Researchers have demonstrated the first single-photon transistor using a semiconductor chip. The 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 behavior 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.
The quantum transistor should be able to perform as a quantum gate between photons. Software running on a quantum computer would use a series of such operations to attain exponential speedup for certain computational problems.
One million of these new transistors could fit inside a single grain of salt. They are able to process 10 billion photonic qubits every second.
A single-photon gate
A long-standing goal in optics is to produce a solid-state alloptical transistor, in which the transmission of light can be controlled by a single photon that acts as a gate or switch. Sun et al. used a solid-state system comprising a quantum dot embedded in a photonic crystal cavity to show that transmission through the cavity can be controlled with a single photon. The single photon is used to manipulate the occupation of electronic energy levels within the quantum dot, which in turn changes its optical properties. With the gate open, about 28 photons can get through the cavity on average, thus demonstrating single-photon switching and the gain for an optical transistor.
Abstract – A single-photon switch and transistor enabled by a solid-state quantum memory
Single-photon switches and transistors generate strong photon-photon interactions that are essential for quantum circuits and networks. However, the deterministic control of an optical signal with a single photon requires strong interactions with a quantum memory, which has been challenging to achieve in a solid-state platform. We demonstrate a single-photon switch and transistor enabled by a solid-state quantum memory. Our device consists of a semiconductor spin qubit strongly coupled to a nanophotonic cavity. The spin qubit enables a single 63-picosecond gate photon to switch a signal field containing up to an average of 27.7 photons before the internal state of the device resets. Our results show that semiconductor nanophotonic devices can produce strong and controlled photon-photon interactions that could enable high-bandwidth photonic quantum information processing.