Single-Photon Sources Can Beat the Limits of Light Measurement Precision

Scientists at the University of Torino, led by G. Gavello, have developed a theoretical model demonstrating that continuously excited two-level single-photon sources can, under specific conditions, generate nonclassical light exhibiting sub-Poissonian statistics. The research challenges the common assumption that such sources invariably emit light following Poissonian statistics, which are characteristic of classical light and limit the precision of optical measurements. Sub-Poissonian light, possessing reduced photon number fluctuations, offers a pathway to surpass the shot-noise limit, a fundamental barrier to sensitivity in classical optics, and potentially enhance the performance of various optical technologies. The team’s modelling also incorporates the effects of realistic detector limitations, such as efficiency and dead time, providing a more comprehensive understanding of the feasibility of observing these quantum effects.

Sub-Poissonian emission enabled by balanced excitation and decay rates

Theoretical modelling reveals that continuously driven single-photon sources can achieve a photon statistics ratio, often denoted as g⁽²⁾(0), of 0.5, representing a significant deviation from the classical limit and a substantial improvement over the previously assumed lower bound of 1 for Poissonian emission. The g⁽²⁾(0) parameter quantifies the degree of photon bunching or antibunching; a value of 1 indicates Poissonian statistics, values greater than 1 indicate bunching, and values less than 1 indicate antibunching, characteristic of sub-Poissonian light. This breakthrough demonstrates that sub-Poissonian statistics, crucial for surpassing classical limits in optical measurements, are indeed possible even with constant excitation, a scenario previously considered unfavourable for generating nonclassical light. It challenges long-held assumptions about single-photon sources and opens avenues for enhanced sensitivity in technologies such as quantum imaging and metrology, where precise photon counting is paramount. The ability to generate sub-Poissonian light from a continuously driven source simplifies experimental setups compared to those relying on pulsed excitation, potentially facilitating wider adoption.

The model treats both excitation and decay processes as stochastic processes, identifying a specific regime where balanced rates, where the rate of excitation equals the rate of decay, yield nonclassical light emission. This balance isn’t simply about equal rates at a single point in time, but rather a dynamic equilibrium maintained throughout the continuous excitation process. Further analysis accounted for practical limitations, including detector efficiency (the probability that a photon is detected) and dead time (the period after a detection during which the detector is unable to register another photon), to assess their impact on observing these quantum effects. These findings build upon earlier work establishing the potential of sub-shot-noise quantum imaging, which leverages quantum correlations to improve image resolution and signal-to-noise ratio, alongside native nitrogen-vacancy centres in diamond as promising single-photon emitters due to their relatively long coherence times and efficient emission at telecommunication wavelengths.

Detailed modelling revealed that comparable rates for excitation and decay are key to this improvement, resulting in nonclassical light emission. The underlying physics involves a reduction in the probability of multiple photon emissions within a given time window, leading to the observed sub-Poissonian statistics. Even with detector inefficiencies and ‘dead time’ preventing the recording of closely spaced photons, the sub-Poissonian behaviour remains observable, as analysis extended to include realistic imperfections. The extent to which these imperfections degrade the observed sub-Poissonian statistics depends on the specific values of detector efficiency and dead time, highlighting the importance of employing high-performance detectors. However, current calculations assume ideal conditions regarding the two-level system itself and do not yet demonstrate a clear pathway to building practical, robust devices capable of consistently generating these effects outside a laboratory setting. The characteristic radiative lifetime of the excited state, represented by 1/τ, governs this behaviour and warrants further investigation, particularly concerning its influence on the balance between excitation and decay rates and the resulting photon statistics.

Continuous excitation enables sub-Poissonian emission from single-photon sources despite detector

Dr. Dirk Englund and Dr. Peter Kok are actively refining techniques to overcome the limitations of traditional optical measurements, seeking to enhance precision beyond what is achievable with conventional light sources. Sub-Poissonian light, exhibiting reduced fluctuations in photon number, offers a clear path forward, but realising this potential with single-photon sources under continuous excitation remains a persistent challenge due to the inherent trade-offs between excitation rate, decay rate, and detector performance. This work demonstrates a theoretical possibility, while practical implementation hinges on overcoming detector limitations, specifically the temporary unresponsiveness of detectors after registering a photon, which can distort the observed photon statistics. Improving detector technology, particularly reducing dead time and increasing efficiency, is therefore crucial for validating these theoretical predictions experimentally.

Enhanced sensitivity in imaging and sensing applications, including biomedical imaging, remote sensing, and materials characterisation, may eventually be realised with improved detectors and optimised single-photon sources. Acknowledging detector limitations is vital, yet this theoretical work establishes a fundamental possibility often overlooked in the field, suggesting that continuous excitation is not necessarily detrimental to achieving sub-Poissonian emission. Demonstrating sub-Poissonian emission from continuously driven single-photon sources, even in principle, broadens the scope for advanced quantum technologies and provides a new avenue for exploring fundamental quantum phenomena. The implications of this finding extend to the development of more sensitive quantum sensors and imaging systems, potentially revolutionising fields reliant on precise optical measurements, and could contribute to advancements in quantum cryptography and secure communication protocols.

The research demonstrated that photon emission from a continuously driven two-level single-photon source can exhibit sub-Poissonian statistics when excitation and decay rates are comparable. This finding challenges the common assumption that continuous excitation inherently leads to Poissonian statistics, opening up possibilities for surpassing the fundamental limits of classical optical measurements. The study also highlights how practical limitations of detectors, such as finite efficiency and dead time, can influence the observation of these nonclassical effects. Researchers are currently refining techniques to address these detector limitations and experimentally validate the theoretical predictions.