As happened in the past with classical computers, researchers are still searching for a good way to implement quantum computers. It isn't so much that researchers don't know what they want to build; it's more of a question of searching for the right materials to create a scalable quantum computer. The current leader in the field—although I'm sure many would dispute this—uses the currents in loops of superconducting material. This has the advantage of being based on very traditional manufacturing processes. And because it is manufactured, many properties are under design control.

This has led to quite rapid progress, but there has been one limitation: long-distance communication. Quantum computers need to be able to communicate with other quantum computers, even with other parts of the same quantum computer. But superconducting loops all speak to each other in the tones of microwaves, which are low energy and easily disrupted. Now, however, a path to using visible light to transfer quantum information between qubits has opened up. This could revolutionize the development of quantum computers based on superconducting currents.

Loopy qubits

The fundamental unit of information in a quantum computer is a qubit. It's not quite analogous to a bit, which has one of two possible values: a one or a zero. While a qubit has two quantum states that we label as a one and a zero, it is incorrect to think of the qubit storing a one or a zero. It is more correct to think of it as holding a one and a zero.

Personally, I think it is more useful to think of qubits as representing probabilities or, more precisely, probability amplitudes (because unlike probabilities, those can be negative or complex). Before measurement, a qubit holds the probability that a measurement of its state will result in a one (or a zero). Computations are not performed directly on the one or zero values, but they modify the probability of obtaining a one or zero after the computation is completed.

A superconducting qubit relies on a minuscule loop of superconducting material with a tiny break in the loop, called a superconducting quantum interference device (SQUID). The current in such a loop cannot take on any arbitrary value; instead it increases in steps. The qubits that we are talking about today are encoded in the amount of the current in a loop. (The current can either go clockwise or anti-clockwise, adding a second potential bit to the device.)

The cool thing is that the current in a SQUID can be set and read electronically at microwave frequencies. The bad thing is that the current in a SQUID is therefore influenced by any stray microwave fields. This has been amply demonstrated by looking at how small-scale quantum computers perform. Each qubit, taken individually, performs really well. If you apply a microwave field to set a particular qubit state, it works with very high probability (say, about 98 percent of the time).

But a computer is based on multiple qubits. Each qubit influences the others, so the exact shape of the microwave pulse needed to set a qubit state is changed in an unpredictable manner by the presence of nearby qubits. As a result, operations performed during a multiple qubit calculation only have a success probability of about 90 percent. At these success rates, it doesn't take too many operations before the qubit is in a completely unexpected state.

Recently, researchers have demonstrated the first step toward coupling SQUID qubits to optical frequencies.

A bouncing qubit

The trick is to use the SQUID to drive a mechanical oscillator—think of a tiny drum or trampoline. That's normally really difficult because their frequencies don't match. Think of a swing: to get a swing moving, you have to push it in time to its swinging motion. But even pretty high frequency mechanical oscillators only have oscillation frequencies of a few MegaHertz; SQUIDs have frequencies of a few GigaHertz. To overcome this problem, researchers used a trick called parametric amplification. Essentially, the SQUID emits a single microwave photon. That photon is combined with a strong microwave signal whose frequency is offset from the SQUID's by the frequency of the mechanical oscillator.

These two signals are mixed, and a photon with a frequency at exactly the difference between the two is produced. This is absorbed by the mechanical oscillator, exciting it. Now, this is known to work with classical signals, and it is commonly used to take audio signals (a kiloHertz) up to the microwave range required for old analog cellphones. And the technique is also commonly used to take extraordinarily weak signals and amplify them to the point of detectability.

Still, it wasn't clear whether the qubit nature of a microwave photon would be preserved under these conditions.

Certainty in uncertainty

The team behind the new research showed that the vibrational modes of a mechanical drum could preserve the qubit state. I won't go into too much detail here, but it relates to the uncertainty principle. In quantum mechanics, properties often come in pairs that cannot be simultaneously measured to arbitrary precision: the very act of measuring one (and, therefore, precisely defining it) makes the other uncertain.

Now, our definition of a qubit is that we use two states to represent a one and a zero. If we put a qubit in the one state, that is a very well-defined state. Which means that some other property of the quantum system has just become highly uncertain. On the other hand, if we set the qubit state so that it has a 50-percent chance of being a one (and 50 percent of being a zero), then we have introduced the maximum possible uncertainty in the qubit state. In that case, some other property has become very well defined.

The relationship will hold as long as the quantum state is properly preserved. If the quantum state is lost, then these states will collapse to some average uncertainty in each property.

In the case of our SQUID qubit, the qubit state is defined by the presence or absence of a microwave photon. No photon equals zero, and one photon equals one. That means the qubit is encoded in the amplitude of the microwave field. The uncertainty principle couples that to the phase. The researchers measured the noise in the amplitude and phase of the mechanical oscillator as they sent different qubit states to it. The noise in these states showed pretty convincingly that the qubit state is transferred to the mechanical oscillator.

Where's my laser?

A mechanical oscillator is not light, you may be thinking. You are correct, it is not. But we already know that we can couple mechanical oscillators to light fields. The new result means that we can use the oscillator as an intermediary to allow qubit states to be transferred from SQUIDs to light. And with light, we can transfer them to, well, anything.

SQUIDs are very convenient for doing computations, but they are dreadful for memory. And the microwaves they use for coupling are really useless for communicating qubits over long distances. Optical frequencies, especially light in the telecoms range (think fiber optic communications), are perfect for the whole long-distance communication thing. There are also a whole range of qubit systems, like atoms, ions, and nitrogen vacancy centers (to name a few) that are better at storing qubits, and these have the added advantage of talking to each other via optical frequencies. This device could be the bridge between all of these different types of architectures.

This research might signal that we are moving away from the idea of finding a least-bad architecture for quantum computing and moving on to combining all the good bits of each architecture. That's an idea that engineers will hate, because it is inherently more complicated. However, it's an idea that may be necessary to end up with a practical quantum computer.

Nature Physics, 2017, DOI: 10.1038/NPHYS4251