Bringing Photonic Quantum Computing to Reality. 
Researchers in China “have demonstrated a way to map and measure large-scale photonic quantum correlation with single-photon sensitivity.”  This capability is critical for making photon-based quantum computing practical.

The new method is dubbed “correlation on spatially-mapped photon-level image or COSPLI.

“COSPLI has the potential to become a versatile solution for performing quantum particle measurements in large-scale photonic quantum computers,” said the research team leader Xian-Min Jin, from Shanghai Jiao Tong University, China. “This unique approach would also be useful for quantum simulation, quantum communication, quantum sensing and single-photon biomedical imaging.”

“One important challenge in the development of quantum computers is finding a way to measure and manipulate the thousands of qubits needed to process extremely large data sets.  For photon-based methods, the number of qubits can be increased without using more photons by increasing the number of modes encoded in photonic degrees of freedom— such as polarization, frequency, time and location — measured for each photon.  This allows each photon to exhibit more than two modes, or states, simultaneously.  The researchers previously used this approach to fabricate the world’s largest photonic quantum chips, which could possess a state space equivalent to thousands of qubits…However, incorporating the new photonic quantum chips into a quantum computer requires measuring all the modes and their photonic correlations at a single-photon level.  Until now, the only way to accomplish this would be to use one single-photon detector for each mode exhibited by each photon.  This would require thousands of single-photon detectors and cost around 12 million dollars for a single computer.”

Reference is found at The Optical Society…