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IEEE International Conference on Microwaves, Communications, Antennas and Electronic Systems (COMCAS)

Quantum emission between the weak and strong coupling regimes

Iñigo Liberal

Published Nov 1, 2019• 0 citations
Publisher

Abstract

The emission properties of single-photon sources (a) attached to photonic nanostructures in the weak and strong coupling regimes have been intensively studied. However, often new opportunities arise in physical systems that operate at intermediate regimes between the more studied extreme cases. Here, we analyze the emission properties of a two-level system coupled to a two-mode cavity. Our analysis demonstrates that there is an intermediate regime, between the weak and strong coupling regimes, which offers an additional degree of freedom in controlling the bandwidth of a single-photon source, even when it is incoherently excited. In turn, this degree of freedom can be harnessed to decouple efficiency and bandwidth, thus bringing additional flexibility in the design of efficient photon sources with (b) controllable bandwidth.

DOI
10.1109/comcas44984.2019.8958309

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The emission properties of single-photon sources (a) attached to photonic nanostructures in the weak and strong coupling regimes have been intensively studied. However, often new opportunities arise in physical systems that operate at intermediate regimes between the more studied extreme cases. Here, we analyze the emission properties of a two-level system coupled to a two-mode cavity. Our analysis demonstrates that there is an intermediate regime, between the weak and strong coupling regimes, which offers an additional degree of freedom in controlling the bandwidth of a single-photon source, even when it is incoherently excited. In turn, this degree of freedom can be harnessed to decouple efficiency and bandwidth, thus bringing additional flexibility in the design of efficient photon sources with (b) controllable bandwidth.
The prototypical system constituted by a two-level atom interacting with a quantized single-mode electromagnetic field is described by the quantum Rabi model (QRM). The QRM is potentially valid at any light-matter interaction regime, ranging from the weak (where the decay rates exceeds the coupling rate) to the deep strong coupling (where the interaction rate exceeds the bare transition frequencies of the subsystems). However, when reaching the ultrastrong coupling regime, several theoretical issues may prevent the correct description of the observable dynamics of such a system: (i) the standard quantum optics master equation fails to correctly describe the interaction of this system with the reservoirs; (ii) the correct output photon rate is no longer proportional to the intracavity photon number; and (iii) they appear to violate gauge invariance. Here, we study the photon flux emission rate of this system under the incoherent excitation of the two-level atom for any light-matter interaction strength and consider different effective temperatures. The dependence of the emission spectra on the coupling strength is the result of the interplay between energy levels, matrix elements of the observables, and the density of states of the reservoirs. Within this approach, we also study the occurrence of light-matter decoupling in the deep strong-coupling regime and show how all of the obtained results are gauge invariant.
© 2019 European Association on Antennas and Propagation. We discuss the role of the classical electromagnetic theory concept of reactive interactions on determining the bandwidth of a single-photon source. Typically, quantum emitters operate in the weak-coupling regime where the bandwidth of the emission spectrum is simply proportional to their decay rates. However, we introduce a first-order correction to the emission spectrum, demonstrating that its bandwidth is also directly affected by the dispersion properties of the reactive interactions of the quantum emitter with its environment. This correction is particularly important in the intermediate region bridging the weak and strong coupling regimes. As an example of the applicability of this theory, we study the behaviour of a quantum emitter decaying through a coupled two-cavity system. Our results suggests that this setup could be utilized for the design of efficient, but narrowband single-photon sources.
We present a systematic study of the properties of systems composed of NNN two-level quantum emitters coupled to a single cavity mode, for light-matter interaction strengths ranging from the weak to the ultrastrong and deep-strong coupling regimes. Beginning with an analysis of the energy spectrum as a function of the light-matter coupling strength, we examine systems with varying numbers of emitters, from a pair to large collections, approaching the thermodynamic limit $(N\ensuremath{\rightarrow}\ensuremath{\infty})$. Additionally, we explore the emission properties of these systems under incoherent excitation of the emitters, employing a general theoretical framework for open cavity-QED systems, which is valid across all light-matter interaction regimes and preserves gauge invariance within truncated Hilbert spaces. Furthermore, we study the influence of the emitter-environment interaction on the spectral properties of the system. Specifically, when each emitter interacts independently with its own reservoir, we observe the emergence of an emission peak at the cavity's resonant frequency for even values of NNN. Our analysis also clarifies the evolution of the system as the number of emitters increases, ultimately converging toward an equivalent system composed of two interacting single-mode bosonic fields.
Reactive power plays a crucial role in the design of small antenna systems, but its impact on the bandwidth of quantum emitters is typically disregarded. Here, we theoretically demonstrate that there is an intermediate domain between the usual weak- and strong-coupling regimes where the bandwidth of a quantum emitter is directly related to the dispersion properties of the reactive power. This result emphasizes that reactive power must be understood as an additional degree of freedom in engineering the bandwidth of quantum emitters. We illustrate the applicability of this concept by revisiting typical configurations of quantum emitters coupled to resonant cavities and waveguides. Analysis of the reactive power in these systems unveils functionalities including the design of efficient but narrow-band photon sources, as well as quantum emitters exhibiting bandwidths narrower than their nonradiative linewidths even under incoherent pumping.
Impressive progress has been achieved in controlling the spontaneous emission rate for emitters in Nanophotonic structures, such as microcavities, photonic crystals and nano-antennas. In all cases, however, the modification of the emission is stationary in time. Thus, the emission rate is time-independent and the distribution of photon emission times are stochastic and follow an exponential distribution in the weak coupling regime. In this paper, by using fast all-optical switching of a microcavity, the cavity resonance is tuned within tens of picosesonds. This drastically changes the local optical density of states at the emission frequency within the emission lifetime of the emitters. This paper theoretically studied an ensemble of two-level sources with a time dependent decay rate and have derived the excited state population and emitted intensity as a function of time for continuous wave and pulsed excitation. Deterministic bursts of dramatically enhanced or inhibited emission intensity within short intervals. Thereby, we control the photon distribution to show behavior that strongly deviates from single exponential decay. This can be used to protect quantum information systems against decoherence and to realize non-Markovian dynamics in cavity quantum electrodynamics.