Dispersive Atom-field Interaction Research Articles

Engineering quantum networks through Bragg diffracted hyperentangled atoms

Hyperentangled states are vitally important for resource economy as they have exponential encoding capabilities per quantum entity. Apart from photonic hyperentanglements, only few proposals exist based on atomic systems. The present article discusses the schematics to generate hyperentangled atomic quantum networks in both internal states and external atomic momenta via cavity QED setup using the Atomic Bragg Diffraction technique. Various graph states forming quantum networks are engineered through hyperentangled correlations encompassing quantized external momenta and internal states of the two-level atoms. First one is a star shaped graph state which is attached to n receivers. The second one is a hybrid of the star and ring graph states, taking three star networks and joining them using dispersive atom-field interactions. Experimental feasibility has also been briefly elaborated under contemporary cavity QED research scenario.

Optical qubit generation via atomic postselection in a Ramsey interferometer

We propose a realizable experimental scheme to prepare a superposition of the vacuum and one-photon states using a typical cavity QED-setup. This is different from previous schemes, where the superposition state of the field is generated by resonant atom-field interaction and the cavity is initially empty. Here, we consider only dispersive atom-field interaction and the initial state of the cavity field is coherent. Then, we determine the parameters to prepare the desired state via atomic postselection. We also include the effect of cavity losses and detection imperfections in our analysis, against which this preparation of the optical qubit in a real Fabry–Pérot superconducting cavity is robust. Additionally, we show that this scheme can be used for the preparation of other photon number Fock state superpositions. In summary, our task is achieved with a high fidelity and a postselection probability within experimental reach.

Doubly tagged delayed-choice tunable quantum eraser: coherence, information and measurement

We present an idea for the doubly tagged delayed-choice tunable quantum eraser in a cavity QED setup, based on fully controlled resonant as well as dispersive atom–field interactions. Two cavity fields, bound initially in the Bell state, are coupled to a three-level atom. Such an atom is initially prepared in the coherent superposition of the lower two levels and is quite capable of exhibiting Ramsey fringes if taken independently. It is shown that the coherence lost due to tagging can not only be retrieved but that the fringe visibility/path distinguishability can also be conditionally tuned in a delayed manner through local manipulation of the entangled cavity fields. The stringent condition here is the retainment of the system’s coherence during successive manipulations of the individual cavity fields. Such a quantum eraser, therefore, prominently highlights the links among all the counterintuitive features of quantum theory including the conception of time, measurement, state vector reduction, coherence and information in an unambiguous manner. The schematics can be straightforwardly extended to a multipartite scenario and employed to explore multi-player quantum games with the payoff being strangely decided through delayed choice setups.

Quantum phase gate through the dispersive atom–field interaction with atoms trapped in optical cavity QED

We propose an experimental feasible scheme for implementing two-qubit quantum phase gate with atoms trapped in an optical cavity. The scheme is based on the dispersive interaction between the optical cavity mode and the three-level atoms in Λ configuration, which has been demonstrated in recent cavity-induced spin squeezing experiment (Leroux et al., 2010) [26]. We also discuss the influence of the cavity decay on the gate fidelity. It is shown that the fidelity of the phase gate is robust to the cavity decay and the high-fidelity quantum phase gate can be implemented with the current experimental technology.

State engineering and information distribution over quantum networks via distant manipulations based on quantized momenta of neutral atoms

We propose experimentally feasible schemes for deterministic state engineering of a superposition as well as a variety of entangled states e.g., Bell, GHZ, Cluster and graph states through exploration of the quantized momenta states of the neutral atoms via distant manipulations. Such decoherence-resistant, distantly contrived momenta states are realized using first order, off-resonant Bragg diffraction of the two-level atoms from quantized cavity fields. The state preparation procedure, along with quantized momenta Bragg interactions, also invokes usual resonant and dispersive atom-field interactions and culminates deterministically into the states having overall good fidelities.

Effective dynamics for two-atom entanglement and quantum information processing by coupled cavity QED systems

We propose a scheme for deterministic generation of entanglement embodied by two \(\Lambda \)-type atoms distributed in two coupled cavities. We study such a system in the dispersive atom-field interactions, where the dynamics of the system operates through the virtual population of both the atomic excited states and the photonic states in the cavities (plus the fiber). We verify the validity of the dynamics, and moreover, study the influences of the decoherence due to the spontaneous emission and photon leakage. We also apply the dynamics for realizing quantum state transfer and quantum phase gates.

Distributed coherent manipulation of qutrits by virtual excitation processes

We propose a scheme for the deterministic coherent manipulation of two atomic qutrits, trapped in separate cavities coupled through a short optical fibre or optical resonator. We study such a system in the regime of dispersive atom–field interactions, where the dynamics of atoms, cavities and fibre operates through virtual population of both the atomic excited states and photonic states in the cavities and fibre. We show that the resulting effective dynamics allows for the creation of robust qutrit entanglement, and thoroughly investigate the influence of imperfections and dissipation, due to atomic spontaneous emission and photon leakage, on the entanglement of the two-qutrit state.

Achieving multipartite entanglement of distant atoms through selective photon emission and absorption processes

We propose two schemes for deterministically generating three-partite Greenberger-Horne-Zeilinger (GHZ) and $W$ states of distant atoms based on the selective photon emission and absorption processes. In our schemes, it is found that the effects of atomic spontaneous decay and photon leakage out of fiber are suppressed effectively due to the presences of dispersive atom-field interaction and strong cavity-fiber coupling intensity. To check the experimental feasibility of our schemes, we also numerically simulated the effects of photon leakage out of the cavities, and the numerical simulation shows that our proposal is good enough to demonstrate the generation of three-partite GHZ and $W$ states of distant atoms with high fidelity. In addition, the present schemes can also be generalized to prepare N-partite $W$ state of distant atoms.

Generation of entanglement between two spatially separated atoms via dispersive atom–field interaction

Based on the dispersive atom–field interaction, a scheme is proposed for deterministically generating entanglement between two Λ-type atoms, which are trapped individually in two spatially separated cavities coupled by an optical fibre. In the present scheme, it is found that the atomic spontaneous decay and photon leakage out of the fibre can be efficiently suppressed via choosing appropriately the frequency detuning of atom–field and the coupling intensity of cavity–fibre, respectively. The influence of photon leakage out of the cavities is also discussed, and the strictly numerical simulation shows that our proposal is good enough to demonstrate the generation of entanglement between two distant atoms with high fidelity.

Dispersive atom-field interaction scheme for three-dimensional entanglement between two spatially separated atoms

We propose a scheme for deterministically generating three-dimensional (3D) entanglement between two distant five-level atoms based on the dispersive atom-field interaction. In our scheme, the two atoms are trapped separately in two spatially separated optical cavities coupled by an optical fiber. To check the experimental feasibility of our scheme, we numerically simulated effects of the atomic spontaneous decay and photon leakage out of the fiber, and the numerical simulation shows that those effects can be suppressed by appropriately choosing the frequency detuning of atom-field and the coupling intensity of cavity fiber, respectively. We also discussed the influence of photon leakage out of the cavities, and the strictly numerical simulation shows our proposal is good enough to demonstrate the generation of 3D entanglement with high fidelity and within the current experimental technology.

Quantum phase gate through a dispersive atom-field interaction

We propose a scheme for realizing a two-qubit quantum phase gate between atoms inside a high-$Q$ optical cavity. In the scheme, the dispersive interaction between atoms and the cavity field induces photon-number Stark shift, which can yield a resonant interaction inside one selected atom-field subspace and leave the others in a highly dispersive regime. Due to the selective atom-cavity interaction, (1) the scheme only requires commonly addressing the atoms inside the optical cavity during the gate operation, and (2) it can be directly used to implement an $N$-qubit conditional phase gate. These properties can significantly reduce the number of operations for implementing quantum computation and quantum algorithms.

Dynamics of dispersive photon-number QND measurements in a micromaser

A numerical analysis of dispersive quantum nondemolition measurement of the photon number of a microwave cavity field is presented. Simulations show that a key property of the dispersive atom-field interaction used in Ramsey interferometry is the extremely high sensitivity of the dynamics of atomic and field states to basic parameters of the system. When a monokinetic atomic beam is sent through a microwave cavity, a qualitative change in the field state can be caused by an uncontrollably small deviation of parameters (such as atom path length through the cavity, atom velocity, cavity mode frequency detuning, or atom-field coupling constants). The resulting cavity field can be either in a Fock state or in a super-Poissonian state (characterized by a large photon-number variance). When the atoms have a random velocity spread, the field is squeezed to a Fock state for arbitrary values of the system’s parameters. However, this makes detection of Ramsey fringes impossible, because the probability of detecting an atom in the upper or lower electronic state becomes a random quantity almost uniformly distributed over the interval between zero and unity, irrespective of the cavity photon number.

Influence of the gravitational field on the quantum-nondemolition measurement of atomic momentum in the dispersive Jaynes–Cummings model

We present a theoretical scheme based on su(2) algebra to investigate the influence of homogeneous gravitational field on the quantum nondemolition measurement of atomic momentum in dispersive Jaynes-Cummings model. In the dispersive Jaynes-Cummings model, when detuning is large and the atomic motion is in a propagating light wave, we consider a two-level atom with quantized cavity-field in the presence of a homogeneous gravitational field. We derive an effective Hamiltonian describing the dispersive atom-field interaction in the presence of gravitational field. We can see gravitational influence both on the momentum filter and momentum distribution. Moreover, gravitational field decreases both tooth spacing of momentum and the width of teeth of momentum.

Sculpturing coherent states to get highly excited Fock states for stationary and travelling fields

An adapted sequential application of dispersive atom–field interaction is used to engineer a field in a highly excited Fock state. A few atoms are used to group into a number component all photons inside a microwave cavity which initially contains a coherent state. The scheme differs from those adding photons to an empty cavity via successive passage of resonant atoms. A conveniently equipped sequence of Mach–Zehnder interferometers is found to be the complementary scheme, extending the result for (optical) travelling waves.

Teleportation of Unknown Superpositions of Collective Atomic Coherent States**The project supported by National Natural Science Foundation of China under Grant No. 60008003

We propose a scheme to teleport an unknown superposition of two atomic coherent states with different phases. Our scheme is based on resonant and dispersive atom-field interaction. Our scheme provides a possibility of teleporting macroscopic superposition states of many atoms first time.

Recurrence formula for generalized optical state truncation by projection synthesis

Several interesting states of the quantized electromagnetic field are studied nowadays. Among them one can cite the number and ~its complementary! phase states @1#, the coherent @2# and squeezed states @3#, superposition states having the previous basic states as components @4#, the interpolating states @5#, which vary between two limiting states, going from one to the other, etc. So, in view of the existence of an abundance of states in quantum optics, it is of interest to know how to prepare these states experimentally, this procedure being known in the literature as ‘‘quantum state engineering’’ @6#. The engineering schemes may consider either the case of stationary waves prepared inside a ~high-Q) cavity @6,7# or the case of traveling waves @8,9#. In the realm of cavity QED phenomena, Vogel et al. @6# employed a resonant atom-field interaction to build up a trapped field in an initially empty cavity, while the proposal in Ref. @10# considers both resonant and dispersive atom-field interactions for the preparation of a general superposition in the empty cavity. An alternative proposal has been presented @11#, named the sculpture of quantum states, where a coherent state is previously injected into the cavity and the Wigner distribution function of the desired state is sculptured, through atom-field interaction, from that representing the initial coherent state, the atoms playing the role of quantum chisels. In recent work by Pegg, Phillips, and Barnett @8#, preparation of an arbitrary running-wave superposition of the vacuum and one-photon states, C0u0&1C1u1&, without using cavities was demonstrated. In this way, a traveling field would be available for further applications, including performing measurements on other field states @12,13#. The scheme in @8# obtains the above mentioned superposition by physical truncation of the photon number superposition making up a coherent state. The proposal requires no additional extension of current experiments and is reasonably insensitive to photodetection efficiency for the fields most likely to be used in practice. Since the scheme works via a truncation of the Hilbert space, it has been called a ‘‘quantum scissors’’ device @8#. As mentioned in @8#, states including superpositions of higher photon numbers might be fabricated by superposing fields prepared as superpositions of zeroand onephoton number states @14#. In this connection, we will present here an alternative way to prepare arbitrary truncated states, i.e., C0u0&1C1u1&1C2u2&1•••1CNuN&, N 51,2,3, . . . . . The method is a direct extension of that in @8#, called the optical truncation of a state by projection synthesis.

Phenomenological-operator approach to dissipation in cavity quantum electrodynamics

We present a phenomenological-operator approach to describe energy dissipation in cavity QED phenomena. This approach, developed for an absolute-zero and a thermal environment, considerably simplifies the introduction of the inevitable errors due to the environmental degrees of freedom when describing processes involving dispersive atom-field interactions. The main result in the present work consists in furnishing a straightforward technique to estimate the fidelity resulting from dispersive atom-field interactions, precluding the necessity of performing the usually extensive ab initio calculations. Furthermore, we expect that the present work can help us account for dissipation in resonant atom-field interactions and even help us achieve a general phenomenological approach to estimate the effects of dissipation in whichever system. To illustrate the universal applicability of the present technique, we calculate the fidelity of a mesoscopic quantum superposition state engineered in a lossy cavity, considering also the excited-state spontaneous decay of the required atom. For the case of a stable atomic excited state, the fidelity computed here is in agreement with a recently announced exact calculation.

A NOTE ON GENERALIZED SUPERPOSITION OF COHERENT STATES

In this comment we present an improvement of our recent proposal [Int. J. Mod. Phys.B12, 1495 (1998)] to generate a generalized superposition of two coherent states. The basic idea here is to use a dispersive atom-field interaction together with a single atom quantum state engineering.

A proposal for testing complementarity with cavity QED techniques

We propose a method to test Bohr complemetarity with cavity QED techniques. It involves an atomic beam passing through two Ramsey zones and a nonresonant cavity initially filled with a strong coherent field. In the absence of the cavity field the probability of finding the atom in a definite state exhibits interference. Due to the dispersive atom-field interaction the paths by which the atom reach the definite state are remarked and thus the interference is destroyed. The associated quantum eraser is also presented.

Proposal for a mesoscopic cavity QED realization of the Greenberger-Horne-Zeilinger state.

A proposal is made for obtaining a mesoscopic realization of the Greenberger-Horne-Zeilinger (GHZ) "all or nothing" contradiction between quantum mechanics and local realism in the context of cavity QED. The proposed experiment involves three separated micromaser cavities, each initially prepared in a coherent state, which are then entangled by dispersive atom-field interactions followed by selective ionization of a single atom that has passed through all three cavities. The cavities are then probed individually by single atoms manipulated to obtain a measurement of the parity of the cavity fields. Using an isomorphism between coherent-state cavity fields and a spin-\textonehalf{} system, we show how this experiment gives rise to violations of local realism in the manner of GHZ.