Two-atom State Research Articles

Effect of Motion of Atoms on the Character of Subradiance of Cold Dilute Ensembles Excited by Resonant Pulsed Radiation

The dynamics of the fluorescence of dilute atomic ensembles excited by resonant pulsed radiation has been analyzed. It has been found that the motion of atoms in ensembles cooled to sub-Doppler temperatures in certain time intervals noticeably enhances subradiance rather than suppressing it. It has also been shown that the time dependence of the spontaneous decay rate can be nonmonotonic. The revealed features occur because the character of the slow decay of the atomic excitation is determined after a certain time mainly by the dynamics of subradiant two-atom quantum states, which are due to the resonant dipole–dipole interaction between atoms.

Effect of spacetime dimensions on quantum entanglement between two uniformly accelerated atoms

We investigate the entanglement dynamics for a quantum system composed of two uniformly accelerated Unruh-DeWitt detectors in different spacetime dimensions. It is found that the range of parameters in which entanglement can be generated is shrunk and the amount of generated entanglement is also decreased with the increasing spacetime dimension, by calculating the evolution of two-atom states using the method for open quantum systems. We study the entanglement evolution between two accelerated atoms for different initial two-atom states, and the influence of corresponding spacetime dimensions for every initial state is discussed. When the spacetime dimensions increase, the change of entanglement becomes slower with time. The influence of spacetime dimensions on the change of entanglement also expands to the case of the massive field. The time delay for entanglement generation is shown in different spacetime dimensions. In particular, entanglement decreases more quickly with the increasing spacetime dimensions compared with that in the case of the massless field. The recently found anti-Unruh effect is discussed, and a novel and interesting phenomenon is found that the Unruh effect in small spacetime dimensions can become the anti-Unruh effect in large spacetime dimensions with the same parameters.

Scattering of edge-state electrons from a two-level atom located near the zigzag edge of a phosphorene nanoribbon

In this study, the general scattering theory based on the Lippmann–Schwinger equation, along with the Green function approach, is applied to investigate the scattering of the mobile electrons from a two-level atom located in a one-dimensional (1D) lattice. We have shown that there is a high similarity between this issue and the scattering of the edge-state electrons from a two-state atom located near the edge of a zigzag nanoribbon of phosphorene. The method is then generalized to the study of this latter case. In both cases, it is assumed that the electron interacts with the electric dipole of the atom and it is shown that the scattering process may lead to the excitation of the atom and change its initial state. For both cases, the elements of the transition matrix, the transmission and reflection coefficients, and the excitation probability of the scattering atom are analytically calculated and discussed.

The polarizability circle

It is shown that the electric dipole polarizability of a small lossless particle lies on a circle in the complex plane, as a consequence of conservation of energy. When expressed in terms of the polarizability volume, this is a parameter-free circle. The center of this polarizability circle is at 3i/4, and its radius is 3/4. The location of the polarizability on the polarizability circle depends on the properties of the particle. For a two-state atom this location is determined by the detuning from resonance. For a material sphere we compare several existing models and check these against the exact solution (Mie scattering), valid for any radius of the particle. We show that when there is dissipation in the particle, the polarizability volume lies asymptotically (large radius) on a reduced polarizability circle. This circle has the same center as the polarizability circle, but it has a smaller radius.

Coherence of resonant light-matter interaction in the strong-coupling limit

We explore the role of quantum fluctuations in the strong-coupling limit of the dissipative Jaynes–Cummings oscillator driven on resonance. For weak excitation, we derive analytical expressions for the spectrum and the intensity correlation function for the photons scattered by the two-state atom coupled to the coherently driven cavity mode. We do so by writing down a birth–death process adding the higher orders in the excitation strength needed to go beyond the pure-state factorization, following the method introduced in Carmichael (2008). Our results for the first and second-order correlation functions are complemented by the numerical investigation of the waiting-time distribution for the photon emissions directed sideways, and the comparison with ordinary resonance fluorescence. To close out our discussion, we increase the driving field amplitude and approach the critical point organizing a second-order dissipative quantum phase transition by depicting the excitation pathways in the intracavity field distribution for a finite system size.

A Novel Practical Quantum Secure Direct Communication Protocol

A novel practical quantum secure direct communication (QSDC) protocol is constructed by utilizing the evolution law of atom via cavity QED. It transmits the atoms from one communicant to the other communicant in a two-step manner. It adopts two-atom product states as the initial quantum resource and the decoy atom technique to guarantee the security against the outside attack. It only needs single-atom measurements, and neither the quantum entanglement swapping nor the unitary operations.

Near-horizon aspects of acceleration radiation by free fall of an atom into a black hole

A two-level atom freely falling towards a Schwarzschild black hole was recently shown to detect radiation in the Boulware vacuum in an insightful paper [M. O. Scully et al., Proc. Natl. Acad. Sci. U.S.A. 115, 8131 (2018)]. The two-state atom acts as a dipole detector and its interaction with the field can be modeled using a quantum optics approach. The relative acceleration between the scalar field and the detector causes the atom to detect the radiation. In this paper, we show that this acceleration radiation is driven by the near-horizon physics of the black hole. This insight reinforces the relevance of near-horizon conformal quantum mechanics for all the physics associated with the thermodynamic properties of the black hole. We additionally highlight the conformal aspects of the radiation that is given by a Planck distribution with the Hawking temperature.

Quantum Secret Sharing via Cavity QED

A novel quantum secret sharing (QSS) protocol is suggested by utilizing the evolution law of atom via cavity quantum electrodynamics (QED) in this paper. It employs two-atom product states rather than entangled states as the initial quantum resource, and adopts single-atom measurements rather than two-atom or multiple-atom joint measurements. It needs neither the unitary operations nor the quantum entanglement swapping. It can resist both the outside attack and the participant attack.

Strong Coupling of Two Individually Controlled Atoms via a Nanophotonic Cavity.

We demonstrate photon-mediated interactions between two individually trapped atoms coupled to a nanophotonic cavity. Specifically, we observe collective enhancement when the atoms are resonant with the cavity and level repulsion when the cavity is coupled to the atoms in the dispersive regime. Our approach makes use of individual control over the internal states of the atoms and their position with respect to the cavity mode, as well as the light shifts to tune atomic transitions individually, allowing us to directly observe the anticrossing of the bright and dark two-atom states. These observations open the door for realizing quantum networks and studying quantum many-body physics based on atom arrays coupled to nanophotonic devices.

Selective Engineering for Preparing Entangled Steady States in Cavity QED Setup

We propose a dissipative scheme to prepare maximally entangled steady states in cavity QED setup, consisting of two two-level atoms interacting with the two counter-propagating whispering-gallery modes (WGMs) of a microtoroidal resonator. Using spontaneous emission and cavity decay as the dissipative quantum dynamical source, we show that the steady state of this system can be steered into a two-atom single state as well as into a two-mode single state. We probed the compound system with weak field coupled to the system via a tapered fiber waveguide, finding it is possible to determine whether the two atoms or two modes are driven to a maximally entangled state. Through the transmission and reflection measurements, without disturbing the atomic state, when the cavity modes are being driven, or without disturbing the cavity field state, when a single atom being driven, one can get the information about the maximal entanglement. We also investigated for both subsystem, two-atom and two-mode states, the entanglement generation and under what conditions one can transfer entanglement from one subsystem to the other. Our scheme can be selectively used to prepare both maximally entangled atomic state as well as maximally entangled cavity-modes state, providing an efficient method for quantum information processing.

Fast and robust generation of singlet state via shortcuts to adiabatic passage

In this paper, we propose a protocol to fast and robustly generate two-atom singlet state by designing the evolution operator with the help of quantum Zeno dynamics. The population of the intermediate state can be controlled by system parameters. The pulses in the protocol can be fitted as Gaussian functions, which are beneficial to the experimental feasibility. Besides, the performance of various decoherence factors, such as spontaneous emission, cavity decay and fiber photon leakage, is discussed by numerical simulations. The results show that the protocol is fast and robust against decoherence and operational imperfection. Finally, the protocol is generalized to realize three-atom singlet state by the same principle.

Evolution of Atomic Entanglement for Different Cavity-Field Statistics in Single-Mode Two-Photon Process

We study the evolution of entanglement for a pair of two-level Rydberg atoms passing one after another into an ideal cavity filled with a single mode radiation field. The atoms interact with the cavity field via two-photon transitions. The initial joint state of two atoms that enter the cavity one after the other is unentangled. Interactions intervened by the single mode cavity photon field brings out the final two-atom mixed entangled type state. We use the well known measure appropriate for the mixed states, i.e. the entanglement of formation to quantify the entanglement. We calculate the entanglement of formation of the joint two-atom state as a function of the Rabi angle, for the Fock state field, coherent field and thermal field respectively inside the cavity. The change in the magnitude of atomic entanglement with cavity photon number has been discussed.

High-fidelity stimulated Raman adiabatic passage analog in optimum three-waveguide system

Abstract We extend Dykhne–Davis–Pechukas (DDP) approximation which describes the nonadiabatic transition probability of a two-state atom system to a three-waveguide coupler. We optimize the traditional Gaussian tunneling rate distributions of optical waveguides based on the parallel adiabatic passage (PLAP) technique which originates from the DDP scheme. The tunneling efficiency can be further improved. We explore the robustness and the tunneling transfer error of the stimulated Raman adiabatic passage (STIRAP) in optical waveguides. The results show that the DDP-optimized scheme can keep the robustness against variations in the shift of propagation constant and hence the width of the channels.

Accelerated and noise-resistant generation of high-fidelity steady-state entanglement with Rydberg atoms

Based on Lyapunov control, a scheme is proposed to accelerate the dissipation dynamics for the generation of high-fidelity entanglement between two Rydberg atoms in the context of cavity quantum electrodynamics (QED). We first use the quantum Zeno dynamics and Rydberg antiblockade to find a unique steady state (two-atom singlet state) for the system. Then, applying additional coherent control (ACC) fields to improve the evolution speed of the dissipative system. The ACC fields are designed based on the target state and they vanish gradually along with increasing of the fidelity thus the system is guaranteed to be finally stable. Besides, the current accelerated scheme is checked to be robust against systematic and amplitude-noise errors.

Adiabatic passage of radio-frequency-assisted Förster resonances in Rydberg atoms for two-qubit gates and the generation of Bell states

High-fidelity entangled Bell states are of great interest in quantum physics. Entanglement of ultracold neutral atoms in two spatially separated optical dipole traps is promising for implementation of quantum computing and quantum simulation and for investigation of Bell states of material objects. We propose a new method to entangle two atoms via long-range Rydberg-Rydberg interaction. Alternatively to previous approaches, based on Rydberg blockade, we consider radiofrequency-assisted Stark-tuned F\"{o}rster resonances in Rb Rydberg atoms. To reduce the sensitivity of the fidelity of Bell states to the fluctuations of interatomic distance, we propose to use the double adiabatic passage across the radiofrequency-assisted Stark-tuned F\"{o}rster resonances, which results in a deterministic phase shift of the two-atom state.

Directly measuring the concurrence of two-atom state via detecting coherent lights

Concurrence is an important parameter for quantifying quantum entanglement, but usually the state tomography must be determined before quantification. In this paper we propose a scheme, based on cavity-assisted atom–light interaction, to measure the concurrence of two-atom pure states and the Collins–Gisin state directly, without tomography. The concurrence of atomic states is encoded in the output coherent optical beams after interacting with cavities and the atoms therein, so the results of detection applied to the output coherent optical beams provide the concurrence data of the atomic states. This scheme provides an alternative method for directly measuring atomic entanglement by detecting coherent light, rather than measuring the atomic systems, which thus greatly simplifies the realization complexity of the direct measurement of atomic entanglement. In addition, as the cavity-assisted atom–light interaction used here is robust and scalable in realistic applications, the current scheme may be realized in the near future.

High-fidelity Rydberg quantum gate via a two-atom dark state

We propose a two-qubit gate for neutral atoms in which one of the logical state components adiabatically follows a two-atom dark state formed by the laser coupling to a Rydberg state and a strong, resonant dipole-dipole exchange interaction between two Rydberg excited atoms. Our gate exhibits optimal scaling of the intrinsic error probability $E \propto (B\tau)^{-1}$ with the interatomic interaction strength $B$ and the Rydberg state lifetime $\tau$. Moreover, the gate is resilient to variations in the interaction strength, and even for finite probability of double Rydberg excitation, the gate does not excite atomic motion and experiences no decoherence due to internal-translational entanglement.

Unraveling mirror properties in time-delayed quantum feedback scenarios

We derive in the Heisenberg picture a widely used phenomenological coupling element to treat feedback effects in quantum optical platforms. Our derivation is based on a microscopic Hamiltonian, which describes the mirror-emitter dynamics based on a dielectric, a mediating fully quantized electromagnetic field and a single two-level system in front of the dielectric. The dielectric is modelled as a system of identical two-state atoms. The Heisenberg equation yields a system of describing differential operator equations, which we solve in the Weisskopf–Wigner limit. Due to a finite round-trip time between emitter and dielectric, we yield delay differential operator equations. Our derivation motivates and justifies the typical phenomenologicalassumed coupling element and allows, furthermore, a generalization to a variety of mirrors, such as dissipative mirrors or mirrors with gain dynamics.

Quantum Zeno Suppression of Intramolecular Forces.

We show that Born-Oppenheimer surfaces can intrinsically decohere, implying loss of coherence among constituent electronic basis states. We consider the example of interatomic forces due to resonant dipole-dipole interactions within a dimer of highly excited Rydberg atoms, embedded in an ultracold gas. These forces rely on a coherent superposition of two-atom electronic states, which is destroyed by continuous monitoring of the dimer state through a detection scheme utilizing the background gas atoms. We show that this intrinsic decoherence of the molecular energy surface can gradually deteriorate a repulsive dimer state, causing a mixing of attractive and repulsive character. For sufficiently strong decoherence, a Zeno-like effect causes a complete cessation of interatomic forces. We finally show how short decohering pulses can controllably redistribute population between the different molecular energy surfaces.

Cavity Carving of Atomic Bell States.

We demonstrate entanglement generation of two neutral atoms trapped inside an optical cavity. Entanglement is created from initially separable two-atom states through carving with weak photon pulses reflected from the cavity. A polarization rotation of the photons heralds the entanglement. We show the successful implementation of two different protocols and the generation of all four Bell states with a maximum fidelity of (90±2)%. The protocol works for any distance between cavity-coupled atoms, and no individual addressing is required. Our result constitutes an important step towards applications in quantum networks, e.g., for entanglement swapping in a quantum repeater.