Quantum computing research scientists at the University of Maryland's A. James Clark School of Engineering have developed the first single-photon transistor using a semiconductor chip.
The research, conducted alongside the Quantum Institute and led by Professor of Electrical and Computer Engineering, Edo Waks, was published on Thursday in the journal Science, and details a transistor so compact that it could fit roughly one million of them inside a single grain of salt.
It is also said to be fast, being able to process 10 billion photonic qubits every second.
"Using our transistor, we should be able to perform quantum gates between photons," said Waks. "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, making it appear much like a honeycomb. It works via light entering the chip, which bounces around and gets trapped by the hole pattern where a small crystal called a "quantum dot".
This sits inside the area where the light intensity is the strongest and stores information about photons as they enter the device. It can thus 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," added Shuo Sun, lead author of the research study. "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 fund 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," the paper explained.
"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.
Waks added 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," he said.
With "realistic engineering improvements", it's hoped that the scientists' work could allow many quantum light transistors to be linked together, leading to speedy, highly connected devices such as compact quantum computers that can process large numbers of photonic qubits.
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