Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

A new phase in reading photons

07.01.2013
A JQI photodetector beats the quantum limit by a factor of 4

"That's not what I meant": human communication is fraught with misinterpretation. Written out in longhand, words and letters can be misread. A telegraph clerk can mistake a dot for a dash. Noise will always be with us, but at least a new JQI (*) device has established a new standard for reading quantum information with a minimum of uncertainty.

Success has come by viewing light pulses not with a single passive detector with but an adaptive network of detectors with feedback. The work on JQI's new, more assured photonic protocol was led by Francisco Becerra and carried out in Alan Migdall's JQI lab. They report their results in Nature Photonics (**). Here are some things you need to know to appreciate this development.

HOW TO MODULATE?

Digital data, in its simplest form, can be read with a process called on-off keying: a detector senses the intensity of incoming bursts of electrons in wires or photons through fibers and assigns a value of 0 or 1. A more sophisticated approach to modulating a signal (not merely off/on) is to encode data in the phase of the pulse. In "phase-shift keying," information is encoded in the amount of phase shift imposed on a carrier wave; the phase of the wave is how far along the wave cycle you happen to be (say, at the top of a crest or the bottom of a trough in a sinusoidal, as in this figure).

WHAT KIND OF ALPHABET?

Larger words can be assembled from a small suite of symbols. The Roman alphabet has 26 letters, the Greek only 24. Binary logic, and most transistors, makes do with just a two-letter alphabet. Everything is a 0 or a 1, and larger numbers and letters and words are assembled from as many binary bits as are necessary. But what if we enlarged the alphabet from two to four? In quaternary logic more data can be conveyed in a single pulse. The cost of this increase is having to write and read 4 states of modulation (or 4 symbols). Even more efficient in terms of packing data, but correspondingly more difficult to implement, is logic based on 6 states, or 8, or any higher number. Digital data at its most basic---at the level of transistor---remains in binary form, but for communicating this data, higher number alphabets can be used. In fact, high-definition television delivery already involves high-level logic.

UNCERTAINTY

No matter what kind of logic is used, errors creep in. A detector doesn't 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, and that is a real problem when the light pulses belong to a set of overlapping states. This is illustrated in the figure below for binary and quaternary phase states.

On the left side of the figure, the measurement of the phase of a light pulse is depicted, where there are only two choices. Is the pulse in the alpha state or the –alpha state? Because the tails of one overlap the other there is a slight ambiguity that leads to uncertainty in which state a measurement indicates. On the right, four possible states are depicted on a complex-number graph (with real (Re) and imaginary (Im) axes). Here the overlap of the states is more complicated, but results in similar ambiguities of the measured states, seen mostly near the borders (decision threshold lines) between the states.

STANDARD QUANTUM LIMIT

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.

But starting in the 1970s with physicist Carl W. Helstrom, some scientists have felt that the standard quantum limit could be circumvented. The JQI researchers do exactly this by using not a single passive photo-detector, but 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 and 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.

ACTIVE DETECTION

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

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. These states are represented as fuzzy distributions arranged at different angles around a circle as seen in the figure above where the angles represent the phase of the light pulses.

The JQI noise-reduction achievement is depicted in the graph below. 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% efficient rather than 100% (with the detector efficiencies being 84%. In the business of detecting single photons, 84% 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% 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.

To conclude, 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.

(*)The Joint Quantum Institute (JQI) is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.

(**) "Experimental demonstration of a receiver beating the standard quantum limit for multiple nonorthogonal state discrimination," by F. E. Becerra, J. Fan, G. Baumgartner, J. Goldhar, J. T. Kosloski, and A. Migdall, Nature Photonics, published online 6 January 2013.

Alan Migdall, migdall@nist.gov, 301-975-2331

Press contact at JQI: Phillip F. Schewe, pschewe@umd.edu, 301-405-0989. http://jqi.umd.edu/

Phillip F. Schewe | EurekAlert!
Further information:
http://www.umd.edu

More articles from Physics and Astronomy:

nachricht Taking a spin on plasma space tornadoes with NASA observations
20.11.2017 | NASA/Goddard Space Flight Center

nachricht NASA detects solar flare pulses at Sun and Earth
17.11.2017 | NASA/Goddard Space Flight Center

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

Im Focus: Researchers Develop Data Bus for Quantum Computer

The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...

Im Focus: Wrinkles give heat a jolt in pillared graphene

Rice University researchers test 3-D carbon nanostructures' thermal transport abilities

Pillared graphene would transfer heat better if the theoretical material had a few asymmetric junctions that caused wrinkles, according to Rice University...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

Antarctic landscape insights keep ice loss forecasts on the radar

20.11.2017 | Earth Sciences

Filling the gap: High-latitude volcanic eruptions also have global impact

20.11.2017 | Earth Sciences

Water world

20.11.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>