Contact: Phillip F. Schewe

pschewe@umd.edu

301-405-0989

Joint Quantum Institute

## JQI Photodetector Beats the Quantum Limit

**Caption:** The error rate is plotted as a function of the mean number of photons used to deliver the information. The standard quantum limit (SQL) is the red line. The light gray line is the SQL line if you take into account that individual photon detector stages used were ~72 percent efficient rather than 100 percent (with the detector efficiencies being 84 percent. In the business of detecting single photons, 84 percent is top of the line.) The error probabilities measured for the system (black points with error bars) fall well below the quantum limit, by about 6 decibels in the center of the curve. This is equivalent to saying that the JQI receiver is performing better than the SQL by a factor of about 4 in determining the phase of an incoming signal. That is, the JQI receiver achieves an error probability that is 4 times lower than the so-called "Standard Quantum Limit." This graph shows results for a system that implements 10 adaptive measurements. The two other lines on the chart show what the expected uncertainty would be for a perfect system (100 percent efficient detectors) and without any of the imperfections that would be encountered in any realistic implementation, and a hypothetical ultimate-limit on uncertainty derived by Helstrom.

**Credit:** NIST

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