Researchers have demonstrated a single-photon switch and transistor that is enabled by solid-state quantum memory. The device consists of a semiconductor spin qubit coupled to a nanophotonic cavity. It is extremely compact (roughly 1 million of the quantum transistors could fit inside a grain of salt) and is able to process 10 billion photonic qubits per second. To create a quantum transistor, a University of Maryland team embedded a quantum dot (QD) in the photonic crystal cavity of its solid-state system. A single photon is used to manipulate the occupation of electronic energy levels within the QD. A long-standing goal in optics has been to produce a solid-state all-optical transistor, in which the transmission of light can be controlled by a single photon that acts as a gate or switch. Light gets trapped in the photonic chip’s honeycomb structure. The QD sits inside the area of the chip where the light intensity is strongest. The QD stores information about photons as they enter the chip, in a way that is analogous to conventional computer memory. The QD can tap into that memory to mediate photon interactions. The single-photon switch and transistor are enabled by the solid-state quantum memory. “In a single-photon transistor the quantum dot memory must persist long enough to interact with each photonic qubit," said researcher Shuo Sun. "This allows a single photon to switch a bigger stream of photons, which is essential for our device to be considered a transistor.” Researchers observed how the device responded to weak pulses containing only one photon and found that the single photon’s presence was registered in the QD. The team further observed that the first light pulse through the device acted like a key, opening the door for a second photon to enter the chip, and that by interacting with the QD, a single photon could control the transmission of a second light pulse through the device. The photonic chip was behaving in a way that was similar to a conventional transistor, where a small voltage controls the passage of current. Researchers say that with the gate open, 27.7 photons can get through the nanophotonic cavity on average before the internal state of the device reset, thus demonstrating single-photon switching and the gain for an optical transistor. Edo Waks, professor of electrical engineering at the University of Maryland, said 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,” Waks said. The team believes that these results show that semiconductor nanophotonic devices can produce strong and controlled photon-photon interactions that could enable high-bandwidth photonic quantum information processing. “Using our transistor, we should be able to perform quantum gates between photons," Waks said. "Software running on a quantum computer would use a series of such operations to attain exponential speedup for certain computational problems.” With realistic engineering improvements, their approach could allow many quantum transistors to be linked together, according to the researchers. The team believes that such fast, highly connected devices could lead to compact quantum computers that process large numbers of photonic qubits. The research was published in Science (doi:10.1126/science.aat3581).