If I were to compare quantum computing to classical computing right now, I would say that we are somewhere beyond the point where Babbage and Lovelace left off but have not yet reached the point where someone has made a Bomba or an ENIAC. We are currently in the phase of trying to figure out what technology is best for implementing a quantum computer. And that choice is not straightforward.

One technology choice for quantum computing is the trapped ion. The challenge is to get as many ions as possible under our control in a trap. Typically, this means about ten ions, but we need more for useful computations. That makes a report of cooling a 2D crystal of nearly 200 ions down to nearly the lowest temperature possible a big step forward.

Floating checkerboard of ions

An ion, for our purposes, is an atom with an electron removed. Because of the missing electron, each ion has a positive charge, so ions repel each other. Likewise, the positive charge means that they can be fenced in with electric and magnetic fields. When ion traps are constructed just right, the ions self-organize into crystals—ions spaced at regular intervals from each other. In quantum computing, these crystals are limited to 1D—a string of evenly spaced ions.

A long 1D string of ions is rather unwieldy, though. The length of the trap becomes its Achilles heel: the ions at the ends of the trap behave quite differently from the ones in the center. This difference makes life difficult for anyone trying to use them to compute.

Enter the 2D crystal. A 2D crystal has all the advantages of a 1D crystal but is a bit more compact. We have not used 2D crystals yet, though, because we can’t cool them down easily. If the crystal is not cold, then the quantum information stored in the ions will be quickly destroyed.

Blinded to a standstill

To understand how physicists cool ions and atoms to near absolute zero, it helps to picture the ions the same way physicists do. As shown below, the ion is nothing more than its possible energetic states. In this diagram, we show four possible states. All the ions start in a single energetic state. However, because the ions are all in motion, the state is blurred: the ions all have a slightly different energy than an ion that is standing still.

Most cooling schemes make use of the Doppler effect. The Doppler effect is what makes the siren of an approaching ambulance higher pitched compared to when the ambulance is moving away.

For ions, it helps to think of Doppler cooling in terms of its opposite. An ion that is not moving can absorb a certain color of light. In absorbing a photon, the ion gets a kick and starts moving away from the light source. Since it's moving, light coming from the same stationary source is no longer the right wavelength to be absorbed. The next photon has to be slightly bluer in color to be absorbed by the ion because of the Doppler shift. If you make the light source slightly more blue, the ion gets another kick and moves slightly faster—we are heating the ion.

We can reverse this process, as shown below, by using light that is slightly redder in color. The only ions that absorb light are moving toward the light source—the Doppler shift makes the light appear bluer to these ions, allowing it to be absorbed. The kick from absorbing the photon slows the ion down.

But the Doppler shift can only take you so far. At a certain point, the heating effect of blue-shifted light balances with the cooling effect of red-shifted light to stop any further cooling. At this point, we need a mechanism to selectively prevent cold ions from being reheated.

Let’s play hide the ion

Hiding the ion is essentially what the researchers do. The ions they use have lots of different quantum states. The laser that performs the Doppler cooling shifts them between two states. One of those two states is then coupled via a pair of lasers to a third state.

As shown in the picture below, the additional pair of lasers is not quite in tune with the ions’ energy levels, either. The combination of their brightness and that slight energy mismatch distorts the ions’ energy levels in such a way that the Doppler cooling laser can only slow faster-moving ions and not accelerate slow-moving ions. In other words, it allows the selection of hotter ions.

As a result, the ion crystal cools to a lower temperature. How cold? Well, the researchers measured the energy in the ions’ collective motion (called a drumhead mode) and showed that about 70 percent of the time the ions were in the lowest possible state (for motion in a particular direction). This counts as something of a failure, because, as the researchers state, they should be able to obtain the lowest possible state 95 percent of the time. They are not sure why their results don’t match theory.

Still, this is exactly the progress required to obtain 2D ion crystals that are ready for the heady world of quantum computing.

Physical Review Letters, 2019, DOI: 10.1103/PhysRevLett.122.053603 (About DOIs)