation fields with classically distinct phases was created and its progressive decoherence observed. The experiment involved Rydberg atoms interacting one at a time with a few photon coherent field trapped in a high $Q$ microwave cavity. The mesoscopic superposition was the equivalent of an `` $\mathrm{atom}+\mathrm{measuring}\mathrm{apparatus}$'' system in which the ``meter'' was pointing simultaneously towards two different directions---a ``Schr\"odinger cat.'' The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement.

Physical Review LettersDec 9, 19961,651 citations

Experiments with single atoms in a cabity: entanglement, Schrödinger's cats and decoherence

S. Haroche, M. Brune, J. M. Raimond

We perform experiments with Rydberg atoms crossing one at a time a superconducting cavity containing a few microwave photons. The coupling between the atoms and the cavity field is either resonant or dispersive. In the resonant case, quantum Rabi oscillations induced by the vacuum or by a small coherent field are observed. These signals reveals in a striking way the quantization of the field. Quantum Rabi oscillations are also used to prepare Fock states of radiation, to transfer information or to produce entanglement between two successive atoms crossing the cavity. Dispersive atom–field coupling is used to prepare and probe coherent superpositions of field states with different phases (Schrödinger's cat states). The progressive decoherence of these states is observed. These experiments constitute fundamental tests of quantum theory and shed light on the transition from quantum to classical in mesoscopic systems.We perform experiments with Rydberg atoms crossing one at a time a superconducting cavity containing a few microwave photons. The coupling between the atoms and the cavity field is either resonant or dispersive. In the resonant case, quantum Rabi oscillations induced by the vacuum or by a small coherent field are observed. These signals reveals in a striking way the quantization of the field. Quantum Rabi oscillations are also used to prepare Fock states of radiation, to transfer information or to produce entanglement between two successive atoms crossing the cavity. Dispersive atom–field coupling is used to prepare and probe coherent superpositions of field states with different phases (Schrödinger's cat states). The progressive decoherence of these states is observed. These experiments constitute fundamental tests of quantum theory and shed light on the transition from quantum to classical in mesoscopic systems.

Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering SciencesDec 15, 199744 citations

Q

microwave cavity. The mesoscopic superposition was the equivalent of an ``

\mathrm{atom}+\mathrm{measuring}\mathrm{apparatus}

'' system in which the ``meter'' was pointing simultaneously towards two different directions---a ``Schr\"odinger cat.'' The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement.