Quantum Computing: Chip To Shoot Lasers At Ions Trapped In Electric Field

One of the biggest hurdles in developing a practical quantum computer is that the technology to create one has not been miniaturized to anything small enough to be called a modern computer. A new paper written by researchers from Massachusetts Institute of Technology (MIT) aims to change that.

Published Monday in the journal Nature Nanotechnology, the paper titled “Integrated optical addressing of an ion qubit” describes “a prototype chip that can trap ions in an electric field and, with built-in optics, direct laser light toward each of them.” A qubit refers to a quantum bit, capable of simultaneously representing 0 and 1, as opposed to a regular bit in classical computing that can represent either 0 or 1 at any given time.

Rajeev Ram, an MIT professor of electrical engineering and one of the senior authors on the paper, said in a statement: “If you look at the traditional 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. Our vision is to take that external laboratory and miniaturize much of it onto a chip.”

A standard cage for trapping ions uses electrodes as bars that generate an electric field, which in turn make ions line up in the center, parallel to the bars. In contrast, the surface trap that researchers are working on “is a chip with electrodes embedded in its surface. The ions hover 50 micrometers above the electrodes.”

According to the statement, while cage traps are inherently limited in size, the surface traps could, at least in theory, “be extended indefinitely,” allowing many more qubits to be trapped inside.

Jeremy Sage co-leads MIT’s Lincoln Laboratory’s trapped-ion quantum-information-processing project. He explained the importance of surface traps to quantum computing: “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. These cage traps work very well, but they really only work for maybe 10 to 20 ions, and they basically max out around there.”

Lasers enter the picture because quantum computations require highly precise control of “the energy state of every qubit independently, and trapped-ion qubits are controlled with laser beams.” However, since the ions are only about 5 micrometers apart in a surface trap, using a laser beam to hit a single ion only is “incredibly difficult.”

An MIT graduate student, Karan Mehta — first author of the new paper — and Ram “designed and built a suite of on-chip optical components that can channel laser light toward individual ions.” The integrated optics were then installed on the surface trap.

David Lucas, a professor of physics at Oxford University not associated with the MIT research, said: “Trapped-ion qubits are well-known for being able to achieve record-breaking coherence times and very precise operations on small numbers of qubits. Arguably, the most important area in which progress needs to be made is technologies which will enable the systems to be scaled up to larger numbers of qubits. This is exactly the need being addressed so impressively by this research.”

Other than researchers from MIT, co-authors of the new paper were from University of Innsbruck in Austria.