Researchers have taken a big step forward in creating a working quantum computer—by using photons manipulated by a silicon chip.
“We made a photonic quantum processor, which creates and manipulates two qubits encoded in photons for universal two-qubit quantum computation,” said Xiaogang Qiang, a research associate at China’s National University of Defense Technology. Xiaogang was the lead author of a paper in Nature Photonics describing the work.
Quantum computers are based on qubits, or quantum bits, which are the basic units of quantum information. While classical computer bits can only output 1s or 0s, qubits can represent both at the same time—which expands their calculating power. Qubits can communicate with each other—known as quantum entanglement—so that measuring one qubit provides information about the state of another. Entanglement is required to do large-scale calculations.
Currently, qubits are created by superconducting wires chilled to near absolute zero or trapped ions held in place by lasers. But as the number of qubits in a system grows, the more likely they are to interact with the outside world—which causes them to lose their quantum state more rapidly and makes them useless for computing.
Xiaogang believes that photons could solve that problem because they don’t interact with the environment. They can be manipulated with precision and can process information at light speed. In addition, a photonic chip can take advantage of the existing silicon-based infrastructure of today’s computer industry.
The chip itself consists of interferometers which split the photons into different spatial modes. Each mode passes through a specific waveguide—an instrument that channels and restricts electromagnetic waves. Having a photon in one waveguide represents a 1, while it represents a 0 in another. Knowing which spatial mode one photon is in tells you which mode its entangled partner is in. The photons are programmed using thermo-optic phase shifters controlled by electrical voltages.
To scale-up the chip for commercial use, the researchers will have to find out how to generate many more entangled photons. This will require fitting enough phase shifters, interferometers and other optical components onto the chip to process those photons.
But according to Xiaogang, silicon photonics has shown the capacity for cramming many devices into tight spaces and getting them all to work with precision. He believes that the chip is “the practical way to implement the ultimate large-scale photonic quantum processor.”
If silicon photonics lives up to its promise, it will bring us significantly closer to quantum computing—which many experts think is still a decade away.
Read more about quantum computing at Single Atoms May Solve Potential Shortage of Data Storage Capacity.