Quantum non-demolition measurement allows physicists to count photons without destroying them

July 9, 2010 By Lisa Zyga, Phys.org feature

The physicists performed a quantum non-demolition measurement, illustrated in this circuit schematic, that could detect single photons without destroying them. The technique allows repeated measurements to be made that give the same result. Image credit: B.R. Johnson, et al. ©2010 Macmillan Publishers Limited.
(PhysOrg.com) -- In a way, the quantum world seems to know when it's being watched. When physicists make measurements on photons and other quantum-scale particles, the measurements always disturb the system in some way. Although an ideal disturbance should still enable physicists to make multiple measurements and get the same result twice, most real measurements cause a greater disturbance than this ideal minimum, and prohibit physicists from making repeated measurements. In a recent study, physicists have demonstrated a new way to make one of the ideal measurements - called quantum non-demolition (QND) measurements - allowing physicists to detect single particles repeatedly without destroying them.

The concept of QND measurements has been around since the beginning of quantum mechanics, and physicists have demonstrated different QND measurement techniques since the ‘70s. In the latest technique, developed by a team of physicists from Yale University, Princeton University, and the University of Waterloo, the scientists have shown how to measure the number of inside a microwave cavity in a way that preserves the photon state 90% of the time; in other words, the method is 90% QND. The physicists explain that, unlike previously reported QND methods, the new technique is strongly selective to chosen photon number states, which could make it useful for applications such as monitoring the state of a photon-based memory in a quantum computer.

In their experiments, the physicists wanted to find out how many photons were in a microwave cavity. To do this without disturbing the system, they coupled a superconducting to a cavity. This cavity stored the photons long enough for them to be measured - or “interrogated” - by using a set of controlled-NOT (CNOT) operations to encode information about the cavity state onto the qubit state. Then the qubit and storage cavity were decoupled, and the qubit state was read out. Because the qubit state now depends on the number of photons in the cavity, measuring the qubit reveals the number of photons.

“Our method takes advantage of the ability to engineer interactions between cavities and qubits in superconducting circuits to make the qubit energies strongly depend on the number of photons in the cavity,” coauthor Blake Johnson of Yale University told PhysOrg.com. “We have made this effect large enough to build a new qubit-photon logic gate which allows us to perform conditional qubit operations based on the cavity state. This type of logic gate is not only applicable to photon readout, but also to some proposals for engineering interactions between photons by using a qubit as a mediator.”

In the new design, the photon read out time is faster than the photon decay time. This timing difference allows the physicists to measure any qubit state several times during the lifetime of photons in the storage cavity. A single interrogation process takes about 550 nanoseconds, which includes the 50-nanosecond to initialize the qubit state. As expected with a high-quality QND method, the results of repeated interrogations are essentially indistinguishable from the first. In contrast, as Johnson explained, a typical quantum measurement would destroy one photon every time, so that repeated interrogations would give different results.

“A typical photon detector, like a CCD or photo-multiplier tube, absorbs photons,” Johnson said. “These detectors don’t work for microwaves because the energy of a microwave photon is too small to generate charges. However, with a setup similar to the one used in our paper, one could measure the photon state by transferring the photon energy into the qubit. This method would destroy exactly one photon every time. In contrast, our detector does not transfer any energy. Instead, we attempt to add energy to the qubit from an external source in such a way that the success or failure of these attempts reveals information about the cavity state. You might worry that this added energy might leak into the cavity and changed the photon number, but we have checked that this does not, in fact, happen.”

Achieving QND measurements of photons, while challenging, could be very useful for the development of quantum information technologies, which require complete control of quantum measurements. As the physicists note in their study, recent progress in manipulating microwave photons in superconducting circuits has increased the demand for a QND detector that operates in the gigahertz frequency range (like the one demonstrated here). In addition, the physicists predict that further research could make it possible to observe quantum jumps of light in a circuit, among other things.

“QND detection in general is interesting because it is the only way that quantum mechanics allows to extract information from a system without modifying its state, and then allowing feedback and manipulation of the same,” Johnson said. “The applications are interesting because if one could implement feedback of a quantum system, one could imagine using these systems for quantum simulation and quantum computation, harnessing toward the goal of practical application.”

Explore further: Yale scientists bring quantum optics to a microchip

More information: B.R. Johnson, et al. “Quantum non-demolition detection of single microwave photons in a circuit.” Nature Physics. Advance Online Publication. DOI:10.1038/NPHYS1710


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Jul 09, 2010
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3 / 5 (4) Jul 09, 2010
how is that spam? Yeah, he has a job, but other than his screen name, there is nothing remotely spamy about that post.

And it WAS an interesting article. 90% is impressive, still not good enough for a quantum computer, but impressive none the less.
3.2 / 5 (6) Jul 09, 2010
The same principle can be used for breaking of quantum cryptography, I'm afraid. If you can read the state of quantum system without its destruction, you can actually do the very same things, like at the case of classical cryptography. The counting of photons isn't still equivalent to counting of photons with particular spin for example, but the difference is infinitesimal here.
Spam? Check his profile
Actually all three comments bellow dedicated to name calling are off-topic spam here as well.
not rated yet Jul 10, 2010
>>allows to extract information from a system without modifying its state

If true, rewrite the textbooks *if* you can test spin... but that's the way science goes...

I'm with Jigga, and I think further Coir man is just out for his business - there are others on this forum that post their titles, businesses, mottos - is it spam - yeah it certainly is as Otto noted and with most likely reason... when Coir man created account, he made two posts (one you saw, the other as:

CoirGreen) on http://www.physor...479.html ... but now I've given him too much airplay...

Further on the article, if you can watch and detail the spookiness without losing quantum information, doesn't that give freedom to transmit data across galaxies, and receive in realtime? ... if true, SETI pay attention...
not rated yet Jul 10, 2010
Quite interesting. So much for unbreakable quantum cryptography.
not rated yet Jul 15, 2010
Its a little hard to tell from the article, but I ~think~ this still changes the timing of the photon. So although it might break some practical implementations of quantum cryptography, in theory one can still design a quantum cryptography implementation that would detect the intrusion simply by noting the timing discrepancy of the photon. If such detection can be done immediately, then the data could be obfuscated/made bogus in order to mislead the intruder.

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