Researchers Use Microwaves to Connect Distant Silicon Quantum Bits

From building an unhackable internet to potentially supercharging artificial intelligence, many experts are envisioning that quantum computing will eventually lead us beyond the current limitations of classical computing. That’s because the peculiar laws of quantum mechanics allow for bits of information in a quantum computer to exist in multiple states simultaneously, rather than only 0 or 1 in the binary system of classical computing — meaning that quantum computers will be able to tackle computations much faster than conventional machines.

Not surprisingly, engineering an affordable and reliable quantum computer is a huge challenge, and some experts are working to develop cheaper and more accessible quantum computers using silicon-based materials, instead of relying on more exotic components like superconducting circuits or floating ions. In what could be a big step forward for mainstreaming future quantum systems, a team of Princeton University researchers have demonstrated that it is possible to get two silicon “spin” quantum bits (or “spin qubits”) to communicate with each other, even when spaced over relatively long distances on a microchip.

‘Resonant Microwave-Mediated Interactions’

The team’s paper in Nature describes how a single microwave photon traveling back and forth within a narrow cavity — much like how signals flow through a fiber optic cable — was used to transmit data from one qubit to another. Each silicon qubit is composed of one electron caught in a double quantum dot — a nano-scaled, semiconductor structure that can trap single electrons so their “spin” can be more easily manipulated.

A crucial part of the experiment was to “couple” the qubits and photons together by getting them to vibrate at the same frequency, yet also allowing for the qubits to be tuned independently of each other, while still remaining coupled to the photon. To do this, the team manipulates the “spins” of the trapped electrons — getting them to spin either “up” or “down” — by subjecting them to a microwave field. Most notably, the two qubits were located about one-half centimeter apart — or to put in proportionally, about the same distance as 750 miles if each qubit were the size of a house.

“Being able to couple qubits with each other is a very important ingredient of a quantum computer, as the power of quantum computing lies in the entanglement of quantum states,” explained Princeton University researcher and paper co-author Felix Borjans. According to the strange laws of quantum physics, particles that are “entangled” together remain connected so that whatever happens to one particle will affect the other, even when they are located far apart.

“Using single electron spins as quantum bits, this coupling has so far been limited to the very short-ranged nearest-neighbor coupling, limiting the interaction range to about 100 nanometers,” added Borjans. ‘This work is a first demonstration that single electron spins can be coupled over distances over four orders of magnitude larger than previous measurements (4 millimeters versus 100 nanometers).”

More importantly, the researchers also observed that the interaction between the two qubits remained “coherent” — meaning that the qubits were able to maintain their phase relationship, which is vital for a quantum computer to function properly. Quantum decoherence can occur when sensitive quantum systems are subjected to environmental factors like temperature fluctuations, resulting in computational errors, and continues to be a major issue, even in state-of-the-art quantum computers that are currently being built.

“The spin of an electron is an incredibly microscopic quantum state,” said Borjans. “Opening up ways to couple a single electron over macroscopic-length scales comes with the challenge that you also allow uncontrolled fluctuations in the environment to change the quantum state in ways that you didn’t anticipate. In order to exchange information coherently and in a well-defined manner, you need to make sure that the controlled interactions outpace any detrimental interactions with the environment. Optimizing these targeted and selective interactions is a key challenge of these experiments.”

Ultimately, Borjan notes that the work points strongly to the prospect of utilizing more versatile and reliable silicon-based qubits in tomorrow’s quantum microchips.

“While the final design of quantum computing processors is not yet determined, being able to transfer quantum information over larger distances on-chip is expected to be an important building block on the path to its larger-scale realization.”

Images: Princeton University