Collective Spontaneous Emission Research Articles

Coherent Amplification of Continuous Laser Field via Superfluorescence.

Superfluorescence (SF) is collective spontaneous emission wherein radiators spontaneously synchronize, resulting in an intense single-pulse emission. The avalanche radiation of photons is initiated by the first photon emitted into the SF propagation mode. Because this process is stochastic, the absolute phase of the SF changes randomly from shot to shot. We demonstrate that this phase can be controlled by seeding the SF with a resonant continuous-wave (CW) laser. The seed light was weak enough not to cause the stimulated emission but strong enough to inject the first photon into the SF propagation mode prior to injection by the radiators themselves. Cross-correlation measurements between the seeded SF and CW laser revealed that the seed light was coherently amplified by the SF. The amplification factor for the instantaneous intensity was estimated to be 7 orders of magnitude. These results will pave the way for the development of new types of quantum optical amplifiers.

Collective photon emission of two correlated atoms in free space

By preparing two distant emitters in entangled Dicke states via detection of a single photon, the subsequent spatiotemporal photon emission is investigated. Depending on the parity of the established quantum state, emission patterns for the second scattered photon are observed featuring spatial superradiance as well as subradiance. We employ ultrafast single-photon resolving cameras with high spatial resolution disclosing the spatiotemporal emission characteristics. By recording the first photon in one direction and the second photon in another, revealing the spatial two-photon cross-correlation function ${g}^{(2)}({\mathbf{r}}_{1},{\mathbf{r}}_{2})$, we characterize the collective spontaneous emission behavior of two ions in free space. We explain the observed contrast of ${g}^{(2)}({\mathbf{r}}_{1},{\mathbf{r}}_{2})$ considering independently derived experimental parameters. Our results show how the detection of a single photon can profoundly modify the collective spontaneous emission dynamics of an atomic ensemble.

Unraveling the Strong Fluorescence Enhancement of HPBI Molecules by ZIF-8 Colloidal Suspensions via Adsorption Analysis.

Enhancing the fluorescence of organic dye by colloidal particles is one of the most promising routes to optimize fluorescence detection. However, in addition to metallic particles, which serve as the most frequently used particles and have been found to employ the plasmonic resonance to provide strong fluorescence enhancement, neither new types of colloidal particles nor new fluorescence mechanisms have been intensively explored in recent years. In this work, strongly enhanced fluorescence was observed when 2-(2-hydroxyphenyl)-1H-benzimidazole (HPBI) molecules were simply mixed with zeolitic imidazolate framework-8 (ZIF-8) colloidal suspensions. Moreover, the enhancement factor ΔI = IHPBI+ZIF-8/IHPBI does not increase accordingly with the increasing amount of HPBI. To find out how the strong fluorescence was triggered and affected by the amount of HPBI, multiple techniques were applied to analyze the adsorption behavior. By combining analytical ultracentrifugation with first-principles calculations, we proposed that HPBI molecules were adsorbed onto the surface of ZIF-8 particles coordinatively and electrostatically, depending on the concentration of HPBI molecules. The coordinative adsorption would result in a new kind of fluorescence emitter. The new fluorescence emitters tend to distribute on the outer surface of ZIF-8 particles periodically. The distance between each fluorescence emitter is fixed and much smaller than the wavelength of the excitation light. Thus, it can be concluded that collective spontaneous emission might be triggered.

A fast, high-order numerical method for the simulation of single-excitation states in quantum optics

We consider the numerical solution of a nonlocal partial differential equation which describes the phenomenon of collective spontaneous emission in a two-level atomic system containing a single photon. We reformulate the problem as an integro-differential equation for the atomic degrees of freedom, and describe an efficient solver for the case of a Gaussian atomic density. The problem of history dependence arising from the integral formulation is addressed using sum-of-exponentials history compression. We demonstrate the solver on two systems of physical interest: in the first, an initially-excited atom decays into a photon by spontaneous emission, and in the second, a photon pulse is used to an excite an atom, which then decays.

Engineering of spontaneous emission in free space via conditional measurements

We study the collective spontaneous emission of three identical two-level atoms initially prepared in the excited states by measuring Glauber's third-order photon correlation function. Assuming two atoms at subwavelength distance from each other such that they are subject to the dipole-dipole interaction while the third one is located several wavelengths away, we observe super- and subradiant decay alike, depending on the direction of observation. Unlike the case where no remote atom is introduced or no conditional measurements are performed, the spontaneous emission behavior of the conditioned three-atom system is strongly modified, i.e., differing from the single-atom and the canonical two-atom configuration. The conditional measurements associated with the three-photon correlation function in combination with the dipole-dipole interaction between the adjacent atoms lead to quantum interference among the different decay channels allowing one to engineer the spontaneous emission in space and time.

Coherent Control of Collective Spontaneous Emission through Self-Interference.

As one of the central topics in quantum optics, collective spontaneous emission such as superradiance has been realized in a variety of systems. This Letter proposes an innovative scheme to coherently control collective emission rates via a self-interference mechanism in a nonlinear waveguide setting. The self-interference is made possible by photon backward scattering incurred by quantum scatterers in a waveguide working as quantum switches. Whether the interference is constructive or destructive is found to depend strongly on the distance between the scatterers and the emitters. The interference between two propagation pathways of the same photon leads to controllable superradiance and subradiance, with their collective decay rates much enhanced or suppressed (also leading to hyperradiance or population trapping). Furthermore, the self-interference mechanism is manifested by an abrupt change in the emission rates in real time. An experimental setup based on superconducting transmission line resonators and transmon qubits is further proposed to realize controllable collective emission rates.

Generating long-lived entangled states with free-space collective spontaneous emission

Considering the paradigmatic case of a cloud of two-level atoms interacting through common vacuum modes, we show how cooperative spontaneous emission, which is at the origin of superradiance, leads the system to long-lived entangled states at late times. These subradiant modes are characterized by an entanglement between all particles, independently of their geometrical configuration. While there is no threshold on the interaction strength necessary to entangle all particles, stronger interactions lead to longer-lived entanglement.

Connecting steady-states of driven-dissipative photonic lattices with spontaneous collective emission phenomena

Recent experimental advances enable the fabrication of photonic lattices in which the light propagates with engineered energy dispersions. When interfaced with quantum emitters, such systems yield strong collective spontaneous emission phenomena, such as perfect sub-radiance, in which the decay into the bath is completely suppressed, forming bound-states-in-the-continuum (BIC). Since such photonic lattices are generally lossy, an alternative way of probing them consists in coherently driving them to an steady-state from which photoluminescence can be extracted. Here, we formalize connections between these two seemingly different situations and use that intuition to predict the formation of non-trivial photonic steady-states in one and two dimensions. In particular, we show that subradiant emitter configurations are linked to the emergence of steady-state light-localization in the driven-dissipative setting, in which the light features the same form than the spontaneously formed BIC. Besides, we also find configurations which leads to the opposite behaviour, an anti-localization of light, that is, it distributes over all the system except for the region defined between the driving lasers. These results shed light on the recently reported optically-defined cavities in polaritonic lattices, and can guide further experimental studies.

Spatial Coherence of Light in Collective Spontaneous Emission

When a quantum system is put into an excited state, it will decay back to the ground state through a process termed spontaneous emission. It is generally assumed that spontaneous emission between different individual emitters would not be coherent with each other; to produce coherent light one would need population inversion and stimulated emission. In this work, we show that an optically-thin ensemble of 11,000 radiating atoms spontaneously organize to produce spatially coherent light. The reason for this coherence is collective-coupling of the individual emitters via Dicke superradiance and subradiance (as opposed to amplification through stimulated emission).

Collective spontaneous emission and kinetic equations for one-photon light in random media

We consider the theory of spontaneous emission for a random medium of stationary two-level atoms. We investigate the dynamics of the field and atomic probability amplitudes for a one-photon state of the system. At long times and large distances, we show that the corresponding average probability densities can be determined from the solutions to a pair of kinetic equations.

Time-resolved interplay between superradiant and subradiant states in superradiance lattices of Bose-Einstein condensates

The collective spontaneous emission of many atoms is significantly different from that of a single atom, depending on the geometry and the phase correlation of atomic ensembles. However, experimental observation of arbitrary superradiant and subradiant states of atoms remains challenging due to the difficulties in both preparation and detection of those states. Here we report the time-resolved observation of superradiance from a timed Dicke state in a momentum-space superradiance lattice of Bose-Einstein condensates, which enables an in situ measurement of the coherent lattice dynamics involving both superradiant and subradiant states. The long-lasting oscillation in the superradiant emission is contributed by population transport from the subradiant states in the superradiance lattice. This work paves the way to prepare and observe subradiant states, which has promising applications in quantum information processing.

Direction dependent photon superbunching effect from an atomic array

The probability of correlated emission of fluorescent photons as a function of detection directions has been investigated. The model system comprises identical two-level atoms arranged in the form of a line. A weak laser field resonantly excites only one of the atoms in the line. Two interaction mechanisms, namely, the vacuum-induced dipole–dipole interaction and the collective spontaneous emission couple the system of atoms. The aim is to observe the emission of a set of photon twins synchronized in time. It is seen that strongly directional emission of pairs of photons can take place due to the interference between the emitters. These highly correlated pairs of photons can be observed in very precise geometric directions. The observation is made based on two different detection procedures. It is found that the superradiant photons always tend to be bunched along the same direction.

Superradiance of bacteriochlorophyll c aggregates in chlorosomes of green photosynthetic bacteria

Chlorosomes are the main light-harvesting complexes of green photosynthetic bacteria that are adapted to a phototrophic life at low-light conditions. They contain a large number of bacteriochlorophyll c, d, or e molecules organized in self-assembling aggregates. Tight packing of the pigments results in strong excitonic interactions between the monomers, which leads to a redshift of the absorption spectra and excitation delocalization. Due to the large amount of disorder present in chlorosomes, the extent of delocalization is limited and further decreases in time after excitation. In this work we address the question whether the excitonic interactions between the bacteriochlorophyll c molecules are strong enough to maintain some extent of delocalization even after exciton relaxation. That would manifest itself by collective spontaneous emission, so-called superradiance. We show that despite a very low fluorescence quantum yield and short excited state lifetime, both caused by the aggregation, chlorosomes indeed exhibit superradiance. The emission occurs from states delocalized over at least two molecules. In other words, the dipole strength of the emissive states is larger than for a bacteriochlorophyll c monomer. This represents an important functional mechanism increasing the probability of excitation energy transfer that is vital at low-light conditions. Similar behaviour was observed also in one type of artificial aggregates, and this may be beneficial for their potential use in artificial photosynthesis.

Collectively driven optical nanoantennas

Optical nanoantennas, i.e., elements transforming localized light or waveguide modes into freely propagating fields and vice versa, are vital components for modern nanophotonics. Optical antennas have been demonstrated to cause the Dicke superradiance effect, i.e., collective spontaneous emission of quantum sources. However, the impact of coherent excitation on the antenna performance, such as directivity, efficiency, and Purcell effect, remains mostly unexplored. Herein, using full-wave numerical simulations backed by a quantum model, we unveil that coherent excitation allows controlling antenna multipoles, on-demand excitation of nonradiative states, enhanced directivity, and improving antenna radiation efficiency. This collective excitation corresponds to the states with nonzero dipole moment in the quantum picture, where the quantum phase is well defined. The results of this work bring another degree of freedom----the collective phase of an ensemble of quantum emitters----to control optical nanoantennas and, as such, pave the way to the use of collective excitations for nanophotonic devices with superb performance. To make the discussion independent of the frequency range, we consider the all-dielectric design and use dimensionless units.

Collective spontaneous emission of two entangled atoms near an oscillating mirror

We consider the cooperative spontaneous emission of a system of two identical atoms interacting with the electromagnetic field in the vacuum state and in the presence of an oscillating mirror. We assume that the two atoms, one in the ground state and the other in the excited state, are prepared in a correlated (symmetric or antisymmetric) Bell-type state. We also suppose that the perfectly reflecting plate oscillates adiabatically, with the field modes satisfying the boundary conditions at the mirror surface at any given instant, so that the time dependence of the interaction Hamiltonian is entirely enclosed in the instantaneous atom-wall distance. Using time-dependent perturbation theory, we investigate the spectrum of the radiation emitted by the two-atom system, showing how the oscillation of the boundary modifies the features of the emitted spectrum, which exhibits two lateral peaks not present in the case of a static boundary. We also evaluate the transition rate to the collective ground state of the two-atom system in both cases of the superradiant (symmetric) and subradiant (antisymmetric) state. We show that it is modulated in time and that the presence of the oscillating mirror can enhance or inhibit the decay rate compared to the case of atoms in vacuum space or near a static boundary. Our results thus suggest that a dynamical (i.e., time-modulated) environment can offer further possibilities to control and manipulate radiative processes of atoms or molecules nearby, such as the cooperative decay, and strongly indicate a similar possibility for other radiative processes, for example, the resonance interaction and the energy transfer between atoms or molecules.

Generation of Ultrashort Attosecond and Terahertz Pulses Based on the Collective Spontaneous Emission from a Thin Resonant Medium (Brief Review)

Generation of Ultrashort Attosecond and Terahertz Pulses Based on the Collective Spontaneous Emission from a Thin Resonant Medium (Brief Review)

Regular and bistable steady-state superradiant phases of an atomic beam traversing an optical cavity

We investigate the different photon emission regimes created by a preexcited and collimated atomic beam passing through a single mode of an optical cavity. In the regime where the cavity degrees of freedom can be adiabatically eliminated, we find that the atoms undergo superradiant emission when the collective linewidth exceeds the transit-time broadening. We analyze the case where the atomic beam direction is slanted with respect to the cavity axis. For this situation, we find that a phase of continuous light emission similar to steady-state superradiance is established providing the tilt of the atomic beam is sufficiently small. However, if the atoms travel more than half a wavelength along the cavity axis during one transit time we predict a dynamical phase transition to a new bistable superradiant regime. In this phase the atoms undergo collective spontaneous emission with a frequency that can be either blue or red detuned from the free-space atomic resonance. We analyze the different superradiant regimes and the quantum critical crossover boundaries. In particular we find the spectrum of the emitted light and show that the linewidth exhibits features of a critical scaling close to the phase boundaries.

Генерация предельно коротких импульсов терагерцового излучения на основе сверхизлучения трехуровневой резонансной среды

The possibility of generating extremely short pulses of terahertz (THz) radiation due to superradiance - collective spontaneous emission of stopped polarization of a thin layer of a three-level resonant medium excited by a pair of attosecond (or femtosecond) pulses is studied theoretically. The source of a terahertz pulse is a pulse of stopped nonlinear polarization of the medium, which occurs in the interval between its excitation and de-excitation. The case of a three-level medium with equidistant energy levels (as in a quantum harmonic oscillator), the transition frequency of which lies in the THz range, is considered. The influence of the populations of the excited levels of the medium on the shape of the terahertz superradiance pulse is discussed.

Environment-mediated charging process of quantum batteries

We study the charging process of open quantum batteries mediated by a common dissipative environment in two different scenarios. In the first case, we consider a quantum charger-battery model in the presence of a non-Markovian environment. Where the battery can be properly charged in a strong coupling regime, without any external power and any direct interaction with the charger, i.e., a wireless-like charging happens. The environment plays a major role in the charging of the battery, while this does not happen in a weak coupling regime. In the second scenario, we show the effect of individual and collective spontaneous emission rates on the charging process of quantum batteries by considering a two-qubit system in the presence of Markovian dynamics. Our results demonstrate that open batteries can be satisfactorily charged in Markovian dynamics by employing an underdamped regime and/or strong external fields. We also present a robust battery by taking into account subradiant states and an intermediate regime. Moreover, we propose an experimental setup to explore the ergotropy in the first scenario.

Geometric Control of Collective Spontaneous Emission.

Dipole spin-wave states of atomic ensembles with wave vector k(ω) mismatched from the dispersion relation of light are difficult to access by far-field excitation but may support rich phenomena beyond the traditional phase-matched scenario in quantum optics. We propose and demonstrate an optical technique to efficiently access these states. In particular, subnanosecond laser pulses shaped by a home-developed wideband modulation method are applied to shift the spin wave in k space with state-dependent geometric phase patterning, in an error-resilient fashion and on timescales much faster than spontaneous emission. We verify this control through the redirection, switch off, and recall of collectively enhanced emission from a ^{87}Rb gas with ∼75% single-step efficiency. Our work represents a first step toward efficient control of electric dipole spin waves for studying many-body dissipative dynamics of excited gases, as well as for numerous quantum optical applications.