Quantum technologies just got a boost from a recent demonstration of an optical device that filters two particles of light (photons) based on the correlations between their polarization that are allowed only in the quantum world. This “entanglement filter” passes the pair of photons only if they inhabit the same quantum state, without the user – or anything else – ever knowing what that state is.The device, demonstrated by a team of physicists and engineers, will have many important applications to quantum technologies, including computers, communication and advanced measurement. Jeremy O’Brien, professor of physics and electrical engineering at Bristol University, together with his collaborators in Japan, has realized an entanglement filter made by combining two state-of-the-art developments in optical technologies with single photons: a special type of mirror that is sensitive to the polarization of light and an optical device that enables stability at the billionths-of-a-meter level.“This is a very exciting development in quantum information science. Because our entanglement filter acts on photonic qubits, it is promising for quantum technologies because photons are the logical choice for communication, metrology and lithography and are a leading approach to information processing,” O’Brien said of the research. “The filter can be used for the creation as well as the purification of entanglement, which will be important in realizing quantum relays and repeaters for long-distance quantum communication.”Filters are one of the most powerful tools available in science and technology, while entanglement is the defining characteristic of quantum information science. An entanglement filter is of fundamental interest and likely will find wide application in quantum information science and technology.Filters that act on the quantum correlations associated with entanglement must operate nonlocally on multiple quantum systems, typically two-level qubits. Such a device has been proposed for photonic qubits, but the technical requirements to build such a device, an optical circuit with two extra photons and multiple quantum gates, requiring both quantum interference and classical interference in several nested interferometers, have been lacking.The entanglement filter will be a key element in the control of multiphoton quantum states, with a wide range of applications in entanglement-based quantum communication and quantum information processing.Quantum technologies aim to exploit the unique properties of quantum mechanics, the physics theory that explains how the world works at very small scales. For example, a quantum computer relies on the fact that quantum particles, such as photons, can exist in a “superposition” of two states at the same time – in stark contrast to the transistors in a PC, which can be only in state “0” or “1.”Photons are an excellent choice for quantum technologies because they are relatively noise-free. The information can be moved around quickly – at the speed of light – and manipulating single photons is easy.Making two photons “talk” to each other to realize the all-important controlled-NOT gate is much harder, but O’Brien and his colleagues at the University of Queensland demonstrated this back in 2003.Last year, O’Brien’s Centre for Quantum Photonics at Bristol showed how such interactions between photons could be realized on a silicon chip, pointing the way to advanced quantum technologies based on photons.Photons also are required to talk to each other to realize the ultraprecise measurements that harness the laws of quantum mechanics – quantum metrology. In 2007, O’Brien and the same Japanese collaborators reported such a quantum metrology measurement with four photons.The work was supported by the EPSRC and by Japanese funding agencies.For more information, visit: www.bristol.ac.uk.