Two-level Quantum Emitters Research Articles

Jaynes-Cummings interaction with a traveling light pulse

The Jaynes-Cummings model provides a simple and accurate description of the interaction between a two-level quantum emitter and a single mode of quantum radiation. Due to the multimode continuum of eigenmodes in free space and in waveguides, the Jaynes-Cummings model should not be expected to properly describe the interaction between an emitter and a traveling pulse of quantum radiation. In this article, we review a cascaded quantum system approach that accurately describes the interaction of a quantum system with an incident quantum pulse of radiation. When applied to the two-level system, different iterations of this approach lead to master equations with different Jaynes-Cummings-like Hamiltonians and damping terms.

Band gap engineering and controlling transport properties of single photons in periodic and disordered Jaynes–Cummings arrays

We theoretically study the single-photon transport properties in periodic and position-disordered Jaynes–Cummings (or JC) arrays of waveguide-coupled microtoroidal ring resonators, each interacting with a single two-level quantum emitter. Employing the real-space formalism of quantum optics, we focus on various parameter regimes of cavity quantum electrodynamics (cQED) to gain better control of single-photon propagation in such a many-body quantum optical setting. As for some of the key findings, we observe that the periodic setting leads to the formation of the band structure in the photon transmission spectra, which is most evident in the strong coupling regime of cQED. However, under resonant conditions with no losses, the application of Bloch’s theorem indicates that the width of forbidden gaps can be altered by tuning the emitter-cavity coupling to small values. Moreover, in the disordered case, we find that the single-photon transmission curves show the disappearance of band formation. However, spectral features originating from cQED interactions observed for the single atom-cavity problem remain robust against weak-disordered conditions. The results of this work may find application in the study of quantum many-body effects in the optical domain as well as in different areas of quantum computation and quantum networking.

Surface Topological Plexcitons: Strong Coupling in a Bi2Se3 Topological Insulator Nanoparticle-Quantum Dot Molecule

Strong coupling of quantum states with electromagnetic modes of topological matter offer an interesting platform for the exploration of new physics and applications. In this work, we report a novel hybrid mode, a surface topological plexciton, arising from strong coupling between the surface topological plasmon mode of a Bi2Se3 topological insulator nanoparticle and the exciton of a two-level quantum emitter. We study the power absorption spectrum of the system by working within the dipole and rotating-wave approximations, using a density matrix approach for the emitter, and a classical dielectric-function approach for the topological-insulator nanoparticle. We show that a Rabi-type splitting can appear in the spectrum suggesting the presence of strong coupling. Furthermore, we study the dependence of the splitting on the separation of the two nanoparticles as well as the dipole moment of the quantum emitter. These results can be useful for exploring exotic phases of matter, furthering research in topological insulator plasmonics, as well as for applications in the far-infrared and quantum computing.

Controlling atom-photon bound states in a coupled resonator array with a two-level quantum emitter.

We consider a one-dimensional (1D) coupled-resonator array (CRA), where a two-level quantum emitter (2LE) is electric-dipole coupled to the modes of two adjacent resonators. We investigate the energy spectrum, the photon probability distribution of the bound states, and the emission process of the 2LE into the CRA vacuum. A quantum phase transition is found which is characterized by the change of the number of the out-of-band discrete levels. The condition for this change is also presented. The photon wave functions of bound states are found to be asymmetry around the position of the 2LE when the coupling strengths between the 2LE and the resonator are not equal, and they have the same preferred directions which are primarily determined by the larger one among the coupling strengths. The presence of the atom-photon bound states is manifested in the form of a stationary oscillation or a non-vanishing constant in the long enough time.

Biphoton routing in few-emitter chiral waveguide quantum electrodynamics ladders

We study the problem of two-photon routing in waveguide QED ladders where a few two-level quantum emitters (QEs) are simultaneously coupled with two chiral waveguides. We analyze the routing probability in two regimes, namely, under a purely plane wave approximation (scattering case) and in the presence of photon-photon bound state formation. Within the scattering case, we examine the two-photon routing in the presence of up to five QEs, considering two possibilities separately: ideal-symmetric coupling and the critical coupling scenario. We examine the photon routing up to the two QEs for the bound state situation and compare the photon redirection efficiency with the corresponding scattering case. Our findings show the potential of utilizing chiral light-matter interactions in multi-photon and multi-emitter-based quantum networking protocols where interlinking among spatially distant nodes is required.

On the simultaneous scattering of two photons by a single two-level atom

AbstractThe interaction of light with a single two-level emitter is the most fundamental process in quantum optics, and is key to many quantum applications. As a distinctive feature, two photons are never detected simultaneously in the light scattered by the emitter. This is commonly interpreted by saying that a single two-level quantum emitter can only absorb and emit single photons. However, it has been theoretically proposed that the photon anticorrelations can be thought of as arising from quantum interference between two possible two-photon scattering amplitudes, which one refers to as coherent and incoherent. This picture is in stark contrast to the aforementioned one, in that it assumes that the atom has two different mechanisms at its disposal to scatter two photons at the same time. Here we experimentally validate the interference picture by showing that, when spectrally rejecting only the coherent component of the fluorescence light of a single two-level atom, the remaining light consists of photon pairs that have been simultaneously scattered by the atom. Our results offer fundamental insights into the quantum-mechanical interaction between light and matter and open up novel approaches for the generation of highly non-classical light fields enabling, for example, Fourier-limited photon-pair sources that approach the theoretical limit in brightness.

Dissipative phase transition in an open Tavis–Cummings model with a two-photon drive

We investigate the steady-state quantum phases and associated collective phenomena of an open Tavis–Cummings (TC) model driven by a two-photon source. The standard (non-dissipative) TC model describes the quantum phases of an ensemble of N q two-level quantum emitters interacting with a single-mode electromagnetic field. The interplay among coherent driving, dissipation, and dipole interactions of the open TC model results in emergent collective phenomena, leading to a dissipative or non-equilibrium phase transition from the normal to the superradiant phase. We solve the Liouvillian equation analytically using the semi-classical mean-field approximation. We carry out stability analysis of the steady-state phases and determine the phase boundary. The use of Holstein–Primakoff transformation in the thermodynamic limit reduces the system to an effective coupled-oscillator model, the solution of which yields collective modes.

Realizing strong photon blockade at exceptional points in the weak coupling regime

We theoretically prove that it is possible to realize strong photon blockade at n-order exceptional points (EPn) in a two-level quantum emitter (QE)–cavity quantum electrodynamics (QED) system even if the emitter–cavity coupling strength is weak. When the single-mode cavity is gain, we show that the ultrastrong single-photon blockade (1 PB) emerges at two-order exceptional points (EP2), avoiding the strong non-linearity of the system. In addition, we first give the pseudo-Hermitian condition for the non-Hermitian cavity QED system and find that the third-order exceptional points (EP3) can be predicted under certain constraints of the parameters. For this case, the pronounced 1 PB at EP3 will be triggered. Furthermore, we also consider the usual EP2-enhanced 1 PB existing in the system with or without the dipole–dipole interaction (DDI) under the pseudo-Hermitian condition. A striking feature is that the system without DDI can realize more obvious 1 PB at EP2 than the case of with DDI. What is important is that both EP2 and EP3 will appear in the weak coupling regime. Our proposal sheds new light on strong EP-engineered photon blockade in the weak coupling regime, providing a unique platform for making high-quality single-photon sources.

Engineering optomechanically induced transparency by coupling a qubit to a spinning resonator

We theoretically study the spectral properties of a pump–probe driven hybrid spinning optomechanical ring resonator optically coupled with a two-level quantum emitter (QE or qubit). Recently, we have shown [Opt. Express 27, 25515 (2019)OPEXFF1094-408710.1364/OE.27.025515] that in the absence of the emitter, the coupled cavity version of this setup is not only capable of non-reciprocal light propagation but can also exhibit slow and fast light propagation. In this work, we investigate in what ways the presence of a single QE coupled with the optical whispering gallery modes of a spinning optomechanical resonator can alter the probe light non-reciprocity. Under the weak-excitation assumption and mean-field approximation, we find that the interplay between the rotational/spinning Sagnac effect and qubit coupling can lead to enhancement of both the optomechanically induced transparency peak value and the width of the transparency window due to the opening of a qubit-assisted backreflection channel. However, compared to the no-qubit case, we notice that such enhancement comes at the cost of degrading the group delay in probe light transmission by a factor of 1/2 for clockwise rotary directions. The target applications of these results can be in the areas of quantum circuitry and in non-reciprocal quantum communication protocols where QEs are a key component.

A study of the effective Hamiltonian method for decay dynamics* *Project supported by the National Natural Science Foundation of China (11964010, 11564013 and 11464014), the Natural Science Foundation of Hunan Province (2020JJ4495), the Scientific Research Fund of Hunan Provincial Education Department (22A0377 and 21A0333), and the Jishou University Innovation Foundation for Postgraduate (Jdy20038).

The decay dynamic of an excited quantum emitter (QE) is one of the most important contents in quantum optics. It has been widely applied in the field of quantum computing and quantum state manipulation. When the electromagnetic environment is described by several pseudomodes, the effective Hamiltonian method based on the multi-mode Jaynes–Cummings model provides a clear physical picture and a simple and convenient way to solve the decay dynamics. However, in previous studies, only the resonant modes are taken into account, while the non-resonant contributions are ignored. In this work, we study the applicability and accuracy of the effective Hamiltonian method for the decay dynamics. We consider different coupling strengths between a two-level QE and a gold nanosphere. The results for dynamics by the resolvent operator technique are used as a reference. Numerical results show that the effective Hamiltonian method provides accurate results when the two-level QE is resonant with the plasmon. However, when the detuning is large, the effective Hamiltonian method is not accurate. In addition, the effective Hamiltonian method cannot be applied when there is a bound state between the QE and the plasmon. These results are of great significance to the study of the decay dynamics in micro-nano structures described by quasi-normal modes.

Degenerate parametric down-conversion facilitated by exciton-plasmon polariton states in a nonlinear plasmonic cavity

We study the effect of degenerate parametric down-conversion (DPDC) in an ensemble of two-level quantum emitters (QEs) coupled via near-field interactions to a single surface plasmon (SP) mode of a nonlinear plasmonic cavity. For this purpose, we develop a quantum driven-dissipative model capturing non-equilibrium dynamics of the system in which incoherently pumped QEs have transition frequency tuned near the second-harmonic response of the SPs. Considering the strong coupling regime, i.e. the SP-QE interaction rate exceeds system dissipation rates, we find a critical SP-QE coupling attributed to the phase transition between normal and lasing steady states. Examining fluctuations above the system’s steady states, we predict new elementary excitations, namely, the exciton-plasmon polaritons formed by the two-SP quanta and single-exciton states of QEs. The contribution of two-SP quanta results in the linear scaling of the SP-QE interaction rate with the number of QEs, , as opposed to the -scaling known for the Dicke and Tavis–Cummings models. We further examine how SP-QE interaction scaling affects the polariton dispersions and power spectra in the vicinity of the critical coupling. For this purpose, we compare the calculation results assuming a finite ensemble of QEs and the model thermodynamic limit. The calculated power spectra predict an interplay of coherent photon emission by QEs near the second-harmonic frequency and correlated photon-pair emission at the fundamental frequency by the SPs (i.e. the photonic DPDC effect).

Control of Localized Single- and Many-Body Dark States in Waveguide QED.

Subradiant states in a finite chain of two-level quantum emitters coupled to a one-dimensional reservoir are a resource for superior photon storage and their controlled release. As one can maximally store one energy quantum per emitter, storing multiple excitations requires delocalized states, which typically exhibit fermionic correlations and antisymmetric wave functions, thus making them hard to access experimentally. Here we identify a new class of quasilocalized dark states with up to half of the qubits excited, which only appear for lattice constants of an integer multiple of the wavelength. These states allow for a high-fidelity preparation and minimally invasive readout in state-of-the-art setups. In particular, we suggest an experimental implementation using a coplanar waveguide coupled to superconducting transmon qubits on a chip. With minimal free space and intrinsic losses, virtually perfect dark states can be achieved for a low number of qubits featuring fast preparation and precise manipulation.

Spontaneous emission of a quantum emitter near a graphene nanodisk under strong light-matter coupling

We investigate the dynamical evolution of the spontaneous emission of a two-level quantum emitter near a graphene nanodisk. We employ the macroscopic quantum electrodynamics methodology to study the reversible population dynamics of the excited state of the quantum emitter in the non-Markovian limit. Our results indicate that the quantum-emitter--nanodisk interaction enters the strong light-matter coupling regime, as manifested in the excited-state probability where Rabi oscillations and population trapping effects are observed. The level of non-Markovianity is estimated by computing three different well-established measures and relate them to the quantum speedup due to the strong-coupling interaction of the emitter with the graphene nanodisk. Importantly, high values of the non-Markovianity and quantum speedup measures are achieved when the emitter-nanodisk interaction is strong, whereas decreasing coupling strength leads to smaller values.

Selective mode excitations and spontaneous emission engineering in quantum emitter-photonic structure coupled systems.

We study the excitation conditions of the supported field modes, as well as the spontaneous decay property of a two-level quantum emitter coupled to photonic structures containing topological insulators (TIs) and left-handed materials. Within the proper field quantization scheme, the spontaneous decay rates of dipoles with different polarizations are expressed in forms of the Green's functions. We find that in the proposed structure, the variation in the topological magnetoelectric polarizability (TMP) has a deterministic effect on the excitation of different field modes. As the result, the spontaneous decay property of the quantum emitter can be engineered. For a dipole placed in different spatial regions, the spontaneous decay feature indicates a dominant contribution from the waveguide modes, the surface plasmon modes or the free vacuum modes. Moreover, a special kind of the surface plasmon modes displaying asymmetric density of states at the interfaces, becomes legal in the presence of nontrivial TIs. These phenomena manifest the feasibility in controlling dipole emissions via manipulations of the topological magnetoelectric (TME) effect. Our results have potential applications in quantum technologies relied on the accurate control over light-matter interactions.

Nonreciprocal Single-Photon Band Structure.

We study a single-photon band structure in a one-dimensional coupled-resonator optical waveguide that chirally couples to an array of two-level quantum emitters (QEs). The chiral interaction between the resonator mode and the QE can break the time-reversal symmetry without the magneto-optical effect and an external or synthetic magnetic field. As a result, nonreciprocal single-photon edge states, band gaps, and flat bands appear. By using such a chiral QE coupled-resonator optical waveguide system, including a finite number of unit cells and working in the nonreciprocal band gap, we achieve frequency-multiplexed single-photon circulators with high fidelity and low insertion loss. The chiral QE-light interaction can also protect one-way propagation of single photons against backscattering. Our work opens a new door for studying unconventional photonic band structures without electronic counterparts in condensed matter and exploring its applications in the quantum regime.

Modulating the temporal dynamics of nonlinear ultrafast plasmon resonances

Surface plasmon-induced nonlinear optical resonances have shown immense potential in advanced optical imaging and nonlinear photonic devices. However, the ultrashort lifetime of these intense nonlinear fields inhibits their effective use in the vast applications of quantum plasmonics. Here, we propose enhancement in the lifetime of fast decaying second harmonic (SH) plasmon mode through a weak and pure resonant interaction with a two-level quantum emitter (QE). We compute the time evolution of SH response under a two-coupled oscillator model, in which we examine the interaction of short-lived SH mode supported by Au nanoparticle (AuNP) with long-lived dark mode (DM) or QE systems. To analyze the effect of spectral and temporal properties of DM and QE on the SH field, we evaluate the lifetime enhancement factor as a function of coupling strength and tuned resonant frequencies. The results show that tiny object like QE with sharp spectral bandwidth, small decay rate, and large oscillating strength is more efficient to control and probe the temporal dynamics of the SH field, as compared to DM which have a wide spectral bandwidth. Also, we control the lifetime of the SH mode after the natural decay time of the fundamental mode (FM), which distinguishes SH mode irrespective of its spatial convolution with elementary modes. Our proposed AuNP-QE coupled plasmonic system supporting nonlinear signal with enhanced temporal character paves its way for designing efficient on-chip nonlinear optical devices and can be a powerful tool in ultrahigh resolution nonlinear optical imaging.

Coherent perfect absorption in Tavis-Cummings models.

We theoretically study the conditions under which two laser fields can undergo Coherent Perfect Absorption (CPA) when shined on a single-mode bi-directional optical cavity coupled with two two-level quantum emitters (natural atoms, artificial atoms, quantum dots, qubits, etc.). In addition to being indirectly coupled through the cavity-mediated field, in our Tavis-Cummings model, the two quantum emitters (QEs) are allowed to interact directly via the dipole-dipole interaction (DDI). Under the mean-field approximation and low-excitation assumption, in this work, we particularly focus on the impact of DDI on the existence of CPA in the presence of decoherence mechanisms (spontaneous emission from the QEs and the leakage of photons from the cavity walls). We also present a dressed-state analysis of the problem to discuss the underlying physics related to the allowed polariton state transitions in the Jaynes-Tavis-Cummings ladder. As a key result, we find that in the strong-coupling regime of cavity quantum electrodynamics, the strong DDI and the emitter-cavity detuning can act together to achieve the CPA at two laser frequencies tunable by the inter-atomic separation which are not possible to attain with a single QE in the presence of detuning. Our CPA results are potentially applicable in building quantum memories that are an essential component in long-distance quantum networking.

Three methods for the single-photon transport in a chiral cavity quantum electrodynamics system

We investigate the single-photon transport problem in the system of a Whispering-Gallery mode microresonator directionally coupled with a two-level quantum emitter (QE). This QE-microresonator coupling system can usually be studied by cavity quantum electrodynamics and the single-photon transport methods. However, we find that if we treat a two-level QE as a single-photon phase-amplitude modulator, we can also deal with such systems using the transfer matrix method. Further, in theory, we prove that these three methods are equivalent. The corresponding relations of respective parameters among these approaches are precisely deduced. Our work can be extended to a multiple-resonator system interacting with two-level QEs in a chiral way. Therefore, the transfer matrix method may provide a convenient and intuitive form for exploring more complex chiral QE-resonator interaction systems.

Quantum interference effect in plasmonic transmission in the presence of quantum emitters

In this paper, we show how to enhance the transmission of the surface plasmon field using several interacting two-level quantum emitters placed in a plasmonic environment. Our model relies on quantum interference effect and, therefore, can be treated as a plasmonic analog of the electromagnetic induced transparency, usually referred to in the context of driven atomic systems. Moreover, our model with multiple interacting quantum emitters, similar to Ising spin chain, can be used for broadband transmission of the surface plasmon field.

On the dissipative dynamics of entangled states in coupled-cavity quantum electrodynamics arrays

We examine the dissipative dynamics of N00N states with an arbitrary photon number N in two architectures of fiber-coupled optical ring resonators (RRs) interacting with two-level quantum emitters (QEs). One architecture consists of a two-way cascaded array of emitter–cavity systems, while in the other architecture, we consider two fiber-coupled RRs, each coupled to multiple dipole–dipole interacting (DDI) QEs. Our focus in this paper is to study how an initially prepared multiple excitation atomic N00N state transfers to the RRs and then how rapidly it decays in these open cavity quantum electrodynamics setups while varying the emitter–cavity coupling strengths, emitter–cavity detuning, and backscattering from cavity modes. We present a general theoretical formalism valid for any arbitrary numbers of QEs, RRs, and N numbers in the N00N state for both schemes. As examples, we discuss the cases of single- and two-excitation N00N states and report the comparison of our findings in both schemes. As one of the main results, we conclude that the array scheme tends to store N00N for longer times, while the DDI scheme supports higher fidelity values. The results of this study may find applications in designing new multiparty entanglement-based protocols in quantum metrology and quantum lithography.