My favorite experiments are not necessarily the groundbreaking ones. I love those too, don't get me wrong; but I like the ones that make me take a good hard look at the way in which I picture the physics.

One critical skill in physics is knowing what to leave out. For instance, if I can predict and describe a physical system with classical physics, why add quantum mechanics? But a recent paper highlights that it is always important to bear in mind that every classical picture has a quantum background. You may be able to neglect that background, but should never forget that it is there.

Cavities and quantum states

The experiment involves thinking about optical cavities, which are sort of on the border between quantum and classical worlds. Normally, we think about optical cavities in terms of the color, or wavelength of light that an optical cavity will accept. The distance between the mirrors must be commensurate with the wavelength. This can be described by both classical and quantum physics. The amount of light in the cavity, however, is almost always thought of in terms of classical physics.

Even when the light intensity drops to the point where we are considering individual photons, the optical cavity doesn't care. Shine the light on the cavity and light will keep building up within it until the rate of incoming photons equals the rate at which they leak out. Again, this is well described by both quantum and classical physics. Although the graininess of intensity jumps at the level of individual photons, in most cases, this can be ignored.

But the optical cavity is still quantum. It accepts one photon, then another, and another, up until the point where the mirrors are destroyed by the light intensity. But, on accepting a photon, the light in the cavity enters a new quantum state. So the infinite acceptance of photons has a caveat: the appropriate quantum state to receive it must exist.

What should happen, though, if there were missing quantum states? Say that a cavity could accept one photon, or two photons, but not three photons because the three photon state was missing? How would the light behave?

A similar experiment gets performed very often, but it's usually done at much higher optical intensities. Say we have an optical cavity that has a block of glass in it. If we shine a laser with the right color on it, the light starts to enter the cavity. As the intensity of the light in the cavity builds up, the glass responds to the intensity by changing its optical properties—typically the refractive index changes slightly.

Once this happens, the light suddenly finds that it has the wrong color and can no longer resonate in the cavity. The laser is also the wrong color, so no new light enters the cavity. The intensity of the light drops sharply, and the refractive index of the glass returns to its normal value. Under the right conditions, you can end up with a stable situation where the intensity of light in the cavity reaches a constant value. But, under other circumstances, the intensity of the light in the cavity fluctuates periodically because the light intensity overshoots and undershoots.

But, that is really a classical physics picture, which works because there are a lot of photons involved. What happens when the quantum nature of the cavity enters the picture?

Going quantum

This is exactly what researchers from France set out to investigate. The researchers did not use optical cavities, but switched to microwave cavities in the form of an aluminum box, which is like a microwave oven. Attached to this was a superconducting qubit (a ring of superconducting material interrupted by a gap of non-superconducting material). The aluminum box is entirely passive and just accepts whatever frequencies its dimensions allow. By itself, it behaves just like an ordinary resonator, so light can just build up to an intensity that depends on the intensity of the driving radiation and how long light stays in the resonator.

The qubit is slightly different. Although it is also a resonator, it can be tuned to resonate different colors of light. Like our block of glass above, that tuning depends on the intensity of the light it experiences, including light from the aluminum cavity. This, in turn, changes the resonant frequency of the aluminum cavity, because the light is influenced by both objects under the right circumstances.

The picture is like this: the researchers decide that they only want a maximum of two photons in the aluminum cavity, so they set the qubit to drive the cavity out of resonance for a light intensity corresponding to three photons. (They actually tried a number of different photon counts in different experiments.) This was done by choosing the qubit drive frequency appropriately.

The researchers found that the number of photons in the cavity was always below the blocking level. If the cavity is set such that it will accept a single photon, then the number of photons in the cavity always varies between zero and one in a periodic fashion. That seems just like the classical behavior, but with a twist. Classically, the resonant frequency of the cavity should only shift if the second photon enters the cavity. But, as far as the measurements show, that never happens. Somehow, the very possibility of a frequency shift prevents the second photon from entering the cavity.

You might be thinking, well, maybe there is a mistake here and, actually, the first photon shifts the frequency enough to prevent the second photon from entering. But we know this isn't true. If the first photon induced a frequency shift large enough to prevent the second from entering, the first photon would itself be quickly lost.

In other words, we would expect that the oscillation between zero and one photon would be much more rapid, because the light intensity in the cavity would decay much more rapidly. That is, the coupling between the microwave source and the cavity, and the reflectivity of the aluminum walls tells us how long we expect a photon to remain in the cavity. If the first photon shifted the resonance frequency, this value would be expected to fall. However, it does not appear to do so.

The researchers tried the experiment for limits of two, three, four, and five photons. In each case, they show that the intensity build up to a photon number that is just one below the set level, and that the intensity then oscillates between that number and zero.

The nice thing is that the whole experiment behaves a bit like an artificial atom being driven through excited states. Being artificial, it can be tuned and used for different experiments. It won't tell us anything much about real atoms, but maybe it can take the place of real atoms for various applications in quantum physics.

Even if it can't be used for other experiments, I don't really care. This is the sort of experiment that just blows my mind: it shows the clear and stark difference between our classical experience and the quantum reality that underlies it.

Science, 2015, DOI: 10.1126/science.1259345

Listing image by Flickr user woodleywonderworks