Quantum Processors for Single Photons

Scientists have realized a photon-photon logic gate via a deterministic interaction with a strongly coupled atom-resonator system.

A team of scientists from the Quantum Dynamics Division of Professor Gerhard Rempe has successfully realized a quantum logic gate where two light quanta play the key actors. Professor Gerhard Rempe is the director at the Max Planck Institute of Quantum Optics. This attempt was highly challenging as photons do not typically interact at all but pass each other uninterrupted


Illustration of the processes that take place during the logic gate operation: The photons (blue) successively impinge on the right onto the partially transparent mirror of a resonator which contains a single rubidium atom (symbolized by a red sphere with yellow electron orbitals). The atom in the resonator plays the role of a mediator which imparts a deterministic interaction between the two photons. The diagram in the background represents the entire gate protocol. Credit: Graphic: Stephan Welte, MPQ, Quantum Dynamics Division

In the experiment presented here two independently polarized photons impinge, in quick succession, onto a resonator which is made of two high-reflectivity mirrors. Inside a single rubidium, an atom is trapped forming a strongly coupled system with the resonator. The resonator amplifies the light field of the impinging photon at the position of the atom enabling a direct atom-photon interaction. As a result, the atomic state gets manipulated by the photon just as it is being reflected from the mirror. This change is sensed by the second photon when it arrives at the mirror shortly thereafter.


After their reflection, both photons are stored in a 1.2-kilometre-long optical fiber for some microseconds. Meanwhile, the atomic state is measured. A rotation of the first photon’s polarization conditioned on the outcome of the measurement enables the back action of the second photon on the first one. “The two photons are never at the same place at the same time and thus they do not see each other directly. Nevertheless, we achieve a maximal interaction between them,” explains Bastian Hacker, a Ph.D. student at the experiment.

The scientists envision that the new photon-photon gate could pave the way towards all-optical quantum information processing. “The distribution of photons via an optical quantum network would allow linking any number of network nodes and thus enable the setup of a scalable optical quantum computer in which the photon-photon gate plays the role of a central processing unit (CPU),” explains Professor Gerhard Rempe.



via: ScienceDaily