A solid-state qubit photon logic gate could harness the quantum nature of light and semiconductors, expanding the capabilities of computers in extraordinary ways. In a computer processor, billions of transistors switch back and forth between two states using logic operations, or “gates” — the building blocks of computational processes in all computers, even the future quantum versions. In optical communications, information from the switches can be encoded onto light, which then travels long distances through glass fiber. Researchers at the Joint Quantum Institute (JQI) and the University of Maryland’s Department of Electrical and Computer Engineering are working to harness this light by performing an ultrafast logic gate on a photon using a semiconductor quantum dot. Photons are a proven transit system for information. In quantum devices, they are ideal information carriers, relaying messages between quantum bits (qubits) such as neutral atoms, ion traps, superconducting circuits, nitrogen vacancy centers and quantum dots. Quantum dots are more attractive to integrate with electronics because they are semiconductors. Professor Edo Waks of JQI and colleagues implemented a conditional logic gate called a Controlled-NOT (CNOT), which flips the state of a second qubit if the control qubit is in a state 1 or does nothing if the control qubit is in state 0. Illustration of the Joint Quantum Institute’s CNOT gate with a semiconductor quantum dot and a photon. Modified version of Figure 1 from paper, courtesy of authors and E. Edwards. “Although this logic operation sounds simple, the CNOT gate has the important property that it is universal, which means that all computational algorithms can be performed using only this simple operation,” Waks said. “This powerful gate can thus be seen as an important step towards implementing any quantum information protocol.” In their experiment, the control qubit was a quantum dot and the second qubit was a photon with two polarization states. The photon could be oriented horizontally or vertically with respect to a defined direction. Just as energy levels for a quantum dot constitute a qubit, the two available polarizations make up a photonic qubit. Light was injected into a photonic crystal cavity containing a quantum dot and a large external magnetic field. The magnetic field shifted the energy levels of the quantum dot, enabling it to simultaneously act as both a stable qubit and a highly efficient photon absorber. The unique energy level structure of the system changed the qubit state of the quantum dot, rendering it completely invisible to the light. Lead author Hyochul Kim works on upgrading the device. Courtesy of E. Edwards/JQI. This property made the CNOT gate possible. Light trapped in a cavity that does not see a quantum dot (in qubit state 1) will eventually leak out, with its polarization flipped. However, if the quantum dot is in qubit state 0, the light is modified so that incoming and outgoing polarizations actually remain the same. In this case, the photonic qubit was not flipped. A sensitive camera collected a fraction of the light that leaked out of the cavity, using special optics to see if a photon’s polarization was flipped. The team controlled the quantum dot qubit’s state, and the photons came from an external laser, so they were not intrinsically connected to the quantum dot through absorption/emission processes. “Using an external photon source has an advantage that the quantum dot state is not destroyed during the process,” said lead author Dr. Hyochul Kim. “Currently, we use a strongly attenuated laser as the photon source, but, eventually, this can be replaced with true single-photon sources.” Quantum dot in photonic crystal cavity is placed in an apparatus such as the one shown here, which is then cooled to a few degrees above absolute zero. A scanning electron microscope images the device. Laser light is guided through fibers and injected into the crystal. Courtesy of E. Edwards/JQI. The demonstration — which happens in a picosecond, making it important when increasing the number of qubits and operations so that a calculation completes before the system’s quantum behavior is lost — paves the way for the next generation of devices that will improve light collection and quantum dot qubit coherence times. “To improve coherence time, we need to trap the electron or hole in the quantum dot and use their spin as a qubit. This is more challenging, and we are currently working on this,” Kim said. The investigators plan to use truly single photons as the light source. “Quantum dots are also excellent single photon sources,” Kim said. “We consider such a system, where single photons are periodically emitted from the neighbor quantum dot, which are then connected to logic devices on the same semiconductor chip.” The research was reported in Nature Photonics (doi: 10.1038/nphoton.2013.48). For more information, visit: jqi.umd.edu