Two trapped and laser-cooled calcium ions can display two-phonon quantum interference, a first in science, according to a research group led by Urabe Shinji and Toyoda Kenji of the Graduate School of Engineering Science of Osaka University, Japan. The phenomenon could have implications for the development of ion traps in quantum information processing and quantum computation.

Pairs of bosons (photons, gluons and the Higgs particle, for example) or pairs of fermions (quarks, electrons, neutrinos and other particles) that are indistinguishable from each other can nevertheless interfere in a quantum mechanical sense. For instance, two photons, which are bosons might enter a beam-splitter with one photon at each input port and then bunch together at either of the two output ports, the researchers report. This causes the coincidence count to disappear, the so-called Hong-Ou-Mandel effect.

This quantum interference effect, well known with photons, has not, until now, been observed with phonons, the units of vibrational energy that arises as atoms oscillate within condensed matter. The Osaka group's demonstration of this phenomenon could allow quantum simulation to be carried out with phonons and allow quantum interface research to be undertaken.

The team's beam-splitter for phonons used the mutual Coulomb repulsion between ions so that the phonons associated with each ion could be made to interference. The team reports that, "We observe an almost perfect disappearance of the phonon coincidence between two ion sites, confirming that phonons can be considered indistinguishable bosonic particles." This purely quantum effect has no classical counterpart the team emphasizes and could be used to demonstrate the existence of quantum entanglement of the phonons as if they were themselves bosons or fermions. As such, the team successfully attempted to generate an entangled state of phonons at the centre of the Hong-Ou-Mandel dip in their ion trap.

"Two-phonon interference, as demonstrated here, proves the bosonic nature of phonons in a trapped-ion system," the team reports. The researchers suggest that their experiments could open the way to establishing phonon modes as carriers of quantum information in their own right. This, in turn, could have implications for how research into the quantum states of bosons is carried out allowing analogue quantum simulations to be undertaken.

Toyoda told Materials Today that the next step will be to increase the number of phonon modes from two to realize an N-mode bosonic system. This would allow quantum simulations or analog quantum computation to be carried out, including the boson-sampling problem, using a multi-mode bosonic system. He adds that another direction will be to observe similar effects in systems which are different from trapped ions, for example in light-controlled micro- and nano-mechanical systems. "By combining these with multi-phonon interference as in our recent work, such systems could eventually be used to build large-scale quantum information processors based on phonons," he suggests.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".