Light particles are loners. They do not normally interact with each other. When using photons for the transmission of information, this solitariness is an advantage. If, however, the particles are to be used to process the information contained in them, this characteristic becomes disadvantageous. Now, physicists Bastian Hacker, Stephan Welte, Stephan Ritter and Gerhard Rempe from the Max Planck Institute of Quantum Optics have leapt this seemingly insurmountable obstacle. The researchers trapped a single rubidium atom between the mirrors of an approximately half-millimeter-long optical resonator. Previous experiments had already shown that a single rubidium atom is an excellent quantum memory.
The physicists made two photons of a preset polarization impinge successively on the rubidium atom. Their plan: that the two photons influence each other through the atom. To begin, the first photon irradiated the atom-resonator system and was reflected by it. The state of the atom changed depending on the polarization of the photon (graphic: Stephan Welte).
Then, the second photon was sent onto the atom. Its interaction with the atom had an influence on its polarization that depended on the state of the atom. Thus, the first photon can indirectly affect the state of the second.
Until this point, the relationship between the two photons was only one-sided. But ultimately, both particles should interact with each other. To enable this, the physicists first performed a measurement on the atom whose state had been influenced by the second photon. In order to be able to change the polarization state of the first photon conditioned on this measurement, the photon – moving at the speed of light – had to be temporarily stored. For this purpose, the researchers sent both reflected photons through a 1.2-kilometer-long optical fiber, which they passed through within six microseconds. After leaving the fiber, the polarization of the first photon was then suitably rotated, whereas the polarization of the second photon remained unchanged. Thus the second photon could also influence the first, although the two photons were never at the same place at the same time.
The physicists are particularly interested in situations where both photons are initially linearly polarized. Here, after the dual reflection, quantum mechanical entanglement occurs. The generation of entangled states of polarization means that the polarizations of the two photons are no longer independent from one another, and that only a common polarization state can be indicated. In this case, measurement of the polarization of the photons leads to correlated results, even if the particles are far apart. Thus, the physicists have achieved photon-photon interaction and created a so-called quantum logic gate.
A quantum logic gate is needed for the construction of a quantum computer. Such a gate performs operations on so-called quantum bits (qubits). Quantum bits can be stored in the polarization of photons, for example. In addition to “1”s and “0”s, qubits can be in all other possible superposition states. In a quantum computer, this would increase computing performance enormously in comparison to performance that is currently possible.
The quantum gate is still in its early stage. Light particles are often lost during reflection from the resonator system. However, the system has the potential to enable efficient, all-optical quantum information processing by linking many of these photon-photon gates together.