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arXiv:2312.04184 · Optica Quantum

Multifold enhancement of quantum SNR by using an EMCCD as a photon number resolving device

Rounak Chatterjee, Vikas S. Bhat, Kiran Bajar, Sushil A. Mujumdar

Published Apr 18, 2024• 12 citations
arXivHTMLPDF

Abstract

Electron multiplying charge-coupled devices (EMCCDs), owing to their high quantum efficiency and spatial resolution, are widely used to study typical quantum optical phenomena and related applications. Researchers have already developed a procedure that enables one to statistically determine whether a pixel detects a single photon, based on whether its output is higher or lower than the estimated noise level. However, these techniques are feasible at extremely low photon numbers (≈0.15 mean number of photons per pixel per exposure), allowing for at most one photon per pixel. This limitation necessitates a very large number of frames required for any study. In this work, we present a method to estimate the mean rate of photons per pixel per frame for arbitrary exposure time. Subsequently, we make a statistical estimate of the number of photons (≥ 1) incident on each pixel. This allows us to effectively use the EMCCD as a photon number resolving device. This immediately augments the acceptable light levels in the experiments, leading to significant reduction in the required experimentation time. As evidence of our approach, we quantify contrast in quantum correlation exhibited by a pair of spatially entangled photons generated by a spontaneous parametric down conversion process. In comparison with conventional methods, our method realizes an enhancement in the signal-to-noise ratio (SNR) by approximately a factor of 3 for half the data collection time. This SNR can be easily enhanced by minor modifications in experimental parameters such as exposure time, etc.

DOI
10.1364/opticaq.518037

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Electron multiplying charge-coupled devices (EMCCDs), owing to their high quantum efficiency and spatial resolution, are widely used to study typical quantum optical phenomena and related applications. Researchers have already developed a procedure that enables one to statistically determine whether a pixel detects a single photon, based on whether its output is higher or lower than the estimated noise level. However, these techniques are feasible at extremely low photon numbers (≈0.15 mean number of photons per pixel per exposure), allowing for at most one photon per pixel. This limitation necessitates a very large number of frames required for any study. In this work, we present a method to estimate the mean rate of photons per pixel per frame for arbitrary exposure time. Subsequently, we make a statistical estimate of the number of photons (≥ 1) incident on each pixel. This allows us to effectively use the EMCCD as a photon number resolving device. This immediately augments the acceptable light levels in the experiments, leading to significant reduction in the required experimentation time. As evidence of our approach, we quantify contrast in quantum correlation exhibited by a pair of spatially entangled photons generated by a spontaneous parametric down conversion process. In comparison with conventional methods, our method realizes an enhancement in the signal-to-noise ratio (SNR) by approximately a factor of 3 for half the data collection time. This SNR can be easily enhanced by minor modifications in experimental parameters such as exposure time, etc.
Electron multiplying charge coupled device (EMCCD) cameras are widely used for single-photon detection in various quantum optics applications. For example, these EMCCD cameras have been extensively used to measure the position and momentum correlations between photon pairs produced by parametric down-conversion in the image and far-field planes of the down-conversion crystal, respectively. The camera's operating parameters are typically set to balance a high quantum efficiency while simultaneously maintaining a low number of erroneous photon detections, the latter being mainly attributable to clock-induced charge (CIC) events in the readout register. The visibility of the measured correlations is an indication of the quantum efficiency of detecting the photon pairs versus the prevalence of the unwanted CIC. In this work, we show that this correlation visibility can be enhanced by operating the EMCCD camera with the electron multiplying (EM) gain set well in excess of its standard operating conditions. This enhancement will be of interest to those using EMCCD detectors in quantum imaging, sensing and processing applications.
The spatial correlations between the photon pairs produced by spontaneous parametric down conversion (SPDC) have been investigated in a number of different configurations over the last two decades (1). The high dimensionality of the transverse spatial degrees of freedom (DOF) of the photon pairs can be explored either by projections onto a discrete (infinite) basis, such as the LG modes, or by using a continuous basis defined by the transverse position or momentum of the photons. For instance, it has been shown that their transverse position and momentum display EPR-type correlations (2). Despite the continuous and unbounded nature of both these bases, the most common measurements to date have relied upon the scanning of a single avalanche photodiode or a very small number of individual detectors. This sequential scanning or small number of detectors negates any information capacity advantage in the use of spatial states. Recent years have seen a rapid advance in imaging technologies, particularly for low light levels. Of these technologies the highest quantum efficiency is achieved using electron multiplying CCDs (EMCCD). Developments in low noise EMCCD cameras suggest they are capable of greater than 80% quantum efficiencies and if operated in an ultra-low light environment whilst being maintained at low temperatures, can provide single photon sensitivity (3). EMCCDs have been used in measurements of sub-shot-noise correlations of intensity fluctuations (4, 5) as well as in intensity correlations measurements (6) in the far field of the photon pairs from SPDC. Here we report the observation of spatial correlations from SPDC using an EMCCD camera in both the image and far-field planes of the down conversion crystal. We pump a BBO crystal at 355nm with a few milli-Watts of optical power and use a simple lens system to relay the down-converted light to an EMCCD array which can be positioned in either the image plane or the far-field of the crystal. Our photon flux is around 1/100 per pixel, which approximately doubles the signal obtained when the light is blocked. We typically record 100,000 individual frames and then sum the auto-correlation of the individual frames. We observed intensity correlation and anti-correlation in the image and far-field planes, respectively. In both cases we make the correlations more visible by subtracting a background which is obtained as the average correlation obtained between successive frames. Using this approach we are able to visualize the strength of the correlations in both position and momentum, showing a variance product nearly one order of magnitude below the classical limit in both x and y directions.
The observation of spatial quantum noise reduction, or spatial squeezing, with a large number of photons can lead to a significant advantage in quantum imaging and quantum metrology due to the scaling of the signal-to-noise ratio with the number of photons. Here we present a systematic study of the limiting factors that play a role on the measurement of spatial squeezing with an electron-multiplying charge-coupled device (EMCCD) camera in the limit of bright quantum states of light generated with a four-wave mixing process in an atomic vapor cell. We detect a total number of photons per beam of the order 108{10}^{8}108 in $1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}$ pulses, which corresponds to a photon flux per beam of the order of 1014{10}^{14}1014 photons per second. We then investigate the role of different parameters, such as cell temperature, pump power, laser detunings, scattered pump background noise, and timing sequences for the image acquisition with the EMCCD camera, on the level of spatial squeezing. We identify critical parameters to obtain an optimum squeezing level and demonstrate that for bright beams it is essential to acquire images at a rate fast enough to overcome the effect of classical technical noise.
The ability to engineer the properties of quantum optical states is essential for quantum information processing applications. Here, we demonstrate tunable control of spatial correlations between photon pairs produced by spontaneous parametric down-conversion, and measure them using an electron multiplying charge coupled device (EMCCD) camera. By shaping the spatial pump beam profile in a type-I collinear configuration, we tailor the spatial structure of coincidences between photon pairs entangled in high dimensions without effect on intensity. The results highlight fundamental aspects of spatial coherence and hold potential for the development of quantum technologies based on high-dimensional spatial entanglement.
use of any of the information contained in it must acknowledge this thesis as the source of the quotation or information. This thesis investigates the measurement of spatial correlations of photon pairs gener-ated through spontaneous parametric down-conversion with single-photon sensitive multi-pixel detectors. A custom designed and fabricated 8×1 fibre array detector for time to position multi-plexing was characterised. This detector was then commissioned in an experiment measuring the spatial correlations of photon pairs in position, momentum and in-termediate bases. The fibre array measured eight positions simultaneously with one Single-Photon Avalanche Diode, which led to an eight-fold increase in the data ac-quisition rate compared to traditional experiments, where a single SPAD was scanned across the detection plane. To capture all of the emitted light, an electron-multiplying CCD (EMCCD) camera was used. The spatial correlations were measured for the first time in momentum and pos-
Spontaneous down conversion (SPDC) occurs in a nonlinear crystal when a pump photon splits in a pair of signal and idler photons. This paper demonstrates detection of single photons in low light level images with an electron multiplying CCD camera (EMCCD). The opposite spatial fluctuations of SPDC radiation are correlated in the quantum regime. Here, correlations between pixels are presented for collinear phase matching. The far field image of the parametric fluorescence is formed in the focal plane of a lens on a back-illuminated EMCCD. A thresholding procedure is applied on each image, in order to decide whether there is one or zero photo-electron on each pixel. We have experimentally demonstrated that spatial SPDC fluctuations between opposite pixels are quantum correlated in the far field.