Weak Coupling Regime Research Articles

Quantum switching between nonclassical correlated single photons and two-photon bundles in a two-photon Jaynes-Cummings model.

We propose a scheme to realize a two-photon Jaynes-Cummings model for a single atom inside an optical cavity. It is shown that the interplay of a laser detuning and atom (cavity) pump (driven) field gives rise to the strong single photon blockade, two-photon bundles, and photon-induced tunneling. With the cavity driven field, strong photon blockade occurs in the weak coupling regime, and switching between single photon blockade and photon-induced tunneling at two-photon resonance are achievable via increasing the driven strength. By turning on the atom pump field, quantum switching between two-photon bundles and photon-induced tunneling at four-photon resonance are realized. More interestingly, the high-quality quantum switching between single photon blockade, two-photon bundles, and photon-induced tunneling at three-photon resonance is achieved with combining the atom pump and cavity driven fields simultaneously. In contrast to the standard two-level Jaynes-Cummings model, our scheme with generating a two-photon (multi-photon) Jaynes-Cummings model reveals a prominent strategy to engineer a series of special nonclassical quantum states, which may pave the way for investigating basic quantum devices to implement in quantum information processing and quantum networks.

Generation of triple-entanglement in second-order optical topological kagome structure

The combination of topological photonics and quantum optics has promising applications. In previous works, the role of the edge states of conventional topological optical structure in the coherent coupling between atoms has been discussed. Besides the optimistic results, we find that if the coupling coefficient between the atom and element-cavity exceeds the coupling coefficient between cavities in the array, atoms cannot be effectively coupled with each other through the edge states. Here, we generalize to higher-order structures and explore the dynamics of three atoms interacting with the cavity array based on the kagome-type optically coupled-cavity array with topologically protected corner states. Three atoms are theoretically coupled in three corner cavities, and various quantum behaviors can be displayed in different parameters. It is shown that three atoms mediated by the corner state can be coherently coupled with each other, without the constraint of the coupling coefficient, and can induce entanglement, which shows that the high-order topological optical structure has more advantages than the conventional topological optical structure in coherent coupling among qubits. In addition, we also studied atoms mediated by edge states. The results show that the atoms mediated by edge states can be coherently coupled only in the weak coupling regime, and the entanglement effect induced by the edge states is not as good as that of the corner state. It indicates that the corner state is more useful than the edge state in the second-order optical topological structure. Our results can offer references for applying optical topological structure in quantum computing and quantum communication.

Photonic heat transport from weak to strong coupling

Superconducting circuits provide a favorable platform for quantum thermodynamic experiments. An important component for such experiments is a heat valve, i.e., a device which allows one to control the heat power flowing through the system. Here we theoretically study the heat valve based on a superconducting quantum interference device (SQUID) coupled to two heat baths via two resonators. The heat current in such a system can be tuned by magnetic flux. We investigate how the heat current modulation depends on the coupling strength $g$ between the SQUID and the resonators. In the weak coupling regime the heat current modulation grows as ${g}^{2}$, but, surprisingly, at the intermediate coupling it can be strongly suppressed. This effect is linked to the resonant nature of the heat transport at weak coupling, where the heat current dependence on the magnetic flux is a periodic set of narrow peaks. At the intermediate coupling the peaks become broader and overlap, thus reducing the heat modulation. At very strong coupling the heat modulation grows again and finally saturates at a constant value.

Higgs boson origin from a gauge symmetric theory of massive composite particles and massless W± and Z0 bosons at the TeV scale

The ultraviolet completion is the Standard Model (SM) gauge-symmetric four-fermion couplings at the high-energy cutoff. Composite particles appear in the gauge symmetric phase in contrast with SM particles in the spontaneous symmetry-breaking phase. The critical point between the two phases is a weak first-order transition. It relates to an ultraviolet fixed point for an SM gauge symmetric theory of composite particles in the strong coupling regime. The low-energy SM realizes at an infrared fixed point in the weak coupling regime. Composite bosons dissolve into SM particles at the phase transition, and in the top-quark channel, they become a composite SM Higgs boson and three Goldstone bosons. Extrapolation of SM renormalization-group solutions to high energies implies that the gauge-symmetric theory of composite particles has a characteristic scale of about 5.1 TeV. We discuss the phenomenological implications of composite SM Higgs boson in the gauge symmetry-breaking phase, and massive composite bosons coupling to massless W± and Z0 gauge bosons in the gauge symmetric phase.

Experimental study of spinon-phonon coupling in spin-chain cuprates

In this paper, we focus on the thermal transport properties of antiferromagnetic spin chains cuprates. The chain magnetic excitations, the spinons, partake in heat transport at low temperature, but spinon heat transport decays well below room temperature, possibly because of a coupling with phonons. By means of inelastic neutron scattering, we thoroughly study the lattice dynamics of spin chain compounds ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}, {\mathrm{Ca}}_{2}{\mathrm{CuO}}_{3}$, along with double spin-chain compounds ${\mathrm{SrCuO}}_{2}$. We come to the conclusion that there are no obvious anomalies in the phonon dispersions, which suggests a weak spinon-phonon coupling regime.

Enhancement of Valley Polarization in Monolayer WSe2 Coupled with Microsphere‐Cavity‐Array

AbstractMonolayer transition metal dichalcogenides (ML‐TMDCs) possess degenerate levels with antiparallel spins in K and K′ valleys, providing the intrinsic valley polarization, which attracts great interest for potential applications on quantum information technology and on‐chip nanophotonics. Unfortunately, it is difficult to distinguish the degree of valley polarization (DoP) near room temperature due to the intensive phonon‐assisted intervalley scattering and the long‐range electron‐hole exchange interaction in ML‐TMDCs, limiting their practical applications. In this study, a novel design is proposed for great promotion of DoP in ML‐WSe2 with a microsphere cavity array, introducing Purcell effect and nanofocusing effect into the system. The radiative decay rate is dramatically enhanced owing to Purcell effect in microcavity in weak coupling regime, thus locking more polarized excitons in the corresponding valley under certain circularly‐polarized pumping. In addition, the nanofocusing effect contributes to increasing the number of charged excitons by suppressing the bright to dark exciton conversion. The present work achieves a great DoP of ML‐WSe2 with a simple configuration and promises broad applications from valleytronic devices to chiral optics in the future.

Exciton–Photon Interactions in Two-Dimensional Semiconductor Microcavities

Investigation of light–matter interactions in two-dimensional (2D) semiconductors is essential to understand many-body effects and explore their applications in photonics. 2D semiconductor microcavities can be formed by the integration of 2D semiconductor active media with planar Fabry–Perot resonant cavities. The emerging exciton–photon interactions between the strongly bound excitons in 2D semiconductors and the cavity photons allow exploring on the wealthy photonic and polaritonic physics of bosonic quasiparticles in the 2D limit. This Perspective focuses on recent advances in exciton–photon interactions of 2D semiconductor microcavities and their inspiring applications. First, we picture the research scope of 2D semiconductor microcavities, sort out the historical development from conventional semiconductor microcavities to the emerged 2D semiconductor microcavities, and illustrate mostly employed device structures. Second, we classify exciton–photon interactions in 2D semiconductor microcavities according to their coupling strengths. In the weak coupling regime, control of spontaneous emission and photon lasing are discussed together with the Purcell effect. In the strong coupling regime, we summarize experimental observations and theoretical predictions on Rabi splitting, Bose–Einstein condensation, polariton lasing, superfluidity, and superconductivity. Third, four types of leading-edge applications are outlined, including on-chip coherent light sources, microcavity-enhanced single-photon sources, topological photonics, and other nonlinear optics. Finally, we highlight the remaining challenges and future opportunities for fundamental physics of 2D semiconductor microcavities and their technological applications.

Topological effects in two-dimensional quantum emitter systems

In this work we discuss particular effects that take place in systems of quantum emitters coupled to two-dimensional bosonic topological insulators. For a single emitter coupled to the Haldane model, we find a "fragile" quasibound state that makes the emitter dynamics very sensitive to the model's parameters, and gives rise to effective long-range interactions that break time-reversal symmetry. We then discuss one-dimensional arrangements of emitters, emitter line defects, and how the topology of the bath affects the effective polariton models that appear in the weak-coupling regime when the emitters are spectrally tuned to a bandgap. In the Harper-Hofstadter model we link the non-monotonic character of the effective interactions to the Chern numbers of the surrounding energy bands, while in the Haldane model we show that the effective models are either gapless or not depending on the topology of the bath. Last, we discuss how the presence of emitters forming an ordered array, an emitter superlattice, can produce polariton models with non-trivial Chern numbers, and also modify the topology of the photonic states in the bath.

Quantum memory effects in atomic ensembles coupled to photonic cavities

This article explores the dynamics of many-body atomic systems symmetrically coupled to Lorentzian photonic cavity systems. Our study reveals interesting dynamical characteristics, including non-zero steady states, super-radiant decay, enhanced energy transfer, and the ability to modulate oscillations in the atomic system by tuning environmental degrees of freedom. We also analyze a configuration consisting of a three-atom chain embedded in a photonic cavity. Similarly, we find a strong enhancement of the energy transfer rate between the two ends of the chain and identified specific initial conditions that lead to significantly reduced dissipation between the two atoms at the end of the chain. Another configuration of interest consists of two symmetrical detuned reservoirs with respect to the atomic system. In the single atom case, we show that it is possible to enhance the decay rate of the system by modulating its reservoir detuning. In contrast, in the many-atom case, this results in dynamics akin to the on-resonant cavity. Finally, we examine the validity of the rotating wave approximation through a direct comparison against the numerically exact hierarchical equations of motion. We find good agreement in the weak coupling regime, while in the intermediate coupling regime, we identify qualitative similarities, but the rotating wave approximation becomes less reliable. In the moderate coupling regime, we find deviations of the steady states due to the formation of mixed photon-atom states.

Single photon controlled steady state electron transport through a resonance DQD-Cavity system in a strong coupling regime

We perform theoretical calculations to study steady-state electron transport in a double quantum dot, DQD, coupled to a quantized cavity photon field both in resonance and off-resonance regimes considering weak and strong coupling. In the resonant strong coupling regime, photon exchanges between the energy states of the DQD and the cavity are found reflecting multiple Rabi-resonances. The electron occupation of the states and the transport current can be smoothly increased by tuning the cavity-environment coupling strength. Making the system off-resonant, but still in the strong coupling regime, the photon exchange is diminished and the electron occupation of the system and the transport current through it are prominent for high cavity-environment coupling strength. In the weak coupling regime between the DQD and the cavity, the system has almost the same response in the resonant and the off-resonant regimes at high values of the cavity-environment coupling strength.

The gaugino condensate from asymmetric four-torus with twists

We calculate the gaugino condensate in SU(2) super Yang-Mills theory on an asymmetric four-torus \U0001d54b4 with ’t Hooft’s twisted boundary conditions. The \U0001d54b4 asymmetry is controlled by a dimensionless detuning parameter ∆, proportional to L3L4 − L1L2, with Li denoting the \U0001d54b4 periods. We perform our calculations via a path integral on a \U0001d54b4. Its size is taken much smaller than the inverse strong scale Λ and the theory is well inside the semi-classical weak-coupling regime. The instanton background, constructed for ∆ ≪ 1 in [1], has fractional topological charge Q=frac{1}{2} and supports two gaugino zero modes, yielding a non-vanishing bilinear condensate, which we find to be ∆-independent. Further, the theory has a mixed discrete chiral/1-form center anomaly leading to double degeneracy of the energy eigenstates on any size torus with ’t Hooft twists. In particular, there are two vacua, ∣0〉 and ∣1〉, that are exchanged under chiral transformation. Using this information, the ∆-independence of the condensate, and assuming further that the semi-classical theory is continuously connected to the strongly-coupled large-\U0001d54b4 regime, we determine the numerical coefficient of the gaugino condensate: 〈0|trλλ|0〉 = ∣〈1|trλλ|1〉∣ = 32π2Λ3, a result equal to twice the known ℝ4 value. We discuss possible loopholes in the continuity approach that may lead to this discrepancy.

Partial synchronization and community switching in phase-oscillator networks and its analysis based on a bidirectional, weighted chain of three oscillators.

Complex networks often possess communities defined based on network connectivity. When dynamics undergo in a network, one can also consider dynamical communities, i.e., a group of nodes displaying a similar dynamical process. We have investigated both analytically and numerically the development of a dynamical community structure, where the community is referred to as a group of nodes synchronized in frequency, in networks of phase oscillators. We first demonstrate that using a few example networks, the community structure changes when network connectivity or interaction strength is varied. In particular, we found that community switching, i.e., a portion of oscillators change the group to which they synchronize, occurs for a range of parameters. We then propose a three-oscillator model: a bidirectional, weighted chain of three Kuramoto phase oscillators, as a theoretical framework for understanding the community formation and its variation. Our analysis demonstrates that the model shows a variety of partially synchronized patterns: oscillators with similar natural frequencies tend to synchronize for weak coupling, while tightly connected oscillators tend to synchronize for strong coupling. We obtain approximate expressions for the critical coupling strengths by employing a perturbative approach in a weak coupling regime and a geometric approach in strong coupling regimes. Moreover, we elucidate the bifurcation types of transitions between different patterns. Our theory might be useful for understanding the development of partially synchronized patterns in a wider class of complex networks than community structured networks.

Quantum critical behaviors and decoherence of weakly coupled quantum Ising models within an isolated global system.

We discuss the quantum dynamics of an isolated composite system consisting of weakly interacting many-body subsystems. We focus on one of the subsystems, S, and study the dependence of its quantum correlations and decoherence rate on the state of the weakly-coupled complementary part E, which represents the environment. As a theoretical laboratory, we consider a composite system made of two stacked quantum Ising chains, locally and homogeneously weakly coupled. One of the chains is identified with the subsystem S under scrutiny, and the other one with the environment E. We investigate the behavior of S at equilibrium, when the global system is in its ground state, and under out-of-equilibrium conditions, when the global system evolves unitarily after a quench of the coupling between S and E. When S develops quantum critical correlations in the weak-coupling regime, the associated scaling behavior crucially depends on the quantum state of E, whether it is characterized by short-range correlations (analogous to those characterizing disordered phases in closed systems), algebraically decaying correlations (typical of critical systems), or long-range correlations (typical of magnetized ordered phases). In particular, different scaling behaviors, depending on the state of E, are observed for the decoherence of the subsystem S, as demonstrated by the different power-law divergences of the decoherence susceptibility that quantifies the sensitivity of the coherence to the interaction with E.

Stationary stochastic Navier–Stokes on the plane at and above criticality

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Quantum state purity versus average phonon number for characterization of mechanical oscillators in cavity optomechanics

Quantum oscillators in Gaussian states are often characterized by average occupation numbers that refer to a basis of eigenstates of the non-interacting oscillator Hamiltonian. We argue that quantum state purity is a more appropriate characteristic of such states, which can be applied to oscillators of any dimensionality. For a one-dimensional oscillator, the state purity is directly related to a thermal occupation number defined with respect to the number state basis in which the oscillator's quantum state is thermal. Thus, it naturally introduces a more versatile definition of an average occupation number. We study optomechanical sideband cooling of one- and two-dimensional mechanical oscillators in particular, and derive exact analytical expressions for the maximal mechanical state purity achievable in the quantum backaction limit. In the case of a one-dimensional oscillator, we show that the thermal occupation number related to purity can be well approximated by the average phonon number in the weak-coupling regime, but that the two differ in the regime of ultrastrong optomechanical coupling or in cases where the oscillator's resonance frequency is strongly renormalized.

Controlling the strong light-matter coupling in metal-dielectric optical resonators using spin-crossover molecules

We report the observation of (ultra)strong light-matter coupling, in the UV spectral region, between optical modes of a metal/dielectric bilayer nanocavity and the electronic excitations of spin-crossover (SCO) molecules. By thermally switching the SCO molecules between their low-spin and high-spin states, we demonstrate the possibility of fine-tuning the light-molecule hybridization strength, allowing a reversible switching between strong- (with Rabi splitting values of up to 550 meV) and weak-coupling regimes within a single photonic resonator. As a result, we show that spin-crossover molecular compounds constitute a novel, promising class of active nanomaterials in the context of tuneable and reconfigurable polaritonic devices.

Edge states and skin effect dependent electron transport properties of non-Hermitian Su-Schrieffer-Heeger chain

In the non-reciprocal Su-Schrieffer-Heeger (SSH) chain, the hopping amplitude of an electron in the intra-cell depends on its hopping direction. Consequently, the non-Hermitian SSH chain has both non-trivial topological edge state and non-Hermitian skin effect. However, how to detect the non-trivial topological edge states and non-Hermitian skin effect has become an important topic in non-Hermitian physics. In this paper, we study the relationships of the non-trivial topological edge states and the non-Hermitian skin effect of non-Hermitian SSH chain with their electron transport properties in the vicinity of the zero energy. It is demonstrated that when the peak value of the electron transmission probability in the vicinity of the zero energy is much smaller than 1, the non-Hermitian SSH chain has a left-non-Hermitian skin effect; while that in the vicinity of the zero energy is much larger than 1, the non-Hermitian SSH chain has a right-non-Hermitian skin effect. In particular, the skin effect of non-Hermitian SSH chain can be further enhanced in the region of non-trivial topological edge states. Moreover, with the increase of the electron tunneling coupling amplitudes between the non-Hermitian SSH chain and the left and right leads from the weak coupling regime to the strong coupling one, the number of the dips of reflection probability in the vicinity of the zero energy will change from two to zero. Therefore, these results theoretically provide an alternative scheme for detecting non-trivial topological edge states and non-Hermitian skin effect types of the non-Hermitian SSH chain.

Analytical solution of the disordered Tavis-Cummings model and its Fano resonances

$\mathcal{N}$ emitters collectively coupled to a quantised cavity mode are described by the Tavis-Cummings model. We present complete analytical solution of the model in the presence of inhomogeneous couplings and energetic disorder. We derive the exact expressions for the bright and the dark sectors that decouple the disordered model and find that, in the thermodynamic limit, the energetic disorder transforms the bright sector to Fano's model that can be easily solved. We thoroughly explore the effects of energetic disorder assuming a Gaussian distribution of emitter transition energies. We compare the Fano resonances in optical absorption and inelastic electron scattering both in the weak and the strong coupling regimes. We study the evolution of the optical absorption with an increase in the disorder strength and find that it changes the lower and upper polaritons to their broadened resonances that finally transform to a single resonance at the bare cavity photon energy, thus taking the system from the strong to the weak coupling regime. Interestingly, we learn that the Rabi splitting can exist even in the weak coupling regime while the polaritonic peaks in the strong coupling regime can represent almost excitonic states at intermediate disorder strengths. We also calculate the photon Green's function to see the effect of cavity leakage and non-radiative emitter losses and find that the polariton linewidth exhibits a minimum as a function of detuning when the cavity leakage is comparable to the Fano broadening.

Two-dimensional Hubbard model at finite temperature: Weak, strong, and long correlation regimes

We investigate the momentum-resolved spin and charge susceptibilities, as well as the chemical potential and double occupancy in the two-dimensional Hubbard model as functions of doping, temperature and interaction strength. Through these quantities, we identify a weak-coupling regime, a strong-coupling regime with short-range correlations and an intermediate-coupling regime with long magnetic correlation lengths. In the spin channel, we observe an additional crossover from commensurate to incommensurate correlations. In contrast, we find charge correlations to be only short ranged for all studied temperatures, which suggests that the spin and charge responses are decoupled. These findings were obtained by a novel connected determinant diagrammatic Monte Carlo algorithm for the computation of double expansions, which we introduce in this paper. This permits us to obtain numerically exact results at unprecedentedly low temperatures $T\geq 0.067$ for interactions up to $U\leq 8$, while working on arbitrarily large lattices. Our method also allows us to gain physical insights from investigating the analytic structure of perturbative series. We connect to previous work by studying smaller lattice geometries and report substantial finite-size effects.

Algebraic approach to a nonadiabatic coupled Otto cycle

Algebraic methods for solving time-dependent Hamiltonians are used to investigate the performance of quantum thermal machines. We investigate the thermodynamic properties of an engine formed by two coupled qubits, performing an Otto cycle. The thermal interaction occurs with two baths at different temperatures, while work is associated with the interaction with an arbitrary time-dependent magnetic field that varies in intensity and direction. For the coupling, we consider the 1D isotropic Heisenberg model, which allows us to describe the system by means of the irreducible representation of the $\mathrm{su}(2)$ Lie algebra within the triplet subspace. We also inspect different settings of the temperatures and frequencies of the cycle, and investigate the corresponding operation regimes. We describe how these regimes can change with the speed of the protocol. Finally, we compare the weak and strong coupling regimes, and conclude that for the weak case the efficiency is optimal and can surpass the value of the classical Otto cycle. For low temperatures of the heat baths, we show that losses due to quantum friction are reduced in the strong coupling limit.