Protoype Photonic Chip Shines a Light on Practical Quantum Computing

Engineers have recently developed a prototype for an integrated photonic chip for quantum computing. This prototype overcomes the scaling limitations of current quantum systems and could help solve the problem of how to create practical quantum computers.

Trapping Ions

Quantum bits, or qubits, are a superposition of two states (0 and 1) that take the place of traditional single state bits (0 or 1). One of the technologies for realizing qubits involves trapped ions, though this has historically required large and complex optical systems. For this reason, scaling the system beyond a few tens of qubits has proven to be a challenge.

The new research offers a way around this problem by taking a different approach to trapping and controlling the ions. Typical ion traps resemble a tiny cage, with electrodes serving as the bars by creating an electric field. Instead of using cage traps, the engineers developed their prototype using surface traps, which involve electrodes embedded in a chip’s surface. The ions hover above the electrodes at a distance of 50 µm.

"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 researcher Jeremy Sage. "These cage traps work very well, but they really only work for maybe 10 to 20 ions, and they basically max out around there."

Photonic Integration

The team’s real breakthrough is in the way they deliver light to the ions. In order to perform a quantum calculation, you have to be able to control the energy state of every qubit. This is accomplished using laser light. However, in surface traps, individual ions are spaced about 5 µm apart, meaning it’s extremely difficult to shine a laser on one without affecting its neighbors.

The researchers solved this problem by designing optical components that can direct laser light to individual ions, and then integrated them directly on the surface trap chip. The chip uses nanophotonic single-mode waveguides and focusing grating couplers to direct the light.

Optical micrograph of the designed ion trap with integrated waveguides and couplers underneath at multiple trap zones; the waveguides and couplers are visible via topography transfer to the metal. Ions are trapped at one of the positions marked by the red dots, 50 μm above the electrodes, with appropriate potentials applied to the d.c. and radio frequency (RF) electrodes. (Image and caption courtesy of Nature Nanotechnology.)

"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 engineer Rajeev 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 team’s successful demonstration of their prototype suggests that large-scale trapped-ion quantum systems could employ similar techniques. However, one remaining barrier is that the integrated photonic system has no mechanism for varying the amount of light delivered to the ions. The researchers are investigating the addition of light modulators to the gratings in order to address this issue.

Quantum computers have been much anticipated, and we’re getting ever closer to realizing them. While they won’t make you a better League of Legends player, they’ll theoretically perform certain types of calculations much quicker than traditional computers. The team’s prototype chip marks another step toward this goal.

To learn more about the potential applications of quantum computing, check out Is Quantum Cryptography the Future of Cybersecurity?