Strong coupling in cavity QED is conventionally achieved at the expense of either high cavity quality factors or ultrasmall mode volumes, a trade-off that fundamentally constrains practical implementations. Here, we circumvent this limitation by introducing two nonidentical auxiliary emitters with opposite detunings into a bad cavity. This hybrid system supports a subradiant mode that significantly squeeze the effective cavity linewidth, creating an ultra-narrow transmission window at the cavity frequency. As a result, a target emitter placed in this engineered environment exhibits prolonged vacuum Rabi oscillations and resolved spontaneous emission splitting, which are clear signatures of strong coupling, even though the bare cavity remains in the weak-coupling regime. Our scheme thus transforms a bad cavity into an effective platform for strong-coupling physics, with potential applications in quantum computation and quantum sensing.
Strong coupling in cavity QED is conventionally achieved at the expense of either high cavity quality factors or ultrasmall mode volumes, a trade-off that fundamentally constrains practical implementations. Here, we circumvent this limitation by introducing two nonidentical auxiliary emitters with opposite detunings into a bad cavity. This hybrid system supports a subradiant mode that significantly squeeze the effective cavity linewidth, creating an ultra-narrow transmission window at the cavity frequency. As a result, a target emitter placed in this engineered environment exhibits prolonged vacuum Rabi oscillations and resolved spontaneous emission splitting, which are clear signatures of strong coupling, even though the bare cavity remains in the weak-coupling regime. Our scheme thus transforms a bad cavity into an effective platform for strong-coupling physics, with potential applications in quantum computation and quantum sensing.
If an excited atom is put in between high-reflectance mirrors, the light emitted by the atom can be bounced back and be re-absorbed and be re-emitted, again and again until the light leaks out of the mirrors or is re-emitted into a wrong direction. This coherent oscillation between atom and light, called vacuum Rabi oscillation, results from interference between two superposition states of the atom and the photon, which have different energies or frequencies, or vacuum Rabi splitting. The vacuum Rabi oscillation and splitting demonstrate the quantum nature of light. They also provide single-photon non-linearity for photonic applications. In recent years, these effects have been considered particularly useful for quantum information processing [1] since the oscillation presents a natural mechanism for information exchange between a stationary quantum bit carried by the atom and a flying quantum bit carried by the photon, making scalable and distributed quantum computing possible. Realization of the vacuum Rabi oscillation and splitting, however, is highly non-trivial. It requires the so-called strong coupling condition, that is, the exchange between the atom and the photon must be faster than the photon leaks out of the space enclosed by the mirrors, i.e. the cavity, and faster than the atom emits the light into a wrong direction. The technique to realize the strong coupling condition is to make the cavity small so that the photon is bounced back more frequently to the atom and to make the mirrors more reflective so that the photon can be bounced more times. That means to decrease the cavity volume V and to increase the quality factor Q. Remarkable progresses have been made to make optical microcavities with small V and high Q, facilitating observation of vacuum Rabi oscillation and splitting in highly dissipative solid-state systems [2,3]. However, there is no systematic method to reduce V/Q—the key factor for realizing strong coupling (see Fig. 1a). The reason appears to be fundamental. When the size of a cavity is made as small as comparable to the light wavelength, small deformation of the cavity would result in large evanescent wave and hence photon leakage [4]. (a) An atom and a small cavity have strong coupling but the cavity leakage is also strong. A large cavity has small photon leakage but its coupling to the atom is small (so emission goes to free space). The atom and the large cavity can have strong coupling, with photons exchanged through but not occupying the small cavity—using the quantum tunneling effect. (b) In the classical world, two lower players have to provide sufficient energy to pass the ball through the higher player. (c) In the quantum world, the two lower players can pass balls through the wall via the higher players but with energy of the balls insufficient to reach the higher player. A recent proposal by [5] provides a general solution, using a divide-and-conquer strategy. A small cavity with relatively low Q is used to enhance the interaction between the emitter and the photon, while the photon stays in a large cavity with high Q. [5] demonstrate the strong coupling by showing Rabi splitting, Rabi oscillation, and single-photon non-linearity of the system. The scheme at the first sight does not seem to work since the emitter has no contact with the high-Q large cavity and if the photon enters the small cavity, it would leak out. The clever point of the scheme is: in the quantum mechanical world, the photon can interact with the emitter without occupying the small cavity! Similar to the ball game in Fig. 1b, a classical particle needs to have sufficient energy to pass a barrier. The small cavity is designed to have a frequency higher than those of the emitter and the large cavity, which would prevent the emitted photons entering the large cavity. However, quantum ball games allow tunneling of a barrier (Fig. 1c). So the emitter and the large cavity can indeed exchange photons. The small cavity would not be occupied since its frequency is not right, which prevents photon leakage. It looks the scheme applies generally to all types of photonic systems—photonic crystals, microspheres, microtoroids, etc. Considering the simplicity of the scheme, we expect applications be found soon.
We calculate the spontaneous-emission spectrum and the spectrum of weakly driven fluorescence for a two-level atom coupled to a resonant-cavity mode. For strong atom-cavity coupling the spectra split into two peaks that can have subnatural linewidths. If the cavity linewidth is negligible, the spontaneous-emission spectrum has half the radiative linewidth of the atom; the spectrum of weakly driven fluorescence shows an additional 36% squeezing-induced narrowing. These effects can be observed using coupled-field and collective-polarization oscillators excited in a cavity containing N two-level atoms.
Large single-atom cooperativity in quantum systems is important for quantum information processing. Here, we propose to exponentially enhance the single-atom cooperativity parameter by exploiting the strongly localized effect of modes in cavity quantum electrodynamics (QED) systems. By increasing the wing width of a cavity with special geometry symmetry, the interference property allows us to exponentially improve the quality factor Q without altering the mode volume V for cavities supporting subwavelength light modes. This effectively overcomes the trade-off between Q and V in conventional subwavelength Fabry-Pérot cavities. Consequently, we demonstrate the occurrence of ultralong vacuum Rabi oscillations and the generation of strong photon blockade by enhancing the single-atom cooperativity parameter. This Letter offers a promising approach for advancing coherent manipulation and holds significant potential for applications in establishing longer-distance quantum communication networks, enhancing the precision and stability of quantum sensors, and improving the efficiency of quantum algorithms.
Our semiconductor normal-mode-coupling microcavities, each containing a single narrow-linewidth quantum well (QW), exhibit record many-oscillator vacuum-field Rabi splitting (VRS). The question addressed here is: can the vacuum field be increased sufficiently by reducing the cavity-mode volume, so that a single quantum dot can exhibit VRS similar to an atom in a high-finesse cavity? The quantum-dot nanocavity has two advantages: a length close to the theoretical minimum and a much larger dipole moment; these permit strong coupling without an ultra high-Q cavity even for such a short length.
The system consisting of a two-level atom coupled strongly to a cavity mode behaves as a two-state system when excited near one of the ``vacuum'' Rabi resonances. With finite-bandwidth squeezed light incident upon the cavity and tuned to one of these resonances, we show that it is possible to realize a two-state system coupled to a squeezed vacuum. This system exhibits subnatural linewidths in the emitted spectra, as described by Gardiner [Phys. Rev. Lett. 56, 1917 (1986)] in a study of spontaneous emission of a two-level atom in a squeezed vacuum, but requires that only a single cavity mode be subject to squeezing rather than the entire three-dimensional vacuum.
The development and physical understanding of high-beta nanolasers operating in regime of cavity-quantum-electrodynamics (cQED) is a highly interdisciplinary field of research, involving important aspects of nanotechnology, quantum optics, and semiconductor physics. Of particular interest is the quantum limit of operation, in which a few or even a single emitter act as gain material. The regime of strong light-matter coupling is typically associated with weak excitation. With current realizations of cQED systems, strong coupling may persevere even at elevated excitation levels sufficient to cross the threshold to lasing. In the presence of stimulated emission, the vacuum-Rabi doublet in the emission spectrum is modified and the established criterion for strong coupling no longer applies. Based on an analytic approach, we provide a generalized criterion for strong coupling and the corresponding emission spectrum that includes the influence of higher Jaynes-Cummings states. The applicability is demonstrated in a theory-experiment comparison of a state-of-the-art few-emitter quantum-dot (QD)–micropillar laser as a particular realization of the driven dissipative Jaynes-Cummings model [1]. Furthermore, we address the question if and for which parameters true single-emitter lasing can be achieved. By using a master-equation approach for up to 8 QDs coupled to the mode, we provide evidence for the coexistence of strong coupling and lasing in our system in the presence of background emitter contributions by identifying signatures in the mean-photon number, the photon-autocorrelation function, and the emission linewidth. [1] C. Gies et al., accepted for publication in PRA, arxiv:1606.05591