Scientific Method —

Divide & conquer makes quantum light a breeze to detect

Classy experiment detects non-classy light, may help with quantum computing.

Aurich Lawson / Slayer (RIP Jeff Hanneman)

These days, when physicists talk about light, they like to divide it into two categories: classical and non-classical. Of course, classical light is the boring, everyday stuff that anyone gets delivered to their doorstep roughly 12 hours every day. But non-classical light is harder to get hold of and, for physicists, obtaining non-classical light states seems to be just one step short of world domination (provided your definition of world domination involves doing quantum cryptography and quantum computing).

But like cheap knock-off goods, genuine non-classical light can be hard to distinguish from ordinary, old-fashioned classical light. Until now, that is. A group of researchers, mainly from Oxford, have figured out a new way to distinguish the two brands of light. It is very clever and relatively simple. So simple that I just have to tell you all about it.

Classical/non-classical? And who cares?

The difference between classical and non-classical light states is one of statistics. Classical light sources behave one way, and non-classical light sources behave another way. The primary example of this is bunched and anti-bunched light.

Bunched light is classical and can be found in a standard light bulb. When you turn on your incandescent light bulb—unless you actually care about your power bill, in which case, you don't—a filament heats up and begins to glow. If we decide to measure this glow, we can set up a detector such that it should only detect one photon per detection interval. To do this, we move the detector far away from the bulb and give it a tiny active area so that the average incident power corresponds to one photon per second.

We would expect a regular series of clicks approximately separated by one second, with a bit of noise in the timing between clicks. Instead, what we would find is that the clicks come in bunches. The flow is 2-3 photons close together, followed by a break of a couple of seconds, then another burst of a few photons. Indeed, if my detector were able to tell me how many photons it detected, it would occasionally report the arrival of two photons at the same time. It seems that photons like to travel together. Hence bunched light.

Every light source from light bulbs to lasers follows this pattern.

Anti-bunched light, on the other hand, does not fit the pattern. An example of this is light from a single atom. We shine a light on it to excite it, and as the atom relaxes, it emits light of a redder color, which we detect. Because there is only a single atom, it can only absorb one photon from the exciting light field and emit a single photon as it relaxes. It cannot emit a second photon until it has been through that process again. As a result, we would never observe two photons arriving simultaneously.

This is just one example. Experimentally, it is a particularly easy example to work with. So in practice, pretty much everyone does so, using single or correlated pairs of photons as their non-classical source. Since the difference is in the statistics of photon arrival times (or more precisely, the fluctuations in the light field's amplitude), we can increase the average number of photons per second and still have the same statistics.

The real reason people work with these non-classical light sources is because you can encode more complicated quantum states in the light field.

Before you try doing so, however, you need to make sure your light source is non-classical. The ultimate way to determine this would be to have a photon-counting detector. Then you would simply calculate the mean number of photons per unit time and the variation in the photon number about that mean, and then you declare the light to be either classical or non-classical. Unfortunately, a detector like this is very difficult to make, so we need to be somewhat clever. Which is exactly what the Oxford group has done.

Divide and conquer

The strategy employed is a kind of divide and conquer approach. Essentially, you divide the light field into many channels of smaller light fields until there is a reasonable chance that some of those channels have no photons at all. The result is that you don't get the photon number of the light field (because a detector click is anything more than zero photons), but the correlations of the detector clicks reveal the statistical variation in the light flow. That variation is enough to reveal whether a light field is classical or non-classical.

Although I won't describe the actual apparatus in detail, it involved a pair of interferometers. These split a single light pulse into two and then each path contributes to two output directions. The researchers set it up so that one light pulse had to travel much farther, and one light pulse becomes two on entering the interferometer and then becomes four on exiting the interferometer (two traveling along each output path). This is then repeated at a second interferometer, creating eight light pulses travelling to two detectors. All that is then required is to measure the correlations between the two detectors.

Light from source gets split at the entrance to the interferometer (first and third cross over points) and mixed at the exits (second and last cross over points). The loops in one of the paths add a timing delay to the photons' arrival at the detectors (far right).
Aurich Lawson, based on the article

To demonstrate that it would work, the researchers used a laser combined with a known non-classical light source so that they could smoothly vary the input light state from classical to non-classical. And guess what—it worked.

The big deal about this is that it should work on light fields that are not at the single-photon level. That doesn't mean we are going to be throwing around powerful non-classical light beams. A one-microwatt light field that is sampled over 50ns will have around 200,000 photons within that time slot. That will require something like 17 sequential divisions to get some channels down to zero photons, which is clearly unfeasible at the moment.

So although we're talking about states that are more complicated and involve more than one or two photons, we're not talking about a huge amount more. However, this is exactly what is needed for better encoding schemes for optical quantum computing and encryption, because more information can be encoded in states that consist of more than one or two photons.

Physical Review Letters, 2013, DOI: 10.1103/PhysRevLett.110.173602

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