Quantum optics with microwaves

[Ichingcarpenter]

Phys.org) —Physicists at ETH Zurich have demonstrated one of the quintessential effects of quantum optics—known as the Hong-Ou-Mandel effect—with microwaves, whose frequency is 100'000 times lower than that of visible light. The experiment takes quantum optics into a new frequency regime and could eventually lead to new technological applications.


Quantum mechanics, famously, is full of effects that defy our basic intuition. A fine example is the Hong-Ou-Mandel effect, which occurs when two light quanta (or, photons) arrive simultaneously at a so-called beam splitter. As its name implies, a beam splitter is a device that splits one beam of light into two, by transmitting one half of the impinging light and reflecting the other half. For a single quantum of light, a photon, this means that it has a 50-percent chance to appear on either side of the device. But when two photons arrive at the same time at the splitter, something unexpected happens: The photons then always emerge as a pair on the same side of the beam splitter, either both on one side or both on the other side. Never do the two photons exit on different sides.

Read more at: http://phys.org/news/2013-05-quantum-optics-microwaves.html#jCp


The Hong–Ou–Mandel effect is a two-photon interference effect in quantum optics. The effect was demonstrated experimentally by Hong, Ou, and Mandel in 1987.

http://en.wikipedia.org/wiki/Hong–Ou–Mandel_effect



Well I guess that clears that up.....LOL

[napoleon_in_rags]

1. I think this next period will be the swiss-cheesification of the "classical limit".

We've been told up until this point that quantum effects only exist on this small scale, and disappear into the averages once a certain scale of observation - the so-called classical limit - is reached. However a number of recent experiments have showed quantum effects magnified up to the macro level of our own existence, and this work seems to be another example of that. A wavelength as long as microwaves shows quantum effects?

I'm no genius physicist, but I've thought a bit about all these phenomena, and I can see how attributes of the core philosophy of quantum mechanics have validity in macro-scale systems - when we're willing to accept the limited knowledge of various observers, instead of arrogantly claiming to enough information to make deterministic proclamations about the future. I think, (in the language of poetic metaphor) that Schrödinger's cat is about to break out it's "classical limit" bounded little box.

There's a LOT to be excited about in the future of science, folks!

PEace

[caraher]

2. What do you want to know about Hong-Ou-Mandel?

I'm going to be using Hong-Ou-Mandel effect in the experiment I'm putting together right now. Exactly what happens at a beamsplitter can be slippery to understand; the Wikipedia "Physical Description" isn't bad. The basic result is that if there's no way to tell whether a photon leaving a given "output" of the beamsplitter came from which "input," then you always get both photons leaving the same side of the beamsplitter, never one on each side (though it will be random which output the pair leaves).

Now you could tell which outgoing photon corresponds to which incoming photon if they had different wavelengths (by putting a filter or prism after the beamsplitter), so you need the same wavelength for the two photons (or more accurately, identical spectra; because a single photon is a wave packet it actually has a sometimes-substantial spread of wavelengths).

Or if they had different polarizations, you might be able to tell which came from which input by putting polarization analyzers on the outputs. So you also need the same polarization.

But you can never get everything perfect; the result is that sometimes you DO get one photon at each output. You can measure this by putting a detector at each output and checking for "coincidences" - instances where both detectors "see" a photon at the same time (usually within a time window of maybe 10 nanoseconds). The degree to which this happens is a measure of how "distinguishable" the input photons are.

Another way photons can be distinguished from one another is if one arrives before the other one. So if you take two otherwise identical photons but you make one arrive at the beamsplitter 1 nanosecond before the other, typically one photon will have left the beamsplitter before the other one even arrives and they therefore cannot interfere with one another. Half the time they both leave the same port but half the time they leave different ports, and in the latter case they'll register as coincidences (since our detectors firing within 10 ns of one another is what we count as a "coincidence" in this example).

If you can vary the distance each photon needs to travel to the beamsplitter you can make them arrive simultaneously, or with a little delay between them. Now the thing to remember is that these photons are wave packets, which means they have a finite temporal width. A typical value for an optical photon is 100 femtoseconds (1/10,000 of a nanosecond). So if you make one photon's travel time 100 fs longer or shorter than the other photon's travel time, they can leave different sides of the beamsplitter and you get coincidences. But as you make the travel times more and more similar, you get fewer and fewer coincidence counts, as you lose the ability to distinguish between the photons based on time. This is the "HOM dip" referred to in the Wikipedia article, and provides a nice way to measure the temporal width of a photon (which is what I'm trying to do).

I think the researchers are slightly overselling the experiment by suggesting that only they can "vary the degree of how distinguishable the two photons are and can, therefore, finely control the appearance and disappearance of the effect." People in optics do that routinely; they just may be able to control things more "finely" under some measure of what it means to have "fine" control. The real advantage is being able to produce single photons on demand; I know a lot of people working very hard to do that at optical frequencies, and it's a big challenge. But the low frequency of microwaves mitigates that advantage; your fundamental frequency is in the GHz range, so you're not going to do any better than that for photon production rates, whereas optical frequencies (and therefore potential production rates) are about 100,000 times greater. (This is the flip side of their "advantage" of working with a wavelength 100,000 times greater.)

But there's a long history of techniques developed in the microwave regime later being converted into the optical domain. Maybe this work will add to that tradition.