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arXiv:2312.11090

Limits for coherent optical control of quantum emitters in layered materials

M. Koch, Vibhav Bharadwaj, Alexander Kubanek

Published Dec 18, 2023• 2 citations
arXivHTMLPDF

Abstract

The coherent control of a two-level system is among the most essential challenges in modern quantum optics. Understanding its fundamental limitations is crucial, also for the realization of next generation quantum devices. The quantum coherence of a two level system is fragile in particular, when the two levels are connected via an optical transition. When such quantum emitters are located in solids the coherence suffers from the interaction of the optical transition with the solid state environment, which requires the sample to be cooled to temperatures of a few Kelvin or below. Here, we use a mechanically isolated quantum emitter in hexagonal boron nitride to explore the individual mechanisms which affect the coherence of an optical transition under resonant drive. We operate the system at the threshold where the mechanical isolation collapses in order to study the onset and temperature-dependence of dephasing and independently of spectral diffusion. The new insights on the underlying physical decoherence mechanisms reveals a limit in temperature until which coherent driving of the system is possible. This study enables to increase the operation temperature of quantum devices, therefore reducing the need for cryogenic cooling.

DOI
10.48550/arxiv.2312.11090

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The coherent control of a two-level system is among the most essential challenges in modern quantum optics. Understanding its fundamental limitations is crucial, also for the realization of next generation quantum devices. The quantum coherence of a two level system is fragile in particular, when the two levels are connected via an optical transition. When such quantum emitters are located in solids the coherence suffers from the interaction of the optical transition with the solid state environment, which requires the sample to be cooled to temperatures of a few Kelvin or below. Here, we use a mechanically isolated quantum emitter in hexagonal boron nitride to explore the individual mechanisms which affect the coherence of an optical transition under resonant drive. We operate the system at the threshold where the mechanical isolation collapses in order to study the onset and temperature-dependence of dephasing and independently of spectral diffusion. The new insights on the underlying physical decoherence mechanisms reveals a limit in temperature until which coherent driving of the system is possible. This study enables to increase the operation temperature of quantum devices, therefore reducing the need for cryogenic cooling.
Abstract The coherent control of a two-level system is among the most essential challenges in modern quantum optics. Understanding its fundamental limitations is crucial, also for the realization of next generation quantum devices. The quantum coherence of a two-level system is fragile in particular, when the two levels are connected via an optical transition, which, at the same time, enables the manipulation of the system. When such quantum emitters are located in solids the coherence suffers from the interaction of the optical transition with the solid state environment, which requires the sample to be cooled to temperatures of a few Kelvin or below. Here, we use a mechanically isolated quantum emitter in hexagonal boron nitride to explore the individual mechanisms which affect the coherence of an optical transition under resonant drive. We operate the system at the threshold where the mechanical isolation collapses in order to study the onset and temperature-dependence of dephasing and independently of spectral diffusion. The insights on the underlying physical decoherence mechanisms reveal a limit in temperature until which coherent driving of the system is possible. This study enables to increase the operation temperature of hBN-based quantum devices, therefore reducing the need for cryogenic cooling.
Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional host for stable single-photon emission owing to its wide bandgap, high photostability, and compatibility with nanophotonic integration. We present a simulation-based study of temperature-dependent spectral dynamics and optical coherence in a mechanically decoupled quantum emitter in hBN. Employing a hybrid stochastic framework that combines Ornstein--Uhlenbeck detuning fluctuations with temperature-dependent, Gaussian-distributed discrete frequency jumps, motivated by experimentally observed spectral diffusion and blinking, we reproduce the measured evolution of inhomogeneous linewidth broadening and the progressive degradation of photon coherence across the relevant cryogenic range (5-30K). The model captures phonon-related spectral diffusion with a cubic temperature dependence and the onset of jump-like spectral instabilities at higher temperatures. By calibrating the hybrid diffusion, jump parameters to the experimentally measured full width at half maximum (FWHM) of the emission line and analyzing the second-order correlation function g(2)(τ)g^{(2)}(τ)g(2)(τ) under resonant driving, we establish a unified phenomenological description that links stochastic detuning dynamics to the decay of optical coherence in a resonantly driven emitter. Analysis of g(2)(τ)g^{(2)}(τ)g(2)(τ) under resonant driving reveals an additional dephasing rate γsd+jγ_{\mathrm{sd+j}}γsd+j​ that rises monotonically with temperature and drive strength, leading to a predicted critical crossover to overdamped dynamics at Tcrit≈25.91T_{\mathrm{crit}} \approx 25.91Tcrit​≈25.91~K. This hybrid framework provides a quantitative connection between accessible spectroscopic observables and the dominant noise mechanisms limiting coherent optical control in mechanically decoupled quantum emitters, exemplified in hBN and generalizable to similar emitters in other materials.
Single-photon sources are crucial components for building quantum networks and communications, but their optical coherent control remains a significant challenge in the development of quantum technologies. Recently, single-photon sources in atomically thin 2D materials such as transition metal dichalcogenides and van der Waals materials have emerged as promising solid-state quantum light emitters [1]. Hexagonal boron nitride (hBN) is a 2D insulator, which hosts optically active states in its band gap that efficiently emit single photons even at room temperature. The wide range of emission wavelengths, narrow emission lines, and convenient tunability of these hBN emitters make them particularly interesting for quantum sensing and wavelength division multiplexed quantum communications. They can also be found in commercially available nanocrystals, making them ideal for creating large arrays of single-photon emitters [2] and efficient coupling to photonic structures, such as 3D-printed microlenses [3].
Coherent generation of indistinguishable single photons is crucial for many quantum communication and processing protocols. Solid-state realizations of two-level atomic transitions or three-level spin-Λ systems offer significant advantages over their atomic counterparts for this purpose, albeit decoherence can arise due to environmental couplings. One popular approach to mitigate dephasing is to operate in the weak-excitation limit, where the excited-state population is minimal and coherently scattered photons dominate over incoherent emission. Here we probe the coherence of photons produced using two-level and spin-Λ solid-state systems. We observe that the coupling of the atomiclike transitions to the vibronic transitions of the crystal lattice is independent of the driving strength, even for detuned excitation using the spin-Λ configuration. We apply a polaron master equation to capture the non-Markovian dynamics of the vibrational manifolds. These results provide insight into the fundamental limitations to photon coherence from solid-state quantum emitters.
Single-photon emitters in the solid state are subject to spectral broadening processes known as spectral diffusion and pure dephasing. Here, the authors show that a combination of quantum optics techniques, namely coherent laser drive and time-resolved photon correlations, can bring both processes to light in a single experimental protocol. This allows an extensive characterization of a large variety of solid-state quantum emitters. Their method is experimentally applied to a quantum emitter in the 2D material hexagonal boron nitride, further establishing this system as an appealing quantum photonics platform.
Single-photon emitters in solid-state systems are important building blocks for scalable quantum technologies. Recently, quantum light emitters have been discovered in the wide-gap van der Waals insulator hexagonal boron nitride (hBN). These color centers have attracted considerable attention due to their quantum performance at elevated temperatures and wide range of transition energies. Here, we demonstrate coherent state manipulation of a single hBN color center with ultrafast laser pulses and investigate in our joint experiment–theory study the coupling between the electronic system and phonons. We demonstrate that coherent control can be performed not only resonantly on the optical transition giving access to the decoherence but also phonon-assisted, which reveals the internal phonon quantum dynamics. In the case of optical phonons, we measure their decoherence, stemming in part from their anharmonic decay. Dephasing induced by the creation of acoustic phonons manifests as a rapid decrease in the coherent control signal when traveling phonon wave packets are emitted. Furthermore, we demonstrate that the quantum superposition between a phonon-assisted process and resonant excitation causes ultrafast oscillations of the coherent control signal. Our results pave the way for ultrafast phonon quantum state control on the nanoscale and open up a new promising perspective for hybrid quantum technologies.