Quantum computers come in many different shapes and forms, but the granddaddy of them all is based on light. This is because it is very easy to create the basic computational unit, called a qubit, from light. The big problem is the memory unit. Light has a pesky habit of traveling quite fast, so by the time you are ready to use your carefully prepared qubit, it is halfway to the Moon, never to return.

A pair of research groups, working independently, showed an effective and reliable memory for light-based qubits. Looking back, a number of competing research groups will be hitting themselves on the forehead, saying, "why didn't we think of that?" Yes, the idea is that simple—though I should caution that there is some distance between an idea and its implementation.

There are three key elements that make a quantum computer special: superposition, coherence, and entanglement. A qubit that is in a superposition state does not represent a logic one or zero; instead, it represents the probability of measuring it as a logic one. Coherence is essentially how the superposition state of different qubits change synchronously with each other. Entanglement takes two or more particles and makes them (mathematically speaking) a single particle. So when we are choosing a qubit system, we must choose physical properties that can be entangled between different particles and remain coherent.

Quantum superposition Superposition is nothing more than addition for waves. Let's say we have two sets of waves that overlap in space and time. At any given point, a trough may line up with a peak, their peaks may line up, or anything in between. Superposition tells us how to add up these waves so that the result reconstructs the patterns that we observe in nature. Read more…

If we limit ourselves to light, there are still many possible ways to encode a qubit on a photon. But practically speaking, polarization is preferred. It is easy to create photon pairs that are entangled through their polarization, and it is easy to perform quantum logic operations on polarization states. And with a bit of care, it is relatively easy to keep polarization-based qubits coherent with each other. In fact, this stuff is all so well-developed that researchers have produced waveguide chips that have arrays of logic gates on them. Truly, this is a promising technology.

Why was memory so difficult?

But memory is a problem. There are only two ways to store a photon: in an optical cavity (between two mirrors) or by having something absorb it. Optical cavities will never really be great for this, because the longer they can store a qubit, the harder it is to get the qubit into the optical cavity in the first place. So that leaves storing the qubit in a physical object.

Quantum entanglement Quantum entanglement is one of the most misused concepts around. Entanglement is delicate, rare, and short-lived. At its heart, quantum entanglement is nothing more or less than a correlation between two apparently separate quantum objects. Having discovered that, you might ask "so what is all the fuss about?" The answer lies deep in quantum mechanics. Read more…

Unfortunately, it's not as simple as sticking a bit of material in the path of the photon and hoping that the photon will be absorbed. It probably will be absorbed, and at some later time, the material may even emit a second photon with the same wavelength. But all the other properties will be completely scrambled.

No, to store the quantum state, one needs to carefully prepare the material. The basic process is similar to electromagnetically induced transparency, where a control light field is used to make the material transparent for the photonic qubit. But once the qubit is in the middle of the medium, you turn off the control light field, trapping the qubit. It is absorbed, but the two light fields and the atomic levels come together to maintain the coherence, entanglement, and superposition state of the qubit.

This works very well in dilute ultra cold gases of metallic atoms. True, you could hook your tiny waveguide chip up to an enormous vacuum system and store the qubit in an ultra cold gas or a trapped ion. But if you are going to do that, why not use the trapped ion or gas as the qubit and forget about the waveguides? A less cumbersome solution was needed.

Crystalline memories

Almost all the properties that we want in a memory can be found in the same sorts of materials used to make lasers. These are oxide crystals, like sapphire, that have small amounts of impurities added to them. When the crystal is cooled, these impurities behave very much like a dilute gas in terms of their optical behavior.

The big problem, though, is that the crystals have impurities with a fixed orientation. This means that they prefer to absorb photons with a particular polarization. You can put in a qubit that was in a superposition of two polarization states—a measurement has a 50 percent chance of reporting a vertically polarized photon. But after attempting to store the qubit and retrieve it from memory, you would have a 99 percent chance of measuring the polarization as vertical. The storage medium destroys the superposition state, making it useless for computation and communication.

Now, two research groups have demonstrated very similar ways around this problem. One group, from China, used two identical crystals. The two crystals were placed one after the other with an optical element that inverts the superposition state in between. Essentially, in our example above, half the qubit is stored in the first crystal and half is stored in the second crystal. The key is that no one knows which crystal is storing what, so the superposition state is preserved.

The researchers demonstrated single photon storage of polarization superposition states for up to 1.5 microseconds. This is certainly long enough to be useful. Furthermore, the fidelity of the memory was very high, reaching something like 99 percent.

The second research group, from Spain, used a slightly more complicated procedure. Instead of two crystals, they used a single crystal and divided the qubit. They separated out the superposition state, so the vertical part would go along one path and the horizontal along another. In the horizontal arm, they rotated the polarization to vertical, so that the crystal stores both parts of the qubit with equal efficiency.

On retrieval, the light in the arm with the rotated polarization was rotated back to its original orientation, and the two polarizations were recombined. Again, the qubit was effectively stored across two different locations, but in this case, the two locations were in the same crystal. They don't report their data in the same way, so I don't know if they can achieve similar storage times—I expect so, though, since it's essentially the same experiment. Their fidelities are also slightly lower, and the experiment is a little more complicated.

In both cases, the efficiency of the memory is the problem. There was only about a 10 percent chance of successfully storing and retrieving a qubit from the crystals. Much of this is limited by things like the efficiency of getting light into and out of fiber optic cables, detector efficiencies, and such. All of these can be expected to improve substantially in the near future.

Both of these results are very impressive. When I look back at all the different quantum computer developments, it's very cool to see the way all the different systems have progressed as each problem is gradually overcome. I now await this new memory module to be combined with waveguide chips to make something that resembles a quantum register for light.

Physical Review Letters, 2012, DOI:10.1103/PhysRevLett.108.190505

Physical Review Letters, 2012, DOI:10.1103/PhysRevLett.108.190504