Can Hybrids Crack Quantum Computing?

There’s no shortage of runners in the race to build scalable quantum computers. Why the fuss, given that the best of these devices can do little more number crunching than would tax a 10-year-old? The promise is that quantum computers will one day make today’s supercomputers look like pocket calculators.

So the question that computer companies and technology investors would very much like to know is which horse to back.

The question boils down to how best to encode, store and process quantum bits or qubits. On the one hand, there are various groups who say that groups of atoms, ions, molecules and even photons are the best bet. These guys store qubits in the quantum properties of these objects, such as their nuclear spin, electronic spin, polarisation or even their mechanical vibrations.

There’s no question that this approach has been successful–many of these systems are remarkably robust at storing and processing qubits. The question mark is over how they can ever be scaled into more complex and capable devices.

One the other hand, other groups have bet on solid state quantum computing. In these devices, qubits are stored as superconducting currents, in phosphorus atoms buried or as the mechanical vibration at very low temperature of silicon nanostructures.

The advantage of these devices is the ease with which they can be fashioned from silicon or other metals in the kind of fabrication facilities that are two-a-penny in the computer industry. Consequently, these devices have the potential not only to be relatively easy to make and hence cheap, but also to link with conventional electronic devices. That’s important.

Their disadvantage is that the qubits tend to be harder to handle: phosphorous atoms to migrate in silicon and superconducting currents are susceptible to outside interference.

It’s far from clear which horse to back in this race. But perhaps we needn’t make a choice, says Margareta Wallquist at the University of Innsbruck in Austria and few pals. Why not marry these approaches in hybrid systems that benefit from the best of both worlds. “With the maturing of the field of experimental quantum information, it seems timely to consider hybrid quantum systems involving atomic and solid state elements with the goal of combining the advantages of the various systems in compatible experimental setups,” they say.

For example, one way of processing superconducting qubits is to manipulate them using microwave photons in a cavity. The problem with this approach is that superconducting currents also couple to any extraneous fields nearby and that allows the quantum information to leak out.

But a hybrid approach could solve this problem, says Wallquist. Surround the superconducting circuitry with polar molecules which also couple to the microwave field and they should act as a kind of quantum memory to store information while the superconducting circuit goes about its work.

Wallquist and company have other examples too. It’s an approach that seems eminently sensible, as long as the devices can be characterized well enough to perform repeatable computations and to be manufactured on a decent scale.

Wallquist is certainly optimistic. “We believe that these ideas are a promising new route in the next generation development of experimental quantum information processing.”

There’s plenty to agree with there. But it’s still possible that one of the early frontrunners alone, such as nonlinear optical quantum computing, will eventually trump allcomers. And that makes it too early to say what role this kind of hybrid thinking will play let alone which approach will be the one to pay out in silver dollars.

Ref: arxiv.org/abs/0911.3835: Hybrid Quantum Devices and Quantum Engineering