MIT Researchers Discover Important Step Toward Practical Quantum Computers

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Quantum computers are principally hypothetical devices capable of performing certain calculations much faster than any existing traditional computers.

Classical computation consists of bits, which represent 0 or 1. Conversely, quantum computers involve quantum bits, or qubits, which are, to some extent, capable of representing 0 and 1 concurrently.

Researchers have demonstrated quantum systems consisting of as many as 12 qubits in the laboratory. However, the qubit technology needs to be miniaturized in order to build quantum computers that have the ability to execute meaningful computations.

The most widely explored qubit technology is trapped ions, but they have traditionally needed a large and complicated hardware apparatus.

A prototype chip capable of trapping ions in an electric field and directing laser light toward each of them with the help of integrated optics has been developed by scientists from MIT and MIT Lincoln Laboratory. This achievement is one step closer to realizing practical quantum computers.

The study results have been reported in the Nature Nanotechnology journal.

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.

Rajeev Ram, Professor of Electrical Engineering, MIT

Caged In

The Quantum Information and Integrated Nanosystems group at Lincoln Laboratory is one of the several research teams involved in the development of simpler, smaller ion traps called surface traps. A typical ion trap resembles a tiny cage, with electrodes as its bars that generate an electric field.

Ions assemble at the center of the cage in parallel to the bars. Conversely, a surface trap is a chip consisting of electrodes that are embedded in its surface, with ions hovering 50 µm above them.

Unlike cage traps that are inherently limited by size, surface traps could principally be expanded indefinitely. With existing technologies, they would still have to be placed in a vacuum chamber, but would have the ability to pack more number of qubits within them.

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.

Jeremy Sage, Lincoln Laboratory

The energy state of each qubit needs to be precisely controlled independently in order to execute a quantum computation. Laser beams are used to control trapped-ion qubits.

The ions are at a distance of only 5 µm from each other in a surface trap. It is a challenging task to hit a single ion using an external laser, without disturbing the neighboring ions. Only a few research groups had earlier attempted it, but their techniques were not practically feasible for large-scale systems.

Getting Onboard

In this study, Ram and Karan Mehta, an MIT graduate student in electrical engineering and first author of the paper, created a set of on-chip optical components capable of channeling laser light toward individual ions. Sage, Chiaverini, and their Lincoln Lab colleagues Colin Bruzewicz and Robert McConnell redesigned their surface trap in order to house the integrated optics without affecting its performance.

Both groups jointly designed and performed the experiments to analyze the new system.

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. 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.

Rajeev Ram, Professor of Electrical Engineering, MIT

The researchers built their new chip on a quartz substrate. The substrate consists of an array of silicon nitride “waveguides” on top of it in order to direct the laser light across the chip. A layer of glass is present over the waveguides, and the niobium electrodes are placed on top of them.

Below the holes in the electrodes, the waveguides split into a string of sequential ridges, a precisely engineered “diffraction grating” to route the light up through the holes and focus it into a beam narrow enough to target a single ion hovering 50 µm above the surface of the chip.

Prospects

The researchers evaluated the performance of the diffraction gratings and the ion traps using the prototype chip. However, no mechanism was available to vary the amount of light received by each ion. In ongoing research, the scientists are exploring the inclusion of light modulators to the diffraction gratings in order to allow the different qubits to receive light with different intensities varying with time.

This capability would allow the qubits to be programmed more efficiently, which is crucial for a practical quantum information system because the “coherence time” of the qubits limits the number of quantum operations that can be performed by the system.

“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,” says 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.”

“Of course, it's important to appreciate that this is a first demonstration,” Lucas adds. “But there are good prospects for believing that the technology can be improved substantially. As a first step, it's a wonderful piece of work.”