Reading Photons In Quantum Information Enters A New Phase
January 7, 2013

Active Detection Process Produces More Accurate Quantum Information Readings

redOrbit Staff & Wire Reports - Your Universe Online

A new device developed by researchers at the Joint Quantum Institute (JQI) uses an active detection process to establish a new, more accurate standard for reading quantum information.

"Decades ago communications theory established a minimal uncertainty for the accurate transmission and detection of information encoded in overlapping states. The hypothetical minimal detection error using conventional schemes is called the standard quantum limit and it depends on things like how many photons of light comprise the signal, how many levels (binary, quaternary, etc.) need to be read out, and which physical property of light is used to encode the information, such as the phase," the JQI researchers explained in a statement on Sunday.

However, approximately four decades ago, physicist Carl W. Helstrom first suggested that this so-called standard quantum limit (SQL) could be bypassed -- a feat that has now been achieved by scientists at the Institute, which is jointly operated by the National Institute of Standards (NIST).

The JQI team was able to accomplish the task by using a multi-stage active detection process instead of just a lone, passive photon detector. During each stage, a light signal hits a mirror that is partially silver, which removes a tiny portion of the pulse for analysis while the rest of it continues on.

"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," the researchers said.

By combining several of these types of stages, as well as using data obtained from each previous stage to adjust the reference wave's phase going forward, scientists are able to obtain a better estimate of the signal phase, they explained. In short, they essentially reduce the error rate and increase the precision of the reading.

"Detecting phase in this adaptive way, and implemented in a feedback manner, the JQI system is able to beat the standard quantum limit for a set of 4 states (quaternary) encoding information as a phase," the researchers said, further explaining that "the JQI photon receiver features an error rate four times lower than perfect conventional receivers, over a wide range of photon number, and with discrimination for four states."

"The only previous detection below the quantum limit was for a very narrow range of photons and with only a 2-state protocol and only slightly below the SQL," they added. Their work is fully explained in the latest edition of the journal Nature Photonics.