For decades, engineers have made computers smaller, faster, and cheaper by improving their fundamental component, the integrated circuit — the generic term that describes the maze of pathways that control the flow of electricity through a silicon chip.
Now, in a new scientific paper, a team led by Professor Jelena Vuckovic describes how it built an integrated circuit to control the flow of light through a diamond chip, helping pave the way for quantum processors that, in theory, could perform some tasks, such as code-breaking, far faster than the fastest electronic computers today.
“Quantum technology is roughly where electronic technology was in the early 1970s,” said Vuckovic, who led the 13-person research team. “Researchers have figured out how to make very basic integrated circuits, but now they have to be scaled and made much better.”
Building an optical integrated circuit in diamond is a practical step toward making quantum technologies useful. Engineers have long known how to design ordinary electronic circuits and control the electrons that help perform computational tasks. But they are still struggling to build quantum circuits with all the necessary pathways to control photons, the basic particles in light.
To meet this design challenge, Vuckovic’s lab used diamond, a crystal that can have atomic impurities that trap electrons. A laser beam can be pointed at one of these trapped electrons, causing it to spin. These spinning electrons — called qubits, or quantum bits — are the basis for performing quantum calculations just as transistors are the basis for performing electronic calculations. In essence, a quantum processor would have an array of qubits connected by light flowing through an optical integrated circuit, just as an electronic computer has transistors connected by current flowing through wires.
To create their optical integrated circuit, Vuckovic’s team, led by graduate student Constantin Dory, developed algorithms that considered the positions of the impurities that form qubits, and the ways that lasers could manipulate these qubits to perform calculations. The algorithms also took into account the capabilities of the equipment that engineers use to make chips. After considering all these variables together using a process called inverse design, the algorithms generated a schematic that the researchers used to fabricate their optical integrated circuit.
To date, the researchers were able to fabricate circuits consisting of six building blocks, potentially enabling interaction of only a few qubits. To build a useful quantum processor, the researchers say they’ll have to design and build a chip with hundreds of interacting qubits, all interconnected with optical pathways, which is extremely challenging, but possible. The Stanford team is also experimenting with other crystals that may prove useful for controlling light and qubits.
Vuckovic foresees one application of their diamond optical chip in the near term — spy-proofing fiber optic networks. Today, all sorts of sensitive data from bank transfers to government secrets flow as a stream of ordinary light through fiber optic cables. Spies or criminals can tap into this stream without leaving any trace. However, if quantum light, such as a single photon, is used for communication, eavesdropping can be detected because any intrusion would leave behind subtle fingerprints.
Challenges remain. For starters, it is difficult to transmit quantum light over large distances. But Vuckovic is working with several other research teams around the world to build quantum repeaters, in which optical chips with a few qubits each, positioned at regular intervals, would be used to transmit tamper-proof quantum signals over long distances, even across continents. “We think a long-distance quantum network is achievable within a five-year time frame,” she said.