Integrated photonics could be key to practical quantum computers

August 10, 2016 // By Graham Prophet
Using nanoscale semiconductor fabrication techniques to construct integrated optics could enable chips that use trapped ions as quantum bits.

Quantum computers, as an account of recent research work at MIT (Cambridge, MA) notes, are still “largely hypothetical devices” that could perform some calculations much more rapidly than 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 miniaturizing qubit technology, much the way the miniaturization 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 toward 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 toward 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 miniaturize 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.