Controlling nonclassical light is key to the development of quantum technologies for such applications as quantum computers and ultraprecise metrology. And researchers from the University of Bristol Centre for Quantum Photonics have brought such technologies a step closer by demonstrating control of four single photons on a silicon chip.Quantum particles such as photons can exist in two states at once, unlike PC transistors, which can exist only in “0” or “1” states at a given time. Photons are not the only choice for quantum technologies, but they are an ideal one because of their relative lack of noise, their resilience to decoherence and their speed – light speed.And manipulating single photons is easy; it’s making them interact with each other that is difficult. Difficult, but important: Nonclassical interference of single photons is critical for generating the entanglement necessary for quantum technologies.Jeremy L. O’Brien, director of the Centre for Quantum Photonics and a professor of physics and of electrical engineering, led the current research. In 2003, he demonstrated the interaction of two photons in a quantum logic gate; in 2008, the Bristol team demonstrated such interaction on a silicon chip.“Despite these impressive advances, the ability to manipulate photons on a chip has been missing,” said Alberto Politi, a doctoral student at the centre.But now, O’Brien, Politi and colleagues have used a microscopic metal electrode lithographically patterned onto a silicon chip to demonstrate precise control of four photons. As in optical fibres, the photons propagate in silica waveguides patterned on the silicon chip, and the electrode is used to manipulate them.“This precise manipulation is a very exciting development for fundamental science as well as for future quantum technologies,” O’Brien said.Using optical fibres, the researchers coupled photons into the CIP Technologies chip and then back out. They changed the temperature of the silica waveguide by applying a voltage across the metal electrode above it; this changed the path along which the photons travelled. To confirm precise manipulation of the photons in the chip, they measured the output of the device.They proved that up to four photons achieved quantum entanglement. “These entangled states are responsible for famously ‘weird’ behaviour arising in quantum mechanics, but are also at the heart of powerful quantum technologies,” said Jonathan Matthews, also a doctoral student at the centre, who carried out the experiments with Politi.Attaining quantum entanglement with four photons means that the state of each individual photon could not be determined; only the collective state of all four could be defined. By linking photons together in this nonclassical manner, Matthews said, dramatic advantages over classical physics can be realized.This is an artist’s impression of a University of Bristol quantum metrology experiment in which researchers made ultraprecise measurements on a chip using photons. Quantum metrology, as with quantum computing, requires photon-to-photon interaction. Image by Will Amery, University of Bristol.Quantum computing is not the only application that could benefit from the Bristol group’s research. The investigators have demonstrated its usefulness for quantum metrology, and there are more possibilities. “As well as quantum computing and quantum metrology, on-chip photonic quantum circuits could have important applications in quantum communication, since they can be easily integrated with optical fibres to send photons between remote locations,” Politi said.“The really exciting thing about this result,” O’Brien said, “is that it will enable the development of reconfigurable and adaptive quantum circuits for photons. This opens up all kinds of possibilities.”The team has reported its results in the June issue of Nature Photonics as well as in a Postdeadline Paper at June’s International Quantum Electronics Conference (IQEC) in Baltimore.A Nature Photonics commentary on the research described this work as a key step on the road to quantum computation, concluding: “The most exciting thing about this work is its potential for scalability. The small size of the [device] means that far greater complexity is possible than with large-scale optics.”