A Princeton University research team has shown that silicon quantum bits (qubits), mediated by a microwave photon, can interact even when spaced relatively far apart on a computer chip. The ability to transmit messages across relatively long distances on a silicon chip using multiple qubits could open new possibilities for quantum computing. The Princeton team, led by professor Jason Petta, connected the qubits via a narrow cavity that carried light in a way similar to the fiber optic cables that deliver internet signals to homes and businesses. The cavity contained a single photon, which could receive a message from one qubit and transmit it to the next qubit. The two qubits were located about one-half centimeter apart. (To put that distance in perspective, if each qubit were the size of a house, one qubit would be able to send a message to another qubit located 750 miles away.) The key to making this approach work, the team said, was finding a way to get the qubits and the photon to vibrate at the same frequency. The team succeeded in tuning both qubits independently of each other while still coupling them to the photon. “You have to balance the qubit energies on both sides of the chip with the photon energy to make all three elements talk to each other,” researcher Felix Borjans said. Researchers at Princeton University showed that a silicon-spin quantum bit (shown in the box) can communicate with another quantum bit located a significant distance away on a computer chip. Their achievement could enable connections between multiple quantum bits to perform complex calculations. Courtesy of Felix Borjans, Princeton University. Each qubit is composed of a single electron trapped in a double quantum dot. By zapping the electron with a microwave field, the researchers were able to flip the electron’s spin up or down to assign the qubit a quantum state of 1 or 0. Resonant microwave-mediated coupling was realized between two electron spins that were physically separated by more than 4 mm. The researchers observed an enhanced vacuum Rabi splitting when both spins were tuned into resonance with the cavity, indicating a coherent interaction between the two spins and a cavity photon. The results indicate that microwave-frequency photons could be used to generate long-range two-qubit gates between spatially separated spins. “This is the first demonstration of entangling electron spins in silicon separated by distances much larger than the devices housing those spins,” said Thaddeus Ladd, senior scientist at HRL Laboratories and a collaborator on the project. “Not too long ago, there was doubt as to whether this was possible, due to the conflicting requirements of coupling spins to microwaves and avoiding the effects of noisy charges moving in silicon-based devices. This is an important proof-of-possibility for silicon qubits because it adds substantial flexibility in how to wire those qubits and how to lay them out geometrically in future silicon-based ‘quantum microchips.’” To build their quantum circuit, the researchers used silicon and germanium — materials heavily used in the semiconductor industry. Today’s prototype quantum computers from Google, IBM, and other companies contain tens of qubits made from a technology involving superconducting circuits, but some technologists view silicon-based qubits as more promising in the long run. “The wiring or ‘interconnects’ between multiple qubits is the biggest challenge toward a large-scale quantum computer,” James Clarke, director of quantum hardware at Intel, said. “Jason Petta’s team has done great work toward proving that spin qubits can be coupled at long distances.” Petta said that the researchers’ ultimate goal is to have multiple quantum bits arranged in a two-dimensional grid that can perform more complex calculations. “The study should help in the long term to improve communication of qubits on a chip as well as from one chip to another,” he said. The research was published in Nature (www.doi.org/10.1038/s41586-019-1867-y).