In the first part of this thesis,we examine the preparation of a single- mode radiation field in arbitrary pure quantum states via resonant inter- action with a sequence of two-level atoms.The preparation is achieved by choosing an appropriate (in general entangled)initial state of the atomic sequence,and does neither require a final state measurement of the atoms,nor a control of the atom-field interaction.Furthermore,the method is applicable also when starting from mixed initial field states. We show how to determine the optimal initial atomic state which pre- pares the desired field state with the maximum fidelity,and prove the feasibility of our state preparation method by numerical calculations. In the second part,we demonstrate the noise-induced control of quantum jumps in a fundamental open quantum system.Here,in addition to the subsequent interaction with a flux of two-level atoms,the quantized field is also coupled to a thermal environment.Under certain experimental conditions,the photon field exhibits a bistable behavior,with quantum jumps between two metastable states.In the presence of a small peri- odic signal (i.e.,a modulation of the initial state of the two-level atoms crossing the single-mode resonator),the best synchronization of these quantum jumps with the signal is achieved at an optimal,nonvanishing temperature of the environment.This stochastic resonance e ffect can be observed in di fferent components of the atomic Bloch vector on exit from the cavity. The third part treats a speci fic problem concerning the characterization of entanglement between two quantum mechanical two-level systems. We consider the optimal decomposition of a two-qubit state into an en- tangled and a separable part,with maximal weight of the latter,and derive necessary and su fficient conditions for the optimality of the de- composition.
In the first part of this thesis,we examine the preparation of a single- mode radiation field in arbitrary pure quantum states via resonant inter- action with a sequence of two-level atoms.The preparation is achieved by choosing an appropriate (in general entangled)initial state of the atomic sequence,and does neither require a final state measurement of the atoms,nor a control of the atom-field interaction.Furthermore,the method is applicable also when starting from mixed initial field states. We show how to determine the optimal initial atomic state which pre- pares the desired field state with the maximum fidelity,and prove the feasibility of our state preparation method by numerical calculations. In the second part,we demonstrate the noise-induced control of quantum jumps in a fundamental open quantum system.Here,in addition to the subsequent interaction with a flux of two-level atoms,the quantized field is also coupled to a thermal environment.Under certain experimental conditions,the photon field exhibits a bistable behavior,with quantum jumps between two metastable states.In the presence of a small peri- odic signal (i.e.,a modulation of the initial state of the two-level atoms crossing the single-mode resonator),the best synchronization of these quantum jumps with the signal is achieved at an optimal,nonvanishing temperature of the environment.This stochastic resonance e ffect can be observed in di fferent components of the atomic Bloch vector on exit from the cavity. The third part treats a speci fic problem concerning the characterization of entanglement between two quantum mechanical two-level systems. We consider the optimal decomposition of a two-qubit state into an en- tangled and a separable part,with maximal weight of the latter,and derive necessary and su fficient conditions for the optimality of the de- composition.
The dynamical evolution of a quantum system composed of two coupled cavities, each containing a two-level atom and a single-mode thermal field, is investigated under different conditions. The entanglement between the two atoms is controlled by the hopping strength and the detuning between the atomic transition and the cavities. We find that when the atomic transition is far off-resonant with both the eigenmodes of the coupled-cavity system, the maximally entangled state for the two atoms can be generated with the initial state in which one atom is in the ground state and the other is in the excited state. When both the two atoms are initially in the excited state, the entanglement exhibits periodical sudden birth and death. By choosing appropriate parameter values, the initial maximal entanglement of the two atoms can be frozen. The relation between the concurrence and the cooperative parameter is calculated.
It has been recently shown [1] that, under suitable conditions, the interaction between a three-level atom and the quantized field of a bimodal microcavity may be effectively reduced to the two-photon coupling between a single two-level atom and the electromagnetic field in the resonator. The corresponding Hamiltonian model is characterized by intensity-dependent Stark shifts due to the presence of an intermediate off-resonance atomic level. Assume the cavity field prepared at in an arbitrary state and let an initially excited atom cross the cavity with an appropriately selected velocity. It is possible to show that, exploiting a single conditional measurement of the atomic state immediately after the atom leaves the resonator, the bimodal field is projected into a new state characterized by the same photon number distribution present at More in detail, the experimental scheme we propose makes it possible manipulating only the phases of the initial probability amplitudes of finding a well defined population in the cavity. We prove that the state into which the cavity collapses after a successful atomic measurement is unitarily related to the initial field state and the explicit form of the accomplishing operator is constructed. We wish to remark that, beside its inherent theoretical interest, this simple experimental scheme might also be of relevance in the socalled field of quantum communication [2], where implementing unitary modifications of a quantum state signal represents an issue of topical and central importance. More in general, our proposal provides a systematic procedure for controlling selected properties of a quantum state. We in fact demonstrate that our experimental scheme may be reliably exploited to generate orthogonal states to a prefixed one or to control the squeezing direction.
The object of this PhD Thesis is light and matter quantum interaction including studies on cavity and circuit quantum electrodynamics (CQED and cQED), quantum information theory and entanglement dynamics in open systems. In recent years both theoretical and experimental efforts have been devoted to study problems regarding the transfer of quantum correlations, quantum memories, entanglement protection against decoherence and new regimes accessible in cQED. One of the fundamental model underlying these topics is the well known Jaynes-Cummings (JC) one for light-matter interaction, introduced in quantum optics already in 1963. This paradigmatic model describes the coherent exchange of a single excitation between a qubit (a two-level system) and a mode of a quantized harmonic oscillator. The JC model describes a lot of purely quantum effects such as Rabi oscillations and generation of entangled quantum states, which were confirmed by several sophisticated experiments in CQED. This model has been then applied in many other frameworks. Analytical results can be derived only under the so-called rotating wave approximation (RWA), which allows to neglect the energy contributions coming from the counter-rotating terms, which describe the simultaneous excitation (or de-excitation) of the qubit and the field mode. In this Thesis a first interesting investigation, based on the JC model, is related to the realization of quantum memories. The main aspect is the transfer of generic quantum entangled states of a propagating radiation to some qubits, such as two-level atoms placed in separated optical cavities. Generalizing previous results for bipartite entangled systems, I investigated the more complex case of tripartite systems, where the very quantification of entanglement is still an open problem. This issue is broadened to open systems in contact with a common environment which induces decoherence effects unavoidable in realistic implementations. The main result of this analysis is that it is possible to map in an optimal way a quantum state from radiation to qubits in the unitary dynamics, and even in the presence of a dissipative environment a significative amount of entanglement can be transferred. This study has been carried on with either theoretical calculations or numerical simulations performed adopting the powerful Monte Carlo wave function method. A second important problem for quantum memories faced in this PhD Thesis, regards the protection of qubits entanglement for long enough times in order to implement quantum information tasks. A possible way to obtain this goal is to add an external coherent field driving two or more two-level atoms interacting with a common mode of a cavity electromagnetic field. In fact it is possible to demonstrate that, under certain conditions, an effective Hamiltonian of the whole system involves, together with the familiar JC terms, also the counter-rotating ones. The main feature that comes out from this analysis is the possibility to freeze, during system dynamics, peculiar quantum entangled states of the atomic qubits in so called decoherence-free subspaces. This important property allows to store the entanglement in matter qubits, isolating the quantum state from environment induced decoherence. Though the system includes dissipation, analytical results have been obtained for multipartite entanglement and decoherence by solving the master equation through a projection on a suitable basis and using the method of characteristics. In particular it is possible to monitor the rate of decoherence via measurements of joint atomic probabilities, shading light on the fascinating border between the quantum and the classical world. A last topic investigated in this Thesis concerns the recently developed and quite promising implementation of the JC model well outside CQED, that is cQED. Here superconductive circuits, which play the role of artificial atoms, interact on-chip with transmission line resonators, instead of the usual electromagnetic cavities. These systems have the advantage of being quite easily engineered exploiting the most advanced available technologies. Moreover they allow a great flexibility in handling the relevant parameters, since the inter-system coupling is here of capacitive or inductive nature, instead of the usual electric dipole interaction in CQED. In this way a strong and ultrastrong coupling regime have become available, giving a direct access to the physics beyond the JC model and its possible applications. I first performed a detailed characterization of system dynamics in order to test the main approximations usually introduced to describe superconductive circuits as qubits. Then I described, both analytically and numerically, the dissipative dynamics of a superconductive qubit coupled to a resonator mode in the deep strong regime.
The generation of entanglement in atomic systems plays a central topic in the fields of quantum information storage and processing. Moreover, a special category of entangled states of multi-atom ensembles, spin squeezed states, have been proven to lead to considerable improvement in the sensitivity of precision measurements compared to systems involving uncorrelated atoms. A treatment of entanglement in open systems is, however, incomplete without a precise description of the process of decoherence which necessarily accompanies it. The theory of entanglement and decoherence are the two main topics of this thesis. Methods are described for the generation of strong correlations in large atomic ensembles using either cavity quantum electrodynamics or measurement outcome conditioned quantum dynamics. Moreover, the description of loss of entanglement resulting from the coupling to a noise reservoir (electromagnetic vacuum) is explored. A spin squeezing parameter is used throughout this thesis as both a measure of entanglement strength and as an indication of the sensitivity improvement above the so-called standard quantum limit (sensitivity obtained with uncorrelated particles) in metrology. The first scheme considered consists of a single mode cavity field interacting with a collection of atoms for which spin squeezing is produced in both resonant and off-resonant regimes. In the resonant case, transfer of squeezing from a field state to the atoms is analyzed, while in the off-resonant regime squeezing is produced via an effective nonlinear interaction (one-axis twisting Hamiltonian). A second, more experimentally realistic case, is one involving the interaction of free space atoms with laser pulses; a projective measurement of a source field originating from atomic fluctuations provides a means of preparing atomic collective states such as spin squeezed and Schrodinger cat states. A new unravelling is proposed, that employs the detection of photon number in a single output channel and is capable of producing mesoscopic Schrodinger cat states in a single step. As a first step to understanding the role of cooperative decoherence, of importance in the case of dense pencil-shaped atomic ensembles, the collective spin decoherence of a two multilevel atom system is derived. This calculation is also relevant to entanglement loss for two qubits manipulated using with reading/writing pulses. Finally, a scheme is proposed in which arbitrarily strong entanglement can be produced in a four-wave mixing setup, where the preparation of the atoms in a dark state limits the decoherence to negligible values.
We provide a detailed description of the response of an experimentally accessible, open, driven quantum system in its stationary state to a weak periodic signal. The system, a single mode of the quantized radiation field in coherent interaction with a sequence of two-level atoms and coupled to an environment of temperature T , is shown to exhibit stochastic resonance on output, under changes of T . Furthermore, at vanishing temperature, the transition rates between the metastable states of the radiation field remain finite and are set by the quantum noise level at . Consequently, signal enhancement and synchronization can be achieved even in this very quantum limit, for a properly tuned signal frequency.
After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essential role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechanics in situations of increasing complexity. Entanglement resulted either from a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not resonant. With two entangled particles (two atoms or one atom and a photon), we have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlated state. This process could be viewed as an elementary measurement, one particle being a ‘‘meter’’ measuring the other. We have performed a ‘‘quantum nondemolition’’ measurement of a single photon, which we detected repeatedly without destroying it. Entanglement is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states (‘‘Schrodinger cat’’) gets rapidly entangled with its environment, losing its quantum coherence. We have prepared a Schrodinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantum/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state systems (qubits) instead of classical bits of information, one can perform logical operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a cavity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectives opened by these experiments for further fundamental studies.