Rabi Coupling Research Articles

Quantum control of classical motion: piston dynamics in a Rabi-coupled Bose–Einstein condensate

We develop a model and explore the dynamics of a hybrid classical-quantum system consisting of a classical piston and a self-interacting pseudospin 1/2 Bose–Einstein condensate with a time-dependent Rabi coupling. We investigate the mechanical work produced by the piston moving as a result of the quantum pressure of the condensate. The time-dependent Rabi field redistributes the condensate density between the spin components and, as a result, causes a time-dependent pressure acting on the piston. Correspondingly, the motion of the piston produces quantum evolution of the condensate mass- and spin density profiles. We show how by optimised design of the time-dependent direction of the Rabi field based on a quasi-stationary quantum pressure approximation, one can control both the position and velocity of the piston.

Vector gap solitons of spin-orbit-coupled Bose-Einstein condensate in square optical lattice

Vector gap solitons in quasi-two-dimensional Bose–Einstein condensate loaded in a square optical lattice with spin-orbit and Rabi coupling are investigated theoretically. The solitons are obtained by the Newton-Conjugate-Gradient method for various physical parameters. The stability properties of gap solitons are theoretically analyzed by direct nonlinear dynamical evolution. It is found that the existence of gap solitons is sensitive to the spin-orbit and Rabi coupling strength. Smaller Rabi coupling strength is beneficial for the excitation of solitons in the semi-infinite gap. Conversely, larger Rabi coupling strength is beneficial for the soliton excitation in the first gap. The dynamical stability of these gap solitons depends on the spin-orbit and Rabi coupling strength, and the location of the soliton in the bandgap. These findings may contribute to understanding the topological excitations in condensed matter systems.

Shortcut-based generation of transmon-magnon entangled states

Optimized generation of entangled states between solid-state qubits is of fundamental significance to quantum information science and technology. Here, we develop an effective scheme to fast and robustly create the entangled states in a hybrid system containing a superconducting transmon qubit and a spin magnon of yttrium iron garnet. The transmon qubit and the magnon system are coupled via a microwave cavity field. Within the framework of invariant-based shortcuts to adiabaticity, the transmon-magnon entangled states can be rapidly induced by engineering the Rabi couplings between the cavity mode and transmon (magnon) with constant amplitudes. Compared with a non-shortcut counterpart, the shortcut-based operations of interest not only considerably shorten the generation time, but also greatly mitigate the decoherence effects. Furthermore, the fast dynamics is well insensitive to the control deviation of Rabi couplings. Thus our approach could lead to advances in information processing and state engineering with composite systems.

Tomography of a Number-Resolving Detector by Reconstruction of an Atomic Many-Body Quantum State.

The high-fidelity analysis of many-body quantum states of indistinguishable atoms requires the accurate counting of atoms. Here we report the tomographic reconstruction of an atom-number-resolving detector. The tomography is performed with an ultracold rubidium ensemble that is prepared in a coherent spin state by driving a Rabi coupling between the two hyperfine clock levels. The coupling is followed by counting the occupation number in one level. We characterize the fidelity of our detector and show that a negative-valued Wigner function is associated with it. Our results offer an exciting perspective for the high-fidelity reconstruction of entangled states and can be applied for a future demonstration of Heisenberg-limited atom interferometry.

Control of Ion-Photoelectron Entanglement and Coherence Via Rabi Oscillations.

We report a theoretical investigation of photoionization by a pair of coherent, ultrashort, fundamental and second-harmonic extreme-ultraviolet pulses, where the photon energies are selected to yield the same photoelectron energy for ionization of two different subshells. This choice implies that the fundamental energy is equal to the difference in energy of the ionic states and that they are therefore coupled by the fundamental photon. By deriving analytical expressions using the essential-states approach, we show that this Rabi coupling creates coherence between the two photoelectron wave packets, which would otherwise be incoherent. We analyze how the coupling is affected by the parameters, such as relative phase, pulse width, delay between the two pulses, Rabi coupling strength, and photoelectron energy. Our discussion mostly considers Ne 2p and 2s photoionization, but it is generally valid for many other quantum systems where photoionization from two different shells is observed.

Phonon and maxon instability in Bose–Einstein condensates with parity-time symmetric spin–orbit coupling

Parity-time () symmetry has drawn great research interest in non-Hermitian physics. Recently, there is an emerging model of -symmetric spin–orbit coupling (SOC) which could be realized with spin-1/2 atomic Bose–Einstein condensates (BECs) when the two spin components are respectively subjected to momentum-dependent gain and loss (Qin et al 2022 New J. Phys. 24 063025). In this model, inter-atom interaction has no influence on the -symmetric plane waves. Thus, collective excitation playing the role of fingerprint of inter-atom interaction is investigated in this paper. For the phonon excitation, it is shown that the repulsive interaction between atoms in different spin states, which tends to drive the atoms to populate in only one spin component, can break the -symmetry, and leads to a phonon instability. Whereas, for the case of the phonon-maxon-roton collective excitation spectrum, the repulsive interaction between atoms in different spin states can lead to a maxon instability, which does not occur in Hermitian BEC systems (dipolar BECs or BECs with Raman induced SOC). Simulation of the time evolution of the plane wave solution against small noise shows that the maxon instability can result in the formation of a supersolid-like stripe pattern at the initial stage, but the non-Hermitian nature of the system finally destroys the pattern. The phase diagram of stability shows that the Rabi coupling and repulsive interaction between the atoms in the same spin state can stabilize the system.

Asymmetric solitons induced by transition and beating effects

We investigate the dynamics of beating solitons in a two-component Bose–Einstein condensate with tunable Rabi coupling strength. Our results demonstrate that the balance between transition and beating effects permits the emergence of a novel family of asymmetric solitons in the symmetric physical settings. We derive the exact analytical solutions for them, which primarily consist of one bright soliton and one dark soliton element. The analytical solutions provide us with precise balance conditions required for the formation of asymmetric solitons. We also show that the degree of asymmetry can be effectively manipulated by adjusting the background density flow of dark soliton element, initial relative phase between two soliton elements, and their width. Furthermore, we discuss the oscillation behavior of asymmetric solitons in a harmonic potential, and the interaction between them.

Error-insensitive preparation of entangled states between a Josephson qubit and microwave photons via invariant-based shortcuts

Optimal preparation of quantum entanglement is of significance to information processing and state engineering. In this paper, an efficient scheme is proposed to implement error-insensitive generation of entangled states between a Josephson qubit and microwave photons by the technique of invariant-based shortcuts to adiabaticity. A superconducting qubit is dispersively coupled to a quantized cavity field of one-dimensional transmission line resonator. Within a considered subspace spanned by three composite states, we deal with an effective interaction of the composite system with two classical drivings. A maximally entangled qubit-photon state can be deterministically induced using a splitting-like quantum state transfer. To nullify the deviation errors of Rabi coupling and frequency detuning, we optimize the driving parameters and then make the entanglement creation insusceptible to these control imperfections. Thanks to the mitigation of deviation effects, robustness against the residual noisy environment could be obtained numerically. The proposed strategy could provide a promising avenue towards fast and robust information processing with superconducting circuit quantum electrodynamics.

Phase transitions in rotating binary Bose–Einstein condensates with Spin–orbit and Rabi couplings

We present the phase transition analysis of binary Bose–Einstein condensates (BECs) with spin–orbit (SO) and Rabi couplings in quasi-two-dimensional system under rotation. In particular, we investigate the superfluid properties induced by rotation and SO coupling within hydrodynamic theory which can explain the emergence of the domain wall. By calculating the evolution of angular momentum with respect to rotation frequency, we discover first-order phase transitions where the domain wall changes into the wall-vortex complex. On the other hand, the angular momentum changes continuously with SO coupling strength accompanying by the elongation of the domain wall along x-direction. When the Rabi coupling strength exceeds a critical value, we observe a transition from the domain wall to vortex lattice resulting from a significant change of angular momentum. A phase diagram demonstrating the boundary regime in the plane of SO and Rabi coupling strengths is obtained. Our result is not only motivated by the search for novel states of matter inaccessible to existing experiments, but also by the need to identify situations where the phase transitions can be benchmarked systematically.

Dominant andreev reflection through nonlinear radio-frequency transport

It is found that Andreev reflection provides a deterministic teleportation process at an ideal normal-superconductor interface, making it behave like an information mirror. However, it is challenging to control the Andreev reflection in a spatially-separated junction due to the mode mixing at the interface. We theoretically propose the laser-induced Andreev reflection between two-component Fermi superfluid and normal states without mode mixing via spatially-uniform Rabi couplings. By analyzing the tunneling current up to the fourth order, we find that the Andreev current exhibits unconventional non-Ohmic transport at zero temperature. The Andreev current gives the only contribution in the synthetic junction system at zero detunings regardless of the ratio of the chemical potential bias to the superfluid gap, which is in sharp contrast to that in conventional junctions. Our result may give a potential impact on theoretical and experimental study of quantum many-body phenomena, and also pave a way for understanding the black hole information paradox through the Andreev reflection as a quantum-information mirror.

Berezinskii-Kosterlitz-Thouless phase transition with Rabi-coupled bosons

We theoretically investigate the superfluid-normal-state Berezinskii-Kosterlitz-Thouless transition in a binary mixture of bosonic atoms with Rabi coupling under balanced densities. We find the nonmonotonic behavior of the transition temperature with respect to the intercomponent coupling and amplification of the transition temperature for finite values of Rabi coupling, but for small intracomponent couplings. We develop the Nelson-Kosterlitz renormalization-group equations in the two-component Bose mixture and obtain the Nelson-Kosterlitz criterion modified by a fractional parameter, which is responsible for half-integer vortices, and by Rabi coupling. Adopting the renormalization-group approach, we clarify the dependence of the Berezinskii-Kosterlitz-Thouless transition temperature on the Rabi coupling and the intercomponent coupling. Analysis of the first and second sound velocities also reveals the suppression of quasicrossing of the two sound modes with a finite Rabi coupling in the low-temperature regime. Our results for a two-dimensional binary Bose superfluid contribute to the understanding of a broad range of multicomponent quantum systems such as two-dimensional multiband superconductors.

Optical bistability and four-wave mixing response of a quantum dot coupled to an optomechanical photonic crystal nanocavity

We theoretically explore the nonlinear optical response including optical bistability and four-wave mixing (FWM) process in a hybrid cavity quantum electrodynamic (C-QED) system. The hybrid C-QED system consists of a quantum dot (QD) which is coherently driven by two-tone laser fields and embedded in an optomechanical photonic crystal (PhC) nanocavity. Our theoretical analysis demonstrates optical bistability, where the steady-state photon number follows a constant rise in the gain of the switch by controlling a strong pump field driving the QD. A strong coupling between quantised fields and matter (e.g. in optical cavities) is a successful approach for conventionally modifying the dressed state and by altering the field parameters, the modulation of QD dressed state is accomplished. In addition, we list the influence of off-resonant detuning, exciton-nanocavity coupling strength and Rabi coupling strength in our proposed system which generates a controllable four-wave mixing (FWM) signal in the output probe field of the system. The intensity of the FWM is significantly changed when strong coupling and high pump power are present simultaneously and results in the formation of an optomechanically induced transparency (OMIT) window and slow light. The investigation of the proposed system have potential application towards on-chip QD-based nanophotonic devices.

Quantum Rabi dynamics of trapped atoms far in the deep strong coupling regime

The coupling of a two-level system with an electromagnetic field, whose fully quantized version is the quantum Rabi model, is among the central topics of quantum physics. When the coupling strength becomes large enough that the field mode frequency is reached, the deep strong coupling regime is approached, and excitations can be created from the vacuum. Here we demonstrate a periodic variant of the quantum Rabi model in which the two-level system is encoded in the Bloch band structure of cold rubidium atoms in optical potentials. With this method we achieve a Rabi coupling strength of 6.5 times the field mode frequency, which is far in the deep strong coupling regime, and observe a subcycle timescale raise in bosonic field mode excitations. In a measurement recorded in the basis of the coupling term of the quantum Rabi Hamiltonian, a freezing of dynamics is revealed for small frequency splittings of the two-level system, as expected when the coupling term dominates over all other energy scales, and a revival for larger splittings. Our work demonstrates a route to realize quantum-engineering applications in yet unexplored parameter regimes.

Dynamics of quantum solitons in Lee-Huang-Yang spin-orbit-coupled Bose-Einstein condensates

We present the numerical results of the structure and dynamics of the self-bound ground state arising solely because of the presence of beyond mean field quantum fluctuation in spin-orbit coupled binary Bose-Einstein condensates in one dimension. Depending upon spin-obit (SO) and Rabi couplings, we observe that the ground state exhibits either quantum-bright (plane) or quantum-stripe soliton nature. We find an analytical soliton solution for non-zero SO coupling that matches quite well with the numerical results. Further, we investigate the dynamical stability of these solitons by adopting three protocols, such as (i) adding initial velocity to each component, (ii) quenching the SO and Rabi coupling parameters at initial and finite time, and (iii) allowing collision between the two spin-components by giving equal and opposite direction velocity to them. Many interesting dynamical features of the solitons, like, multi-fragmented, repelling, and breathing in time and space-time, are observed. For given Rabi coupling frequency, the breathing frequency of the soliton increases upon the increase in SO coupling, attaining a maximum at the critical SO coupling where the phase transition from the bright to stripe soliton occurs. We observe that the maximum breathing frequency exhibits power law dependence on the Rabi coupling frequency with an exponent $\sim 0.16$.

Rabi Coupled Fermions in the BCS–BEC Crossover

We investigate the three-dimensional BCS–BEC crossover in the presence of a Rabi coupling, which strongly affects several properties of the system, such as the chemical potential, the pairing gap and the superfluid density. We determine the critical interaction strength, below which the system is normal also at zero temperature. Finally, we calculate the effect of the Rabi coupling on the critical temperature of the superfluid-to-normal phase transition by using different theoretical schemes.

Resonant superfluidity in the Rabi-coupled spin-dependent Fermi-Hubbard model

We investigate the ground-state phase diagram of the one-dimensional attractive Fermi-Hubbard model with spin-dependent hoppings and an on-site Rabi coupling using the density matrix renormalization group method. In particular, we show that even in the limit of one component being immobile the pair superfluidity can be resonantly enhanced when the Rabi coupling is on the order of the interaction strength just before the system starts to strongly polarize. We derive an effective spin-1/2 XXZ model in order to understand the ground-state properties in the strong attraction limit.

Spontaneous symmetry breaking induced by interaction in linearly coupled binary Bose–Einstein condensates

The spontaneous symmetry breaking (SSB) induced by a specific component of a linearly coupled binary Bose–Einstein condensate was analyzed. The model is based on linearly coupled Schrödinger equations with cubic nonlinearity and double-well potential acting on only one of the atomic components. By numerical simulations, symmetric and asymmetric ground states were obtained, and an induced asymmetry in the partner field was observed. In this sense, it is adequately demonstrated that the linear coupling mixing the two-field component (Rabi coupling) promotes the (in)balance between the atomic species, as well as the appearance of the Josephson and SSB phases.

Undamped Rabi oscillations due to polaron-emitter hybrid states in a nonlinear photonic waveguide coupled to emitters

The collective dynamics of two non-interacting two-level emitters, which are coupled to a structured wave guide that supports two-photon bound states, is investigated. Tuning the energy of the two emitters such that they are in resonance with the two-photon bound state energy band, we identify parameter regimes where the system displays fractional populations and essentially undamped Rabi oscillations. The Rabi oscillations, which have no analog in the single-emitter dynamics, are attributed to the existence of a collective polaron-like photonic state that is induced by the emitter-photon coupling. The full dynamics is reproduced by a two-state model, in which the photonic polaron interacts with the state $|e,e,\text{vac} \rangle$ (two emitters in their excited state and empty wave guide) through a Rabi coupling frequency that depends on the emitter separation. Our work demonstrates that emitter-photon coupling can lead to an all-to-all momentum space interaction between two-photon bound states and tunable non-Markovian dynamics, opening up a new direction for emitter arrays coupled to a waveguide.

Superfluid transition temperature and fluctuation theory of spin-orbit- and Rabi-coupled fermions with tunable interactions

We obtain the superfluid transition temperature of equal Rashba-Dresselhaus spin-orbit- and Rabi-coupled Fermi superfluids, from the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) regimes in three dimensions for tunable $s$-wave interactions. In the presence of Rabi coupling, we find that spin-orbit coupling enhances (reduces) the critical temperature in the BEC (BCS) limit. For fixed interactions, we show that spin-orbit coupling can convert a first-order (discontinuous) phase transition into a second-order (continuous) phase transition, as a function of Rabi coupling. We derive the Ginzburg-Landau free energy to sixth power in the superfluid order parameter to describe both continuous and discontinuous phase transitions as a function of spin-orbit and Rabi couplings. Lastly, we develop a time-dependent Ginzburg-Landau fluctuation theory for an arbitrary mixture of Rashba and Dresselhaus spin-orbit couplings at any interaction strength.

Rabi Frequency Management of Collapsing Quasi-Two-Dimensional Bose-Einstein Condensates with Pseudospin-1/2

The collapse of quasi-two-dimensional pseudospin-1/2 Bose-Einstein condensate of attracting atoms with intra- and cross-spin interaction is studied in the presence of the Rabi coupling. The condensate dynamics is presented as a function of the self-interaction and Rabi frequency. The evolution of two components of the condensate by using the Gross-Pitaevskii equations is investigated. The initial Gaussian ansatz for two-component wave functions is selected for the better interpretation of the numerical results. The intra-spin-coupling modifies the critical number of atoms causing the collapse while the collapse is observed only in a single pseudospin component. It is demonstrated that for cross-spin-coupling only double spin-components collapse can occur.