Semiconductor quantum transistor opens the door for photon-based computing

Researchers from the University of Maryland claim to have demonstrated the first single-photon transistor using a semiconductor chip.

Making a quantum transistor triggered by light has been a previous challenge because it requires that the photons interact with each other, the researchers explain.

According to the team, the device is compact: roughly one million of these new transistors could fit inside a single grain of salt. It is also fast and able to process 10billion photonic qubits every second.

"Using our transistor, we should be able to perform quantum gates between photons," says Professor Edo Waks of the University of Maryland's A. James Clark School of Engineering and Joint Quantum Institute. "Software running on a quantum computer would use a series of such operations to attain exponential speedup for certain computational problems.

The photonic chip is made from a semiconductor with numerous holes in it. Light entering the chip bounces around and gets trapped by the hole pattern; a quantum dot sits inside the area where the light intensity is strongest.

Analogous to conventional computer memory, the dot stores information about photons as they enter the device. The dot can effectively tap into that memory to mediate photon interactions, meaning that the actions of one photon affect others that later arrive at the chip.

"In a single-photon transistor the quantum dot memory must persist long enough to interact with each photonic qubit," says Shuo Sun, lead author of the new work. "This allows a single photon to switch a bigger stream of photons, which is essential for our device to be considered a transistor."

To test that the chip operated like a transistor, the researchers examined how the device responded to weak light pulses that usually contained only one photon. In a normal environment, such dim light might barely register, however, in this device, a single photon gets trapped for a long time, registering its presence in the nearby dot.

The team observed that a single photon could, by interacting with the dot, control the transmission of a second light pulse through the device. The first light pulse acts like a key, opening the door for the second photon to enter the chip. If the first pulse didn't contain any photons, the dot blocked subsequent photons from getting through. This behaviour is similar to a conventional transistor where a small voltage controls the passage of current through its terminals. Here, the researchers successfully replaced the voltage with a single photon and demonstrated that their quantum transistor could switch a light pulse containing around 30 photons before the quantum dot's memory ran out.

Prof Waks says that his team had to test different aspects of the device's performance prior to getting the transistor to work. "Until now, we had the individual components necessary to make a single photon transistor, but here we combined all of the steps into a single chip.”

Sun says that with realistic engineering improvements their approach could allow many quantum light transistors to be linked together. The team hopes that such speedy, highly connected devices will eventually lead to compact quantum computers that process large numbers of photonic qubits.