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arXiv:1808.04884 · Physical review. B./Physical review. B

Dynamical multistability in a quantum-dot laser

Mattia Mantovani, Andrew D. Armour, Wolfgang Belzig, Gianluca Rastelli

Published Jan 30, 2019• 31 citations
arXivHTMLPDF

Abstract

We study the dynamical multistability of a solid-state single-atom laser implemented in a quantum-dot spin valve. The system is formed by a resonator that interacts with a two-level system in a dot in contact with two ferromagnetic leads of antiparallel polarization. We show that a spin-polarized current provides high-efficiency pumping leading to regimes of multistable lasing, in which the Fock distribution of the oscillator displays a multipeaked distribution. The emergence of multistable lasing follows from the breakdown of the usual rotating-wave approximation for the coherent spin-resonator interaction which occurs at relatively weak couplings. The multistability manifests itself directly in the charge current flowing through the dot, switching between distinct current levels corresponding to the different states of oscillation.

DOI
10.1103/physrevb.99.045442

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We study the dynamical multistability of a solid-state single-atom laser implemented in a quantum-dot spin valve. The system is formed by a resonator that interacts with a two-level system in a dot in contact with two ferromagnetic leads of antiparallel polarization. We show that a spin-polarized current provides high-efficiency pumping leading to regimes of multistable lasing, in which the Fock distribution of the oscillator displays a multipeaked distribution. The emergence of multistable lasing follows from the breakdown of the usual rotating-wave approximation for the coherent spin-resonator interaction which occurs at relatively weak couplings. The multistability manifests itself directly in the charge current flowing through the dot, switching between distinct current levels corresponding to the different states of oscillation.
We study a single-level quantum dot strongly coupled to a superconducting lead and tunnel coupled to a normal electrode which can exchange energy with a single-mode resonator. We show that such a system implements a single atom laser. We employ both a semiclassical treatment and a quantum master-equation approach to characterize the properties of this laser. In particular, we find that this system can be operated with efficiency approaching unity; that is, a single photon is emitted into the cavity for every Cooper pair participating in the charge current. We find also that lasing in the proposed setup is clearly identifiable in the transport properties: in the lasing state, the electrical current through the quantum dot is pinned to the maximum value achievable in this hybrid nanostructure, and hence, the onset of lasing can be detected simply by a current measurement.
Two experiments observe the so-called Mollow triplet in the emission spectrum of a quantum dot—originating from resonantly driving a dot transition—and demonstrate the potential of these systems to act as single-photon sources, and as a readout modality for electron-spin states. Single-quantum emitters emit only one photon at a time1,2, but the properties of the photon depend on how the emitter is excited3. Incoherent excitation is simple and broadly used with solid-state emitters such as quantum dots, but does not allow direct manipulation of the quantum state. Coherent, resonant excitation on the other hand is used in pump–probe techniques to examine the quantum state of the emitter4, but does not permit collection of the single-photon emission. Coherent control with simultaneous generation of photons has been an elusive goal in solid-state approaches, where, because of strong laser scattering at the detection wavelength, measurement of resonant emission has been limited to cross-polarized detection5 or Stokes-shift techniques6,7. Here we demonstrate that a semiconductor quantum dot in a microcavity can be resonantly driven and its single-photon emission extracted background free. Under strong continuous-wave excitation, the dot undergoes several Rabi oscillations before emitting, which are visible as oscillations in the second-order correlation function. The quantum-dot states are therefore ‘dressed’, resulting in a Mollow-triplet emission spectrum. Such coherent control will be necessary for future high-efficiency sources of indistinguishable single photons3,8, which can be used for quantum key distribution9 or through post-selection10 to generate entangled photon pairs11,12.
We present a theoretical study of spin multistability and hysteresis in resonantly pumped exciton–polaritons within semiconductor microcavities. This multistable behavior arises from the nonlinear, polarization-dependent anisotropy of polariton–polariton interactions. Unlike conventional optical bistability in spinless nonlinear optical systems, the dynamics are governed by spin-selective coupling. We calculate the steady-state polariton density and degree of circular polarization of the intracavity polariton field, demonstrating that for a fixed pump polarization, wave vector, and detuning, the degree of polarization could take on multiple values depending on the pump strength. This spin-sensitive multistability lays the groundwork for multichannel optical switches governed not just by total pump intensity but also by its pump polarization.
The optically-induced coherent spin dynamics in a charged quantum dot (QD) is studied theoretically using a new dynamical model for rigorous description of circularly polarized ultrashort optical pulse resonant interactions with the electron-trion system. Generalized pseudospin master equation is derived for description of the time evolution of spin coherences and spin populations in terms of the real state pseudospin (coherence) vector including dissipation in the system through spin relaxation processes. The equation is solved in the time domain self-consistently with the vector Maxwell equations for the optical wave propagation coupled to it via macroscopic medium polarization. Using the model the long-lived electron spin coherence left behind a single resonant ultrashort optical excitation of the electron-trion transition in a charged QD is simulated in the lowand high-intensity Rabi oscillations regime. Signatures of the polarized photoluminescence (PL) resulting from the numerical simulations, such as the appearance of a second echo pulse after the excitation and a characteristic PL trace shape, specific for initial spin-up orientation, are discussed for realization of high-fidelity schemes for coherent readout of a single spin polarization state.
The steady state behaviour describing the interaction of a mesoscopic system of coherently injected two-level Rydberg atoms with a coherently driven single-mode cavity is investigated in both resonant and non-resonant regimes. Multiple-switching effects between the output field states are predicted with the atomic coherent excitation (θ) and relative coherent phase parameter (Φ r ) of the atomic coherent state . The relation between the transmitted field ( x ) and the atomic cooperation parameter ( C ) shows multistable behaviour for some values of the polar and azimuthal angles (θ,ϕ). The transverse field feature in the form of a Gaussian beam profile tends to degrade the multistable structure in the ( C – x ) and the input–output field ( x – Y ) relations. Micromaser action is analysed for the system with atomic coherence within and without the plane wave approximation.
We model the population and coherence dynamics of a singly charged quantum dot driven by a constant optical field. Using a separation of time scales we predict the occurrence of quantum jumps in the spin state of the excess electron or hole. Our analysis extends the description of intermittent fluorescence from a three-level to a four-level description; the former is common in atomic systems, the later occurs in many solid-state systems. The statistics of the quantum jumps as function of the intensity of the driving field provide detailed information on physical processes that limit the ground-state coherence of these systems and their use as a solid-state quantum bit. In particular it enables a discrimination of coherent spin coupling, induced by the nuclear magnetic field, from incoherent spin flips; the coherent coupling can gradually be suppressed by increasing the pump intensity due to the quantum Zeno effect.