Scatterings image

Less than a centimeter in width, the device from the Princeton-HRL team can achieve coherent coupling of a single photon and an electron in a double quantum dot, electrically controlled by nanowire gate electrodes. [Image: Princeton University]

In the past several years, silicon-hosted semiconductor quantum dots (QDs) have emerged as an intriguing potential platform for a solid-state quantum computing. But actually passing information from one QD-based quantum bit, or qubit, to another in a silicon-based quantum circuit, using the most natural information carriers—photons—has proved a hurdle.

A team of U.S. scientists led by Jason Petta of Princeton University, N.J., USA, has now built a chip-scale silicon device that reportedly can achieve strong coupling between a single electron in a silicon-hosted double QD and a single microwave photon in an overlying resonant cavity (Science, doi: 10.1126/science.aal2469). The scheme—which effectively allows for quantum information transfer between the electron and the photon, and the potential shunting of that information to another qubit as much as a centimeter away—represents, in the view of the team, “an important milestone toward the development of silicon-based quantum processors.”

Defeating charge noise

While much recent progress in quantum computing has focused on platforms such as diamond nitrogen-vacancy centers, superconductors and trapped ions (see “Quantum computing: How close are we?,” OPN, October 2016), quantum computing in silicon has a natural draw. One reason is that so much of today’s chip-manufacturing infrastructure is already silicon based; another is that recent research has shown the potential for quantum spin coherence times of seconds to minutes for electrons in QDs hosted in high-purity silicon.

The problem comes in trying to pass quantum information from one such electron qubit to another. To do so requires coupling the electron to a single photon that would act as the information carrier. And such coupling runs aground owing to charge noise—fluctuations in the occupation of available quantum states in the semiconductor that can perturb the local electric field and get in the way of strong electron-photon coupling.

Double quantum dots

To hammer down charge noise and get to better electron-photon coupling, Petta’s team, which included other scientists at Princeton and at HRL Laboratories, Calif., began with a semiconductor heterostructure consisting of silicon and silicon-germanium layers that could trap a single layer of electrons below the chip’s surface. They then topped this device with three overlapping layers of aluminum nanowires that acted as gate electrodes.

The aluminum electrodes defined a double QD in the heterostructure that could capture individual electrons in a tightly defined potential well. But the electrodes had another virtue: they formed an electromagnetic shield to protect the double QD-qubit, bringing about a several-orders-of-magnitude reduction in charge noise from the surrounding semiconductor.

Superconducting cavity

Finally, on top of the double-QD structure, the researchers fabricated a tiny, half-wavelength niobium superconducting cavity to trap microwave photons. By using current in the aluminum electrodes to tune the energy level of the electron to match that of the photon, they were able to achieve strong electron-photon coupling. They confirmed the coherent hybridization of the light and matter quantum states by observing a key indicator of such coupling, vacuum Rabi splitting (the emergence of two resolvable normal modes in the cavity transmission spectrum, separated by the vacuum Rabi frequency).

The researchers write that the strong coupling between a single silicon-hosted electron and a microwave photon “paves the way toward … long-range coupling of [silicon] qubits.” As a next step, the team will work to extend the device to couple one quantum property in particular, electron spin, to photons.

“In the long run, we want systems where spin and charge are coupled together to make a spin qubit that can be electrically controlled,” Petta noted in a press release accompanying the paper. “We've shown we can coherently couple an electron to light, and that is an important step toward coupling spin to light.”