Quantum transistor made from a semiconductor

Researchers at the University of Maryland have made one giant leap towards quantum computing with their demonstration of the first single-photon transistor using a semiconductor chip.

Transistors are tiny switches that route electrical signals around inside the computers that power our smartphones, tablets and other devices. Quantum computers will need analogous hardware to manipulate quantum information. But the design constraints for this new information technology are stringent, and today’s most advanced processors can’t be repurposed as quantum devices. That’s because quantum information carriers, or qubits, have to follow the radically different rules laid out by quantum physics.

Scientists can use many kinds of quantum particles as qubits, even the photons that make up light. Photons have added appeal because they can swiftly shuttle information over long distances, and they are compatible with fabricated chips. However, making a quantum transistor triggered by light has been challenging because it requires that the photons interact with each other, something that doesn’t ordinarily happen.

Now, researchers from Maryland’s Joint Quantum Institute (JQI) have used a quantum memory to make photons interact, creating the first single-photon transistor made from a semiconductor. Writing in the journal Science, the researchers reveal that around 1 million of these transistors could fit inside a single grain of salt. The device is also fast and able to process 10 billion photonic qubits every second.

The transistor device has numerous holes in it, making it appear much like a honeycomb. Light entering the chip bounces around and gets trapped by the hole pattern. A small crystal sits inside the area where the light intensity is strongest and, analogous to conventional computer memory, this crystal stores information about photons as they enter the device. It can then effectively tap into that memory to mediate interactions with other photons that later arrive at the chip.

The team observed that a single photon could, by interacting with the crystal, 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 crystal 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 device’s memory ran out.

“Using our transistor, we should be able to perform quantum gates between photons,” said Edo Waks, who co-led the research. “Software running on a quantum computer would use a series of such operations to attain exponential speed-up for certain computational problems.”

According to lead author Shuo Sun, who was a grad student at Maryland at the time of the research, engineering improvements could enable 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.

Image caption: Researchers used a single photon, stored in a quantum memory, to toggle the state of other photons. Image credit: E Edwards/JQI.