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New Detector Reads Photons Better

Photonics.com
Jan 2013
COLLEGE PARK, Md., Jan. 7, 2013 —  A photodetector developed at the Joint Quantum Institute (JQI) has established a new standard for reading quantum information with a minimum of uncertainty, beating the previous limit by a factor of four.

Created by Francisco Becerra and carried out in the lab of Alan Migdall, the device circumvents a previous communications theory by viewing light pulses with an adaptive network of detectors with feedback, instead of with a single passive detector.

Digital data at its most basic remains in binary form, assigning a value of 0 or 1, but for communicating more data in the phase of a pulse, or phase-shift keying, introducing higher number alphabets can lead to errors and uncertainty. A detector does not just unequivocally measure a 0 or a 1; the reading process is imperfect. And even worse, the state of the light pulse is inherently uncertain, a significant problem when the light pulses belong to a set of overlapping states.


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 four in determining the phase of an incoming signal. 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 and without any of the imperfections that would be encountered in any realistic implementation, and a hypothetical ultimate-limit on uncertainty derived by Helstrom. Courtesy of NIST.

A communications theory developed decades ago established a minimal uncertainty for the accurate transmission and detection of information encoded in overlapping states. The hypothetical minimal detection error using conventional schemes — called the standard quantum limit’ — depends on how many photons of light comprise the signal, how many levels must be read out and which physical property of light is used to encode the information, such as the phase. In the 1970s, scientists began questioning whether this standard quantum limit could be circumvented.

The JQI researchers did exactly that by using an active detection process involving a series of stages. At each stage, the current light signal strikes a partially-silvered mirror, which peels off a fraction of the pulse for analysis, while the rest goes on to subsequent stages. At each stage, the signal is combined with a separate reference oscillator wave used as a phase reference against which the signal phase is determined. This is done by shifting the reference wave by a known amount and letting it interfere with the signal wave at the beamsplitter. By altering that known shift, the interference pattern can reveal something about the phase of the input pulse.

By combining many such stages and using information gained from previous stages to adjust the phase of the reference wave in successive stages, a better estimate of the signal phase can be obtained.

This adaptive manner of phase detection, implemented in a feedback processr, enabled the JQI system to beat the standard quantum limit for a set of four states encoding information as a phase. Previous detection below the quantum limit was for a very narrow range of photons and with only a two-state protocol and only slightly below the standard quantum limit.

The findings appeared in Nature Photonics (doi:10.1038/nphoton.2012.316). 

For more information, visit: www.jqi.umd.edu


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