Atom-cavity System Research Articles

Two-photon phase gate with linear optical elements and atom–cavity system

We propose a protocol for implementing $$\pi $$ź phase gate of two photons with linear optical elements and an atom---cavity system. The evolution of the atom---cavity system is based on the quantu...

Photon bunching and anti-bunching with two dipole-coupled atoms in an optical cavity**Project supported by the National Natural Science Foundation of China (Grant Nos. 11305037, 11347114, and 11374054) and the Natural Science Foundation of Fujian Province, China (Grant No. 2013J01012).

We investigate the effect of the dipole–dipole interaction (DDI) on the photon statistics with two atoms trapped in an optical cavity driven by a laser field and subjected to cooperative emission. By means of the quantum trajectory analysis and the second-order correlation functions, we show that the photon statistics of the cavity transmission can be flexibly modulated by the DDI while the incoming coherent laser selectively excites the atom–cavity system’s nonlinear Jaynes–Cummings ladder of excited states. Finally, we find that the effect of the cooperatively atomic emission can also be revealed by the numerical simulations and can be explained with a simplified picture. The DDI induced nonlinearity gives rise to highly nonclassical photon emission from the cavity that is significant for quantum information processing and quantum communication.

Pfaffian states in coupled atom-cavity systems

Coupled atom-cavity arrays, such as those described by the Jaynes-Cummings Hubbard model, have the potential to emulate a wide range of condensed matter phenomena. In particular, the strongly correlated states of the fractional quantum Hall effect can be realised. At some filling fractions, the fraction quantum Hall effect has been shown to possess ground states with non-abelian excitations. The most well studied of these states is the Pfaffian state of Moore and Read, which is the groundstate of a Hall Liquid with a 3-body interaction. In this paper we show how an effective 3-body interaction can be generated within the Cavity QED framework, and that a Pfaffian-like groundstate of these systems exists.

Universal quantum gates for photon-atom hybrid systems assisted by bad cavities.

We present two deterministic schemes for constructing a CNOT gate and a Toffoli gate on photon-atom and photon-atom-atom hybrid quantum systems assisted by bad cavities, respectively. They are achieved by cavity-assisted photon scattering and work in the intermediate coupling region with bad cavities, which relaxes the difficulty of their implementation in experiment. Also, bad cavities are feasible for fast quantum operations and reading out information. Compared with previous works, our schemes do not need any auxiliary qubits and measurements. Moreover, the schematic setups for these gates are simple, especially that for our Toffoli gate as only a quarter wave packet is used to interact the photon with each of the atoms every time. These atom-cavity systems can be used as the quantum nodes in long-distance quantum communication as their relatively long coherence time is suitable for multi-time operations between the photon and the system. Our calculations show that the average fidelities and efficiencies of our two universal hybrid quantum gates are high with current experimental technology.

Counterfactual entanglement distribution using quantum dot spins

Counterfactual quantum communication allows distant users to share a message without any physical particles traveling through the quantum channel. Here, we present a counterfactual scheme to realize multiparty entanglement distribution. We first realize bipartite counterfactual entanglement distribution to create photon–electron entanglement by using interaction-free measurements and quantum dots. Then the atom–cavity systems are used to create many photon–photon entanglement pairs, which can connect photon–electron pairs with the help of an entanglement beam splitter. Consequently, the electron spins in distant optical microcavities are entangled by the coherent exchange of single flying photons. We also discuss the implementation issues to show that the presented scheme can be realized successfully in experiment. The numerical analysis about the efficiency of multiparty entanglement distribution indicates that there is a trade-off between the entanglement fidelity and the number of users.

Realising a quantum absorption refrigerator with an atom-cavity system

An autonomous quantum thermal machine comprising a trapped atom or ion placed inside an optical cavity is proposed and analysed. Such a machine can operate as a heat engine whose working medium is the quantised atomic motion or as an absorption refrigerator that cools without any work input. Focusing on the refrigerator mode, we predict that it is possible with state-of-the-art technology to cool a trapped ion almost to its motional ground state using a thermal light source such as sunlight. We nonetheless find that a laser or a similar reference system is necessary to stabilise the cavity frequencies. Furthermore, we establish a direct and heretofore unacknowledged connection between the abstract theory of quantum absorption refrigerators and practical sideband cooling techniques. We also highlight and clarify some assumptions underlying several recent theoretical studies on self-contained quantum engines and refrigerators. Our work indicates that cavity quantum electrodynamics is a promising and versatile experimental platform for the study of autonomous thermal machines in the quantum domain.

Optical-bistability-enabled control of resonant light transmission for an atom-cavity system

The control of light transmission through a Fabry-Perot cavity containing atoms is theoretically investigated, when the cavity mode beam and an intersecting control beam are both close to specific atomic resonances. A four-level atomic system is considered and its interaction with the cavity mode is studied by solving for the time dependent cavity field and atomic state populations. The conditions for optical bistability of the atom-cavity system are obtained in steady state limit. For an ensemble of atoms in the cavity mode, the response of the intra-cavity light intensity to the intersecting resonant beam is understood for stationary atoms (closed system) and non-static atoms (open system). The open system is modelled by adjusting the atomic state populations to represent the exchange of atoms in the cavity mode, with the thermal environment. The solutions to the model are used to qualitatively explain the observed steady state and transient behaviour of the light in the cavity mode, in Sharma et. al. [1]. The control behaviour with three- and two-level atomic systems is also studied, and the rich physics arising out of these systems, for closed and open atomic systems is discussed.

Observation of Fano-Type Interference in a Coupled Cavity-Atom System**Supported by the National Basic Research Program of China under Grant No 2012CB922101, and the National Natural Science Foundation of China under Grant No 11404375. Yi-Fu Zhu was supported by the National Science Foundation of USA under Grant No 1205565.

We present the experimental observation of the Fano-type interference in a coupled cavity-atom system by confining the laser-cooled 85Rb atoms in an optical cavity. The asymmetric Fano profile is obtained through quantum interference in a three-level atomic system coherently coupled to a single mode cavity field. The observed Fano profile can be explained by the interference between the intra-cavity dark state and the polariton state of the coupled cavity-atom system. The possible applications of our observations include all-optical switching, optical sensing and narrow band optical filters.

Temporal behavior of an atom-cavity system in two distinct regimes

The time-dependent treatment of a three-level atom in the V configuration confined in an optical cavity as well as the dynamics of the cavity field in two different regimes, in the presence and absence of the nonlinear mirror, has been discussed. The time-dependent infinite coupled differential equations of motion are solved by the matrix continued fraction method. In deriving the numerical inverse Laplace transform, the quotient difference algorithm and the fast Fourier transform have been applied. The nonlinear effects of the system over the time evolution of the population inversion, the mean photon number and the second-order correlation function for some dimensionless parameters are delineated. Ultimately the reduction of the three-level atom to an effective two-level one under specific conditions and in a weak-driving limit has been performed.

Triangular and honeycomb lattices of cold atoms in optical cavities

We consider a two-dimensional homogeneous ensemble of cold bosonic atoms loaded inside two optical cavities and pumped by a far-detuned external laser field. We examine the conditions for these atoms to self-organize into triangular and honeycomb lattices as a result of superradiance. By collectively scattering the pump photons, the atoms feed the initially empty cavity modes. As a result, the superposition of the pump and cavity fields creates a space-periodic light-shift external potential and atoms self-organize into the potential wells of this optical lattice. Depending on the phase of the cavity fields with respect to the pump laser, these minima can either form a triangular or a hexagonal lattice. By numerically solving the dynamical equations of the coupled atom-cavity system, we have shown that the two stable atomic structures at long times are the triangular lattice and the honeycomb lattice with equally-populated sites. We have also studied how to drive atoms from one lattice structure to another by dynamically changing the phase of the cavity fields with respect to the pump laser.

Low light level all-optical switching in a four-level atom-cavity system

We report on an all-optical switching in a double ∧ four-level atom-cavity system both theoretically and experimentally. In this system, an extra coherence between two ground states is induced by two coupling lasers, thus the loss of the cavity field decreases. Then, we can use one weak field to control another weak field at low light levels. Compared to the three-level atom-cavity system, the power of the switching laser can be much weaker in the four-level atom-cavity system.

Self-ordered stationary states of driven quantum degenerate gases in optical resonators

We study the role of quantum statistics in the self-ordering of ultracold bosons and fermions moving inside an optical resonator with transverse coherent pumping. For few particles we numerically compute the nonequilibrium dynamics of the density matrix towards the self-ordered stationary state of the coupled atom-cavity system. We include quantum fluctuations of the particles and the cavity field. These fluctuations in conjunction with cavity cooling determine the stationary distribution of the particles, which exhibits a transition from a homogeneous to a spatially ordered phase with the appearance of a superradiant scattering peak in the cavity output spectrum. While the ordering threshold is generally lower for bosons, we confirm the recently predicted zero pump strength threshold for superradiant scattering for fermions when the cavity photon momentum coincides with twice the Fermi momentum.

Vacuum-induced dressing movement of the multi-wave mixing in a composite atom–cavity system

We report experimental studies on the vacuum-induced dressing movement of multi-wave mixing in a composite atom–cavity system. Strong linear movement is observed in accordance with the two-photon condition, while slight movement is explained by using doubly dressing theory. On the other hand, we deduce the vacuum-induced moving rule of FWM and SWM, which gives a proper explanation of the phenomena at two cases for scanning cavity detuning and dressing-field detuning, respectively. This investigation may find applications in all-optical switching and optical communication.

Architecture dependence of photon antibunching in cavity quantum electrodynamics

We investigate numerically the architecture dependence of the characteristics of antibunched photons generated in cavity quantum electrodynamic systems. We show that the quality of antibunching [the smallness of the second-order intensity correlation function at zero time ${g}^{(2)}(0)$] and the generation efficiency significantly depend on the configurations: the arrangements of single-mode optical cavities and waveguides. We found that for certain class of architecture, when the Jaynes-Cummings system (the atom-cavity system) couples to two terminated waveguides, there exists a fundamental tradeoff between high transmission and low ${g}^{(2)}(0)$, and is sensitive to dissipation. We further show that optimal antibunching can be achieved in two alternative cavity quantum electrodynamic configurations operating in the dissipatively weak coupling regime such that the two-photon transmission can be two orders of magnitude higher for the same ${g}^{(2)}(0)$.

Vacuum-induced suppression and enhancement of four-wave mixing in an optical cavity

We report on an experimental study of vacuum- induced suppression and enhancement of four-wave mix- ing (FWM) signal in a composite atom-cavity system. By scanning the additional dressing field, the suppression ratio of the FWM signal can reach 90 % compared with 40 % without cavity. We attribute the enhanced suppression and enhancement to the atom-cavity coupling arising from a vacuum-induced Raman process, which amplifies the dressing effect from the additional field. Also, the dress - ing asymmetry of the atom-cavity coupling is discussed and used to estimate the nonlinearity of atomic medium in the cavity. The suppression and enhancement can be inter- preted by a dressed-state picture and agree with theoreti- cal calculations. The investigation may find applications in optical switch and quantum memory controlled by cavity.

Long-distance entanglement distribution using individual atoms in optical cavities

Individual atoms in optical cavities can provide an efficient interface between stationary qubits and flying qubits (photons), which is an essentiel building block for quantum communication. Furthermore, cavity assisted controlled-not (CNOT) gates can be used for swapping entanglement to long distances in a quantum repeater setup. Nonetheless, dissipation introduced by the cavity during the CNOT may increase the experimental difficulty in obtaining long-distance entanglement distribution using these systems. We analyse and compare a number of cavity-based repeater schemes combining various entanglement generation schemes and cavity assisted CNOT gates. We find that a scheme, where high-fidelity entanglement is first generated in a two-photon detection scheme and then swapped to long distances using a recently proposed heralded CZ-gate exhibits superior performance compared to the other schemes. The heralded gate moves the effect of dissipation from the fidelity to the success probability of the gate thereby enabling high-fidelity entanglement swapping. As a result, high-rate entanglement distribution can be achieved over long distances even for low cooperativities of the atom-cavity systems. This high-fidelity repeater is shown to outperform the other cavity-based schemes by up to two orders of magnitude in the rate for realistic parameters and large distances (1000 km).

Long distance atomic teleportation with as good success as desired

Long distance atomic teleportation (LDAT) is of prime importance in long distance quantum communication. Scheme proposed by Bose et al. (1999) in principle enables us to have LDAT using cavity decay. However it gives message state dependent fidelity and success rate. Here, using interaction of entangled coherent states with atom–cavity systems and a two-step measurement, we show how, LDAT can be achieved with unit fidelity and as good success as desired under ideal conditions. The scheme is unique in that, the first measurement predicts success or failure. If success is predicted then second measurement gives perfect teleportation. If failure is predicted the message-qubit remains conserved therefore a second attempt may be started. We found that even in presence of decoherence due to dissipation of energy our scheme gives message state independent success rate and almost perfect teleportation in single attempt with mean fidelity of teleportation equal to 0.9 at long distances. However if first attempt fails, unlike ideal case where message-qubit remains conserved with unit fidelity, in presence of decoherence the message-qubit remains conserved to some degree, therefore mean fidelity of teleportation can be increased beyond 0.9 by repeating the process.

Polariton states of cavity coupled three-level atoms

We study a cavity quantum electrodynamics system in which a cavity mode couples simultaneously with two separate transitions of three-level atoms, and show that three polariton states are created in the coupled cavity-atom system. We report the observation of such polariton states in an experiment with cold Rb atoms confined in a cavity. The experimental results agree with theoretical calculations based on a semiclassical analysis. The three-level cavity-atom system may be used for a variety of fundamental studies and practical applications. (C) 2015 Optical Society of America

Experimental investigation of the statistical distribution of single atoms in cavity quantum electrodynamics

The Hanbury Brown–Twiss experiment for a beam of photons or atoms can be performed using counting experiments. We present the statistical distribution of single 133Cs atoms detected by a high finesse microcavity, which acts as a point-like single-atom counter. The distribution of the arrival times of the atoms and the correlation between the atoms was obtained based on the full counting statistics of the beam emitted from the cavity. The bunching behavior of the thermal atomic beams is clearly observable using this type of atom–cavity system. The correlation between the cesium atoms depends on the temperature of the atom cloud, and the corresponding parameters may be found by fitting an experimentally measured curve using the theory of multimode thermal light.

Generation and stabilization of entanglement in a cascaded atoms–cavity system

We discuss the generation and stabilization of entanglement between two distant atoms in a driven cascaded two-cavity system with homodyne-based feedback control. As the scheme works freely, the two atoms can fall into a steady state close to the Bell state $$\left| \varPhi _{+}\right\rangle $$?+ with an amount of entanglement inverse to its decay rate in a narrow parameter region. The homodyne feedback enlarges this region and makes the system robust against the unexpected imperfections.