According to a recent research conducted by MIT, quantum computers remain to be "largely hypothetical devices," that can out-calculate conventional computers. Although quantum systems with as many as 12 qubits have been demonstrated in laboratory conditions, a fully practical quantum computer will require the handling of qubits to be “tamed” into a manageable and repeatable form of technology; or, as the MIT statement puts it; “... will require miniaturising qubit technology, much the way the miniaturisation of transistors enabled modern computers.”

Trapped ions are probably the most widely studied qubit technology, but they’ve historically required a large and complex hardware apparatus. As reported in the journal Nature Nanotechnology, researchers from MIT and MIT Lincoln Laboratory describe work that promises a potential route towards practical quantum computers. Their paper describing a prototype chip that can trap ions in an electric field and, with built-in optics, direct laser light towards each of them.

The work centres around applying integrated photonics – the combination of nano-scale electronic circuitry and optical waveguides – to the task of confining and observing ions whose quantum states are the basis of the qubit.

“If you look at the traditional [lab-scale qubit confinement system] assembly, it’s a barrel that has a vacuum inside it, and inside that is this cage that’s trapping the ions. Then there’s basically an entire laboratory of external optics that are guiding the laser beams to the assembly of ions,” says Rajeev Ram, an MIT professor of electrical engineering and one of the senior authors on the paper. “Our vision is to take that external laboratory and miniaturise much of it onto a chip.”

The Quantum Information and Integrated Nanosystems group at [MIT’s] Lincoln Laboratory was one of several research groups already working to develop simpler, smaller ion traps known as surface traps. A standard ion trap looks like a tiny cage, whose bars are electrodes that produce an electric field. Ions line up in the centre of the cage, parallel to the bars. A surface trap, by contrast, is a chip with electrodes embedded in its surface. The ions hover 50 microns above the electrodes.

Cage traps are intrinsically limited in size, but surface traps could, in principle, be extended indefinitely. With current technology, they would still have to be held in a vacuum chamber, but they would allow many more qubits to be crammed inside.

“We believe that surface traps are a key technology to enable these systems to scale to the very large number of ions that will be required for large-scale quantum computing,” says Jeremy Sage, who together with John Chiaverini leads Lincoln Laboratory’s trapped-ion quantum-information-processing project. “These cage traps work very well, but they really only work for maybe 10 to 20 ions, and they basically max out around there.”

Performing a quantum computation, however, requires precisely controlling the energy state of every qubit independently, and trapped-ion qubits are controlled with laser beams. In a surface trap, the ions are only about 5 microns apart. Hitting a single ion with an external laser, without affecting its neighbours, is incredibly difficult; only a few groups had previously attempted it, and their techniques weren’t practical for large-scale systems.

Next: Designing on-chip optical components for quantum computers