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arXiv:2408.01799

Signatures of superradiance in intensity correlation measurements in a two-emitter solid-state system

Madhura Ghosh Dastidar, Aprameyan Desikan, Gniewomir Sarbicki, Vidya Praveen Bhallamudi

Published Aug 3, 2024• 0 citations
arXivHTMLPDF

Abstract

We perform intensity correlation ($g^{(2)}(τ)$) measurements on nitrogen-vacancy (NV) emitters embedded in diamond nanopillars. We observe an increase in transition rates from both the singlet and triplet states by a factor of $\approx 6$, indicating cooperative effects between the multiple emitters in the pillar, at room temperature. We simultaneously observe a $g^{(2)}(0) > 0.5 (\to 1$) as opposed to $g^{(2)}(0) < 0.5$ for others (and as expected for single emitters), indicating the presence of at least two emitters. Furthermore, we observe a triple exponential behaviour for the $g^{(2)}$ in contrast to the standard double exponential behaviour seen for single NV emitters. To understand our experimental observations, we developed a theoretical model. We solve the Lindblad master equation, tailored for single and two NV centers, to study their dissipative dynamics when coupled to a common electromagnetic field, at a finite temperature. Through this, we identify superradiant emission from a two-emitter system as the most likely explanation for our observed data. We also find that random number generation using the coupled emitter system performs better under the NIST test suite and explain it in terms of an entropy-driven model for a coupled emitter system. Our results provide a new signature for multiphotonic states, such as superradiant states, using intensity correlation measurements, that will become important for quantum photonic technologies progress.

DOI
10.48550/arxiv.2408.01799

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We perform intensity correlation (g(2)(τ)g^{(2)}(τ)g(2)(τ)) measurements on nitrogen-vacancy (NV) emitters embedded in diamond nanopillars. We observe an increase in transition rates from both the singlet and triplet states by a factor of ≈6\approx 6≈6, indicating cooperative effects between the multiple emitters in the pillar, at room temperature. We simultaneously observe a g(2)(0)>0.5(→1g^{(2)}(0) > 0.5 (\to 1g(2)(0)>0.5(→1) as opposed to g(2)(0)<0.5g^{(2)}(0) < 0.5g(2)(0)<0.5 for others (and as expected for single emitters), indicating the presence of at least two emitters. Furthermore, we observe a triple exponential behaviour for the g(2)g^{(2)}g(2) in contrast to the standard double exponential behaviour seen for single NV emitters. To understand our experimental observations, we developed a theoretical model. We solve the Lindblad master equation, tailored for single and two NV centers, to study their dissipative dynamics when coupled to a common electromagnetic field, at a finite temperature. Through this, we identify superradiant emission from a two-emitter system as the most likely explanation for our observed data. We also find that random number generation using the coupled emitter system performs better under the NIST test suite and explain it in terms of an entropy-driven model for a coupled emitter system. Our results provide a new signature for multiphotonic states, such as superradiant states, using intensity correlation measurements, that will become important for quantum photonic technologies progress.
Abstract Superradiance (SR) is a cooperative phenomenon which occurs when an ensemble of quantum emitters couples collectively to a mode of the electromagnetic field as a single, massive dipole that radiates photons at an enhanced rate. Previous studies on solid-state systems either reported SR from sizeable crystals with at least one spatial dimension much larger than the wavelength of the light and/or only close to liquid-helium temperatures. Here, we report the observation of room-temperature superradiance from single, highly luminescent diamond nanocrystals with spatial dimensions much smaller than the wavelength of light, and each containing a large number (~ 10 3 ) of embedded nitrogen-vacancy (NV) centres. The results pave the way towards a systematic study of SR in a well-controlled, solid-state quantum system at room temperature.
Nitrogen-vacancy (NV) centers in diamond have been successfully coupled to various optical structures to enhance their radiation by the Purcell effect. The participation of many NV centers in these studies may naturally lead to cooperative emission and superradiance, and our recent experimental study with a diamond membrane in a fiber-based ultranarrow optical cavity demonstrated nonlinear radiation and fast photon bunching as signatures of the collective effects. In this theoretical article, we go beyond the simple model used in the previous study to address more phenomena, such as the appearance of bunching shoulders in second-order correlation function, Rabi splitting in steady-state spectrum, and population dynamics on excited Dicke states, which for moderate pumping explains the observed collective effects. Overall, our results can guide further experiments with NV centers, and they are also relevant for other solid-state color centers, such as silicon-vacancy centers in diamond and silicon carbide, boron-vacancy centers, and carbon-related centers in hexagonal boron nitride.
The rapid growth of quantum technologies has driven the development of quantum defects for both single photon sources as well as for spin qubit and sensor applications. These artificial atoms in wide bandgap solid-state materials such as diamond and hBN offer the possibility of a truly quantum deterministic source. These single photon emitting quantum defects exhibit antibunching that can be quantified by performing the Hanbury-Brown Twiss (HBT) experiment. The HBT experiment allows one to evaluate the second-order autocorrelation (g(2)(τ )) function, which is a central measure in determining the quality of single photon emission. To enhance the various emission parameters such as intensity, wavelength, and directionality from these defects the development of photonic structures become essential. Here, we study single photon emission from a diamond nanopillar array containing Nitrogen Vacancy (NV) centers. We spatially map the g(2)(τ ) function and radiative lifetimes in these samples and correlate the two quantities. Crucially, we observe that the measured antibunching varies as a function of position of the objective relative to the nanopillar, even across the same pillar. This likely shows the dependence of the apparent antibunching to the angle of excitation and detection in such photonic structures. Our results highlight the importance of structural and geometric factors in g(2)(τ ) measurements of emitters, especially when embedded in photonic structures. These should be carefully understood in evaluating the quality of the single photon sources.
Abstract Individual quantum emitters form a fundamental building block for emerging quantum technologies. Collective effects of emitters, such as superradiance, might improve the performance of applications even further. When scaling materials to larger sizes, however, the optical density of states is modified by the surrounding material, and the collective coupling in small domains might be covered by transitions to bulk properties due to the presence of multiple collectively emitting domains, which inhomogeneously add. Here, we probe the optical properties of nitrogen vacancy centers in agglomerates of nanodiamonds. We quantify the transition from individual emitters to bulk emission by fluorescence lifetime measurements, and find a transition to occur on a length scale of ∼ 3 wavelengths around the emitter. While our lifetime measurements are consistent with superradiant decay, the second-order correlation function, which is a standard measure to reveal collective properties, fails to probe collective effects for our case of an ensemble of collectively contributing domains to the emission. Therefore, we propose and apply a new measure to trace collective effects based on the intensity fluctuations of the emitted light. Our work points toward systematically studying collective effects in a scalable solid-state quantum system, and using them for quantum optical applications in agglomerates of highly-doped nanodiamonds.
When an ensemble of quantum emitters couples to a common radiation field, their polarizations can synchronize and a collective emission termed superfluorescence can occur. Entering this regime in a free-space setting requires a large number of emitters with a high spatial density as well as coherent optical transitions with small inhomogeneity. Here, we show that, by coupling nitrogen-vacancy centers in a diamond membrane to a high-finesse microcavity, also few, incoherent, inhomogeneous, and spatially separated emitters—as are typical for solid state systems—can enter the regime of collective emission. We observe a superlinear power dependence of the emission rate as a hallmark of collective emission. Furthermore, we find simultaneous photon bunching and antibunching on different timescales in the second-order autocorrelation function, revealing cavity-induced interference in the quantized emission from about 15 emitters. We develop theoretical models for mesoscopic emitter numbers to analyze the behavior in the Dicke state basis and find that the population of collective states together with cavity enhancement and filtering can explain the observations. Such a system has prospects for the generation of multiphoton quantum states, the preparation of entanglement in few-emitter systems, and enhancement of signals in quantum sensing. Published by the American Physical Society 2024
The photoluminescence of a single nickel–nitrogen NE8 color center in a diamond nanocrystal is investigated at room temperature under pulsed excitation by scanning confocal optical microscopy. Photon arrival times are analyzed in terms of the temporal intensity correlation function. Antibunching at short times and bunching behavior for longer times is observed, associated with sub- and super-Poissonian statistics, respectively. The behavior is interpreted by a ‘on–off’ state model, and transition rates between these two states are inferred from intensity correlation measurements realized at different excitation powers. NE8 color center photoluminescence quantum yield is evaluated to be 65%, a value higher than achieved for a nitrogen-vacancy color center, which is, up to now, the most practical single emitter to build a reliable single-photon source at room temperature.