Researchers found a way to make two photons interact through an ultra-thin glass fiber, a finding that could aid in the development of quantum technology.
In free space, two photons do not interact and light waves pass through each other without one having an influence on the other, Vienna University of Technology reported. Photon interaction is crucial for quantum technology advances, such as transmitting information through quantum channels or for building logic gates. Researchers have now successfully established a strong interaction between two single photons.
"In order to have light interact with light, one usually uses so-called nonlinear media," said Professor Arno Rauschenbeutel (Vienna Center for Quantum Science and Technology, Institute for Atomic and Subatomic Physics, TU Wien).
This light can influence the properties of the materials, and the material can also affect the light, leading to a coupling of photons. In the past systems have only been able to be used at high light intensities where multiple photons are coupled. This new system can create a bond between only two photons that is so strong their phase is changed by 180 degrees.
"It is like a pendulum, which should actually swing to the left, but due to coupling with a second pendulum, it is swinging to the right. There cannot be a more extreme change in the pendulum's oscillation," Rauschenbeutel said. "We achieve the strongest possible interaction with the smallest possible intensity of light."
The system works by pairing an ultra-thin glass fiber with a "bottle-like" optical resonator, which allows light to enter, move in circles, and return to the resonator. This sequence inverts the phase of the photon, causing a "wave crest" to appear where one would usually find what is called a "wave trough." When a single rubidium atom is coupled to the resonator, the system is altered and hardly any light is able to enter; this process changes when two photons arrive at the same time.
"The atom is an absorber which can be saturated," Rauschenbeutel said. "A photon is absorbed by the atom for a short while and then released again into the resonator. During that time, it cannot absorb any other photons. If two photons arrive simultaneously, only one can be absorbed, while the other can still be phase shifted."
When the photons hit the resonator at the same time they are indistinguishable and experience a joint phase shift of 180 degrees, creating a much different behavior than what would be seen in a single photon.
"That way, a maximally entangled photon state can be created," Rauschenbeutel said. "Such states are required in all fields of quantum optics - in quantum teleportation, or for light-transistors which could potentially be used for quantum computing."
