Hong–Ou–Mandel interference of two phonons in trapped ions

• Published in: Nature (London) Vol. 527; no. 7576; pp. 74 - 77
• Main Authors: Toyoda, Kenji, Hiji, Ryoto, Noguchi, Atsushi, Urabe, Shinji
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 05.11.2015; Nature Publishing Group
• Subjects: 639/766/483/1139; 639/766/483/3926; 639/766/483/481; Atoms & subatomic particles; Humanities and Social Sciences; Interference (Light); Ions; letter; multidisciplinary; Observations; Phonons; Properties; Quantum physics; Quantum theory; Science
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature15735

The Hong–Ou–Mandel effect is a quantum phenomenon that involves the interference of bosonic particles and demonstrates their indistinguishability; this effect has been demonstrated previously for photons and neutral atoms, and is now demonstrated for phonons, using a system of trapped ions that are promising building blocks for quantum computers. Hong-Ou-Mandel effect in captivity The Hong-Ou-Mandel effect is a quantum phenomenon that involves the interference of bosonic particles and demonstrates their indistinguishability. The effect has been demonstrated for photons and neutral atoms. Kenji Toyoda and colleagues now demonstrate Hong-Ou-Mandel interference for phonons, which are quasiparticles associated with quantized vibrational modes in a system. The demonstration is performed in a system of trapped ions, which are promising building blocks for quantum computers. In addition, the authors attempt to create an entangled state with the phonons. While phonons have previously only played a supporting role in quantum-computing experiments with trapped ions, this result opens new perspectives for establishing phonons as quantum information carriers in their own right. The quantum statistics of bosons and fermions manifest themselves in the manner in which two indistinguishable particles interfere quantum mechanically. When two photons, which are bosonic particles, enter a beam-splitter with one photon in each input port, they bunch together at either of the two output ports. The corresponding disappearance of the coincidence count is the Hong–Ou–Mandel effect 1 . Here we show the phonon counterpart of this effect in a system of trapped-ion phonons, which are collective excitations derived by quantizing vibrational motions that obey Bose–Einstein statistics. We realize a beam-splitter transformation of the phonons by employing the mutual Coulomb repulsion between ions, and perform a two-phonon quantum interference experiment using that transformation. We observe an almost perfect disappearance of the phonon coincidence between two ion sites, confirming that phonons can be considered indistinguishable bosonic particles. The two-particle interference demonstrated here is purely a quantum effect, without a classical counterpart, hence it should be possible to demonstrate the existence of entanglement on this basis. We attempt to generate an entangled state of phonons at the centre of the Hong–Ou–Mandel dip in the coincidence temporal profile, under the assumption that the entangled phonon state is successfully generated if the fidelity of the analysis pulses is taken into account adequately. Two-phonon interference, as demonstrated here, proves the bosonic nature of phonons in a trapped-ion system. It opens the way to establishing phonon modes as carriers of quantum information in their own right 2 , 3 , 4 , and could have implications for the quantum simulation of bosonic particles 5 , 6 and analogue quantum computation via boson sampling 7 .