Solid-state systems which mimic two-level atoms are being actively developed. Improving the quantum coherence of these systems, for instance spin qubits or single photon emitters using semiconductor quantum dots, involves dealing with noise. The sources of noise are inherent to the semiconductor and are complex. Charge noise results in a fluctuating electric field, spin noise in a fluctuating magnetic field at the location of the qubit, and both can lead to dephasing and decoherence of optical and spin states. We investigate noise in an ultra-pure semiconductor using a minimally-invasive, ultra-sensitive, local probe: resonance fluorescence from a single quantum dot. We distinguish between charge noise and spin noise via a crucial difference in their optical signatures. Noise spectra for both electric and magnetic fields are derived. The noise spectrum of the charge noise can be fully described by the fluctuations in an ensemble of localized charge defects in the semiconductor. We demonstrate the "semiconductor vacuum" for the optical transition at frequencies above 50 kHz: by operating the device at high enough frequencies, we demonstrate transform-limited quantum dot optical linewidths
We probe local charge fluctuations in a semiconductor via laser spectroscopy on a nearby self-assembled quantum dot. We demonstrate that the quantum dot is sensitive to changes in the local environment at the single charge level. By controlling the charge state of localized defects, we are able to infer the distance of the defects from the quantum dot with ±5 nm resolution. The results identify and quantify the main source of charge noise in the commonly-used optical field-effect devices. Based on this understanding we achieve routinely close-totransform-limited quantum dot optical linewidths.PACS numbers: 73.21. La and 78.67.Hc Condensed matter systems, notably quantum dots in III-V semiconductors and color centers in diamond, are very attractive as the building blocks for quantum light sources [1] and spin qubits [2]. For instance, an InGaAs quantum dot is a robust, high repetition rate, narrow linewidth source of ondemand single photons and polarization-entangled photons, properties not shared by any other emitter. In the future, the demands placed on the quality of the single photons will increase. For instance, the creation of remote entanglement via photon interference and associated applications as a quantum repeater require Fourier-transform-limited single photons, i.e. wavepackets with a spectral bandwidth determined only by the radiative lifetime. This is hard to achieve in a semiconductor. On the one hand, a quantum dot is extremely sensitive to the local electric field via the Stark effect [3,4] leading to a stringent limit on the acceptable charge noise. Charge noise can also lead to spin dephasing [5,6]. On the other hand, phonons in the host semiconductor can lead to dephasing [7]. However, at low temperature and with weak optical excitation, phonon scattering is suppressed in a quantum dot by the strong quantum confinement [8,9], and the remaining broadening arises from relatively slow fluctuations of the environment leading to spectral fluctuations [10]. Transform-limited lines have not been routinely achieved, with typical optical linewidths a factor of at least 2 or 3 above the theoretical limit [10-13]. While spectral fluctuations in self-assembled quantum dots have been investigated with non-resonant excitation [14,15], their origin in the case of true resonant excitation is not known with any precision and are potentially complex with contributions from a number of sources. Furthermore, spectral fluctuations are a common feature in condensed matter systems, arising also in diamond [16], semiconductor nanocrystals [17] and nanowires [18].We report new insights into local charge fluctuations in a semiconductor. High resolution laser spectroscopy on a single quantum dot is used as an ultra-sensitive sensor of the local environment. We observe single charge fluctuations in the occupation of a small number of defects located within ∼ 100 nm of the quantum dot. We control the occupation of these close-by defects with an additional non-resonant excitation. Once the defects are fully occupied, t...
We report the realization of a spatial and spectrally tunable air-gap Fabry-Pérot type microcavity of high finesse and cubic-wavelength-scale mode volume. These properties are attractive in the fields of opto-mechanics, quantum sensing and foremost cavity quantum electrodymanics. The major design feature is a miniaturized concave mirror with atomically smooth surface and radius of curvature as low as 10 µm produced by CO 2 laser ablation of fused silica. We demonstrate excellent mode-matching of a focussed laser beam to the microcavity mode and confirm from the frequencies of the resonator modes that the effective optical radius matches the physical radius. With these small radii, we demonstrate sub-wavelength beam waists. We also show that the microcavity is sufficiently rigid for practical applications: in a cryostat at 4 K, the root-mean-square microcavity length fluctuations are below 5 pm.
We investigate the strong coupling regime of a self-assembled quantum dot in a tunable microcavity with dark-field laser spectroscopy. The high quality of the spectra allows the lineshapes to be analyzed revealing subtle quantum interferences. Agreement with a model calculation is achieved only by including exciton dephasing which reduces the cooperativity from a bare value of 9.0 to the time-averaged value 5.5. In the pursuit of high cooperativity, besides a high-Q and low modevolume cavity, we demonstrate that equal efforts need to be taken towards lifetime-limited emitter linewidths.Cavity quantum electrodynamics (QED) involves an exchange of energy quanta between a single emitter and a cavity photon. The coupling ratehg = µ 12 E vac , depending on the emitter's dipole moment µ 12 and the vacuum electric field at the location of the emitter E vac , sets the relevant timescale of the coupled dynamics. If g is considerably smaller than the emitter relaxation rate γ or the cavity photon decay rate κ, on resonance the cavity mode acts as an additional decay channel to the emitter giving rise to an enhanced spontaneous emission rate (the Purcell effect of the weak coupling regime). If g is much larger than the energy loss rates, a coherent exchange of energy quanta takes place giving rise to new eigenstates, "polaritons", split in energy by 2hg (the strong coupling regime). The efficacy of the coherent coupling is commonly denoted by the cooperativity parameter C = 2g 2 /(κγ), the figure of merit for this work. The coherent exchange was first realized with single Cs atoms in a high finesse cavity [1].The strong coupling regime is a potentially powerful tool in quantum information processing [2], notably in quantum networks [3], since it enables for instance atomatom entanglement [4] or the distribution of quantum states [5]. Furthermore, strong coupling enables a nonlinear photon-photon interaction and hence the observation of photon blockade [6,7], a prerequisite for the creation of a single photon transistor [8,9].It is clearly desirable to implement cavity-QED in the solid-state as the solid-state host acts as a natural trap for the emitter. Furthermore, on-chip integration of multiple elements is feasible. As emitter, self-assembled quantum dots have desirable properties: high oscillator strength, narrow linewidths and weak phonon coupling [10]. As host, a semiconductor such as GaAs is very versatile: heterostructures can be realized; there is a wide array of post-growth processing techniques. Photoluminescence experiments on single InGaAs SAQD coupled to a photonic crystal cavity or a micropillar cavity revealed an anticrossing, the signature of the strong coupling regime [11][12][13]. For micropillars, recent experiments exhibit cooperativity values of around C 3 [14]. For photonic crystal cavities, a much higher C is achieved [15] but C is skewed by the fact that g γ yet g > ∼ κ.The photon decay rate κ at the emitter wavelength is relatively high in both geometries, limiting the cooperativity. In addition, ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.