Using a scheme based on quantum mechanics called unambiguous state discrimination (USD), the lowest error rate yet was achieved for a photodetector deciphering a four-fold phase encoding of information. The fundamental probabilistic nature of light makes it impossible to perfectly distinguish light from dark at very low intensity. Low power and high fidelity in reading data is especially important for transmitting and processing quantum information for secure communications and quantum computation; to facilitate such quantum capabilities, it is crucial for a detector that sees well in the dark. The photodetection system demonstrated at the Joint Quantum Institute can make highly accurate readings of incoming information at the single-photon level by allowing the detector to not give a conclusive answer in some instances. Instead of the traditional two states — zeros and ones — the researchers used a more sophisticated data encoding scheme of four states — 0, 1, 2 and 3. These states correspond to four different phases of the light pulse, but with some overlap, which can produce ambiguity when trying to distinguish from which state you have received information, the investigators say. This inherent overlap means that the measurement system can sometimes provide an incorrect answer. By implementing a USD of four-fold phase-encoded states, Alan Migdall and colleagues were able to eliminate all but one possible value for the input state, achieving an error rate four times lower than what is possible with conventional measurement techniques. Previous measurements were done using a minimum error discrimination (MED) scheme. A scheme devised by scientists at the Joint Quantum Institute for carrying out unambiguous state discrimination (USD). Inset (i) shows the four nonorthogonal symmetric coherent states with phases equal to Φ = {0, π/2, π, 3π/2}. The state under measurement, |ai>, has vertical (V) polarization, and the phase reference, |LO>, has horizontal (H) polarization. The pulse is distributed among four elimination stages using mirrors (M) and beamsplitters (BS). Each elimination stage uses phase shifters (PS), a polarizer (Pol) and a single-photon detector (SPD) to eliminate one possibility for the phase of the input state |ai>. Courtesy of JQI. “The former is a technique that always gets an answer, albeit with some probability of being mistaken, while the latter is designed to get answers that in principle are never wrong, but at the expense of sometimes getting an answer that is the equivalent of ‘don’t know,’” Migdall said. The USD scheme makes a series of measurements that rule out each state. By process of elimination, you can figure out at which state the information was received. However, sometimes inconclusive results are possible; for example, when the measurements eliminate less than three of the four possibilities. The approach will be useful for quantum information processing, quantum communications with many states, and fundamental quantum measurement studies at low-light levels, the investigators say. The study appeared in Nature Communications (doi: 10.1038/ncomms3028). For more information, visit: http://jqi.umd.edu