Massively Parallel Coincidence Counting of High-Dimensional Entangled States
Quantum entangled states of light are essential for quantum technologies and fundamental tests of physics. While quantum information science has relied on systems with entanglement in 2D degrees of freedom, e.g. quantum bits with polarization states, the field is moving towards ever-higher dimensions of entanglement. Increasing the dimensionality enhances the channel capacity and security of quantum communication protocols, gives rise to exponential speed-up of quantum computation, and is necessary for quantum imaging. Yet, characterization of even bipartite quantum states of high-dimensional entanglement remains a prohibitively time-consuming challenge, as the dimensionality of the joint Hilbert space scales quadratically with the number of modes. Here, we develop and experimentally demonstrate a new, more complete theory of detection in CCD cameras for rapid measurement of the full joint probability distribution of high-dimensional quantum states. The theory spans the intensity range from low photon count to saturation of the detector, while the massive parallelization inherent in the pixel array makes measurements scale favorably with dimensionality. The results accurately account for partial detection and electronic noise, resolve the paradox of ignoring two-photon detection in a single pixel despite collinear spatial entanglement, and reveal the full Hilbert space for exploration. For example, use of a megapixel array allows measurement of a joint Hilbert space of 1012 dimensions, with a speed-up of nearly four orders of magnitude over traditional methods. We demonstrate the method with pairs, but it generalizes readily to arbitrary numbers of entangled photons. The technique uses standard geometry with existing technology, thus removing barriers of entry to quantum imaging experiments, and open previously inaccessible regimes of high-dimensional quantum optics.