Already the interference of a few photons presents a very complex problem which now has been structured by focusing on the photons’ permutation symmetry. The results present an important step towards larger and thus more powerful optical quantum processors. Interference of light is an important tool in science and industry and the principle behind some of the most precise measurements devices on earth. This important effect persists, regardless of whether the source is a bright laser beam or single particles of light (photons). For single photons however, a second, completely different kind of interference owed solely to their quantum nature can occur. Such quantum interference of photons proved to be fundamental for optical quantum computation, precision measurements and information processing.

**A different kind of interference**

Single photons are unique among the quantum particles due to an inherent robustness against disturbance from the environment. This makes them ideal information carriers for quantum communication and information processing, but proves to be a challenge in quantum computation where interactions with other photons are required. “This challenge can be overcome with several experimental tricks, but at one point nearly all of them require the quantum interference of some photons” explains Philip Walther, head of the research group at the University of Vienna. The quantum interference of photons was observed nearly 30 years ago when researchers at the University of Rochester were measuring the properties of single photons. They were sending one photon in each of the two input modes of a 50/50 beam splitter, a small piece of glass that transmits light in 50% of the cases and reflects light in the other 50%. When these photons are prepared to be completely indistinguishable some output combinations, the ones where the two photons exit the beam splitter in different modes, completely vanish. “It is amazing that this effect proved to be so useful for photonic quantum computation, although it was originally intended as a precision measurement of the photons’ ‘length’ ” adds Max Tillmann, first author of the publication.

**A few more photons make the difference**

Due to the vast progress in photonic quantum technology scientists envision to double or even triple the number of photons that are processed in their quantum laboratories. One of their goals is to build integrated interferometers, so called photonic chips, for the quantum interference of a few dozen photons. In this regime the interference gets very complex and such quantum processors are seen as promising candidates to outperform their classical counterparts. To reach such a historic landmark a precise understanding of the quantum interference effect for an increased number of photons and nested photonic chips is mandatory. Researchers from the University of Vienna, together with their colleagues from Canada, Germany and Singapore now showed that in general the quantum interference can be traced back to the permutation symmetry of the interfering photons. The team carried out an experiment in which they manipulated the temporal delay of three photons propagating through an optical chip to test their theory. Their work allows to understand and interpret such complex processes with a minimal set of permutation possibilities which is an important step in the quest for optical quantum processors of tomorrow.

**Publication in "Physical Review X":**

Generalized Multiphoton Quantum Interference;

Max Tillmann, Si-Hui Tan, Sarah E. Stoeckl, Barry C. Sanders, Hubert de Guise, René Heilmann, Stefan Nolte, Alexander Szameit, and Philip Walther.

Phys. Rev. X 5, 041015 – Published 27 October 2015. doi:10.1103/PhysRevX.5.041015