We have heard a lot about quantum computers over the past years—those basically hypothetical devices that could potentially perform calculations much more rapidly than conventional computers. Quantum computers run on quantum bits (quibits) instead of traditional bits that classic computers run on. The difference being bits represent 0 or 1, while qubits can represent 0 and 1 simultaneously.
So far, quantum systems with as many as 12 qubits have been demonstrated in a lab setting, but building quantum computers that are capable enough to perform useful computations will require miniaturizing qubit technology, in the same way there is a need to shrink down many components for modern electronics.
Part of the qubit technology that is commonly studied is the trapping of ions, which normally requires a large and complex hardware apparatus. But now, researchers from MIT and MIT Lincoln Laboratory have taken a step toward practical quantum computers, with a prototype chip that can trap ions in an electric field and then, employing its built-in optics, direct laser light toward each of them.
“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,” said 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 Lincoln Laboratory was one of several research groups that have been working to develop simpler and smaller ion traps—chips with electrodes embedded in their surface, over which ions hover 50 micrometers above.
“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,” said Jeremy Sage, who together with John Chiaverini, leads Lincoln Laboratory’s trapped-ion quantum-information-processing project.
One of the key challenges in using a surface trap is that performing a quantum computation requires precisely controlling the energy state of every qubit independently. Because trapped-ion qubits are controlled with laser beams and the ions are only about five micrometers apart, hitting a single ion with an external laser, without affecting its neighbors, is incredibly difficult. There have been some groups that have attempted to do so, but their techniques were not practical for large-scale systems.
To overcome this, the MIT team designed and built a suite of on-chip optical components that could channel laser light toward individual ions.
“Typically, for surface electrode traps, the laser beam is coming from an optical table and entering this system, so there’s always this concern about the beam vibrating or moving,” said Ram. “With photonic integration, you’re not concerned about beam-pointing stability, because it’s all on the same chip that the electrodes are on. So now everything is registered against each other, and it’s stable.”
The new chip, which is built on a quartz substrate, is still being studied by the team to determine how to change the amount of light that is delivered to each ion.
If different qubits could simultaneously receive light of different, time-varying intensities, it would make programming the qubits more efficient, which is important for a practical quantum information system because the number of quantum operations the system can perform is limited by the “coherence time” of the qubits.
“As far as I know, this is the first serious attempt to integrate optical waveguides in the same chip as an ion trap, which is a very significant step forward on the path to scaling up ion-trap quantum information processors [QIP] to the sort of size which will ultimately contain the number of qubits necessary for doing useful QIP,” said David Lucas, a professor of physics at Oxford University. “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.”