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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

High-dimensional spatial entanglement observed with an electron multiplying CCD camera

D. S. Tasca, M. Edgar, Frauke Izdebski, Ryan E. Warburton, Jonathan Leach, Megan Agnew, Gerald S. Buller, Robert W. Boyd, Miles J. Padgett

Published Nov 19, 2012• 0 citations
Publisher

Abstract

Using an electron multiplying CCD camera we observe both image plane (position) and far field (momentum) correlations between photon pairs produced from spontaneous parametric down-conversion when using a 201 x 201 bi-dimensional array of pixels and a flux of around 0.02 photons/pixel. After background subtraction we characterize the strength of signal and idler correlations in both transverse dimensions by applying entanglement and EPR criteria, showing good agreement with the theoretical predictions. The application of such devices in quantum optics could have a wide range, including quantum computation with spatial degrees of freedom of single photons.

DOI
10.1117/12.981414

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Using an electron multiplying CCD camera we observe both image plane (position) and far field (momentum) correlations between photon pairs produced from spontaneous parametric down-conversion when using a 201 x 201 bi-dimensional array of pixels and a flux of around 0.02 photons/pixel. After background subtraction we characterize the strength of signal and idler correlations in both transverse dimensions by applying entanglement and EPR criteria, showing good agreement with the theoretical predictions. The application of such devices in quantum optics could have a wide range, including quantum computation with spatial degrees of freedom of single photons.
The light produced by parametric down-conversion shows strong spatial entanglement that leads to violations of EPR criteria for separability. Historically, such studies have been performed by scanning a single-element, single-photon detector across a detection plane. Here we show that modern electron-multiplying charge-coupled device cameras can measure correlations in both position and momentum across a multi-pixel field of view. This capability allows us to observe entanglement of around 2,500 spatial states and demonstrate Einstein–Podolsky–Rosen type correlations by more than two orders of magnitude. More generally, our work shows that cameras can lead to important new capabilities in quantum optics and quantum information science. Measuring the entanglement between down-converted photons is central to many quantum optical experiments, and is normally performed by scanning detectors stepwise across a plane. Edgaret al. use a CCD camera to measure the entire entangled light field, finding strong correlations in position and momentum.
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.
Cameras with single-photon sensitivities can be used to measure the spatial correlations between the photon-pairs that are produced by parametric down-conversion. Even when pumped by a single-mode laser, the signal and idler photons are typically distributed over several thousand spatial modes yet strongly correlated with each other in their position and anti-correlated in their transverse momentum. These spatial correlations enable applications in imaging, sensing, communication, and optical processing. Here we show that, using a photon-number resolving camera, spatial correlations can be observed after only a few 10s of seconds of measurement time, thereby demonstrating comparable performance with previous single photon sensitive camera technologies but with the additional capability to resolve photon-number. Consequently, these photon-number resolving technologies are likely to find wide use in quantum, low-light, imaging systems.
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.
We demonstrate EPR entanglement by detecting purely spatial quantum correlations in the near and far-field of spontaneous parametric down conversion generated in a type II BBO crystal. Full field imaging is performed in the photon counting regime with an electron-multiplying CCD (EMCCD) camera. The data are used without any post-selection and we obtain a violation of Heisenberg inequalities with inferred quantities in both transverse dimensions taking into account all the biphoton pairs in both the near and far field, by integration on the entire two dimensional transverse planes. This ensures a rigorous demonstration of the EPR paradox in its original position-momentum form.