(Image: Tu Delft)

Spin or superconductor? That’s the “Apple or Android?” of the quantum computing world. Superconducting qubits have been around longer, but spin’s the ultracold new thing – and there are a few wild-card options to boot. Here’s what you need to know.

Superconductor or spin? That’s like deciding between Apple or Android

Superconducting qubits:

This is the grandaddy of all quantum computer tech. Back in 1962, Cambridge physicist Brian Josephson showed that putting a small gap into a strip of superconductor – a material that has zero resistance to the flow of electricity at low temperatures – has a surprising effect. For example, superconducting loops incorporating such a “Josephson junction” let current flow clockwise and anticlockwise simultaneously. That’s a superposition of states – just what you need for a qubit.


What’s more, these systems are manufactured on the mainstay material of the tech industry: silicon. “That allows you to use standard lithographic tools,” says Steffen. “You’re not in thrall to natural systems, and once you can do a handful of qubits on a chip reliably, you should be able to put many more onto the same chip.” That makes superconducting a good choice for the buyer looking for a tried-and-tested solution to their quantum computing needs.

If you’re sold on this approach, you still have a choice to make: transmon or Xmon? Transmons are loop-shaped and, at the moment, up to five of them can be linked together. A standard transmon can maintain its coherence for around 50 microseconds – long enough to be used in quantum circuits. What’s more, coherence times twice that length, and transmon arrays of 10 to 20 loops, are just around the corner, according to Göran Wendin of Chalmers University in Gothenburg, Sweden.

The Xmons created by a team at the University of California, Santa Barbara (UCSB), are cross-shaped superconducting qubits made from sapphire sitting on aluminium. The UCSB group can connect up five of these to create an array that corrects its own errors, and are working on a nine qubit array. UCSB’s John Martinis, who has just embarked on a collaboration with Google, thinks they can power ahead now: “My challenge to the group is to double the number of qubits every year.” That’s a big ask: the architecture and the kinds of algorithms the machine will run still need work.

Spin qubits:

If you’re more of an early adopter, you might check out what Andrea Morello’s group is up to at the University of New South Wales (UNSW) in Sydney, Australia. Their single atom of phosphorus sitting inside a silicon chip may not sound as impressive as a quintuplet of superconducting loops, but it certainly has its advantages.

Morello’s team is able to put the atom’s spin in a superposition, manipulate this blurred quantum state and then read it out by applying a microwave pulse. Morello says the team has maintained coherence for “tens of seconds” – plenty of time to run the kinds of quantum apps being dreamed up. There’s scope for going longer, too. Researchers at Simon Fraser University in Burnaby, Canada, have managed to get their phosphorus-in-silicon rig to hold for nearly 40 minutes at room temperature. They also preserved the superposition while cycling the material between room temperature and 4.2 kelvin.

At the moment, the UNSW group is squeezing two qubits out of a single atom, using the phosphorus nucleus as the first and one of its electrons as the second. In June, they announced that they could now couple two atoms together and read out all four spins, although they haven’t yet managed to manipulate them. Once they can, they aim to have a handful of qubits that will let them start making quantum calculations. But that will take three to four years, and the wait for apps will be even longer.

(Image: UCSB)

Some researchers are looking for a more high-end solution – which is where those diamonds come in. When certain diamonds are formed, a nitrogen atom can sneak into the place of a carbon atom to give the gem a slight pink colouring and, at the same time, leave an empty space nearby in the crystal lattice. This combination of nitrogen plus “vacancy” (NV) can be used to create a qubit; the vacancy has distinct quantum energy levels that can be put into superposition using a pulse of laser light. Further pulses can manipulate the state and read out the result of the operation.

In May this year, researchers from the Delft University of Technology in the Netherlands managed to teleport quantum information between two diamonds that were 3 metres apart. This is a baby step towards quantum computing in the cloud and a quantum internet.

Information has been quantum teleported between two diamonds

Before we get ahead of ourselves, there’s a catch. You can’t manufacture these blingtastic NV qubits to order. Putting hundreds of them together, as would be necessary for a useful quantum computer, would give a noisy output. But that’s not a deal-breaker. Last year, a team led by Simon Benjamin at the University of Oxford showed that you can put just a few NV qubits together in a “cell”, then link those cells together with photons that act as the input and output bits. Even at room temperature, the cell can maintain coherence for around a second, and the noisy photon network that connects cells can tolerate a 10 per cent error rate without crashing.

Also available:

If you’re really into bleeding-edge technology, there are a few other options to consider. Ion trap quantum computing is actually the market leader when it comes to entanglement. This technique has linked together a whopping 14 qubits made from ions – typically ytterbium – held in carefully shaped electromagnetic fields and manipulated with laser or microwave pulses. But that’s still a long way from a useful computer, and researchers are finding it tricky to scale up.

You would have to be brave to put all your money on photonic quantum computing, too. Photons look like they would make good qubits: they are easily superposed and stay coherent for good lengths of time. But although it’s possible to work with particles that move at the speed of light, it isn’t easy.

Topological quantum computing, where qubits are encoded in the way subatomic particles move past one another, has its pluses too – it’s particularly resistant to environmental disturbances for one thing. Microsoft is beginning to invest heavily in it, but it won’t be in the shops any time soon. Maybe one to consider when your first quantum computer starts to look a little vintage.

Verdict: Superconducting qubits might attract those who like to play it safe, but spin could just overtake it during the next decade. Everything else is for die-hard experimenters only.

Read more: “Quantum computers: The world’s first buyers’ guide“