According to research published Thursday in Science, physicists at Princeton University have designed a device that allows a single electron to pass its quantum information to a photon in what could be a big breakthrough for silicon-based quantum computers.
"We now have the ability to actually transmit the quantum state to a photon," said Xiao Mi, a graduate student in Princeton's Department of Physics. "This has never been done before in a semiconductor device because the quantum state was lost before it could transfer its information."
The device designed by the Princeton researchers is the result of five years of research and works by trapping an electron and a photon within a device built by HRL laboratories, which is owned by Boeing and General Motors. It is a semi-conductor chip made from layers of silicon and silicon-germanium, materials that are inexpensive and already widely deployed in consumer electronics.
Across the top of this wafer of silicon layers were laid a number of nanowires, each smaller than the width of a human hair, which were used to deliver energy to the chip. This energy allowed the researchers to trap an electron in between the silicon layers of the chip in microstructures known as quantum dots.
The electrons function as the smallest units of data used in computing which are known as bits. In a regular computer, a bit can have one of two values: either a 0 or a 1. But in a quantum computer, the smallest units of data are known as qubits, which can have a value of both 0 and 1 simultaneously. The ability to manipulate data as qubits in a quantum computer will allow for faster computing because the machine can calculate many problems simultaneously instead of one at a time.
"In our device, the state of the qubit is encoded in the position of the electron," Jason Petta, a professor of physics at Princeton, told Motherboard. "The electron is trapped in a double well potential where the electron can occupy the left well, the right well, or be in a superposition state: both left and right at the same time. The information is therefore stored in the position of a single electron."
The thing about quantum information, however, is that it is extremely fragile. Indeed, just measuring a quantum state can corrupt it, so figuring out a way to pass information from electron to electron without destroying it in the process was no small task.
The researchers settled on photons as the medium of exchange between electrons since they are less sensitive to disruption from their environment and could potentially be used to carry quantum information between quantum chips, rather than within the circuits on a single quantum chip. The ability to scale up this device would mean that photons could be used to pass quantum information from electron to electron in order to form the circuits for a quantum computer.
"Just like in human interactions, to have good communication a number of things need to work out —it helps to speak the same language and so forth," said Jason Petta, a professor of physics at Princeton. "We are able to bring the energy of the electronic state into resonance with the light particle, so that the two can talk to each other."
Following their successful experiment, the Princeton researchers hope to fine tune their device so that it is also able to electrically manipulate the spin of the trapped electrons giving them even greater control over the exchange of information between qubits.
"Our next step is to couple spin to light," said Petta. "The spin of the electron, or its magnetic moment, has some advantages, as the spin state of an electron in silicon can remain coherent for a much longer time. So in principle, one can perform many gate operations electrically before the spin superposition state collapses."