Scientists from Paderborn University, the University of Basel, and Ruhr University Bochum have achieved 90% quantum photon interference visibility using semiconductor nanostructures. This advance addresses a major bottleneck in quantum communication by producing nearly identical photon pairs via biexciton decay inside an optical microcavity.
Researchers Use Biexciton Decay for Quantum Information Processing
Photons serve as an ideal medium for transmitting information in quantum information processing. However, deploying them for complex calculations requires strict indistinguishability. Until recently, sources suffered from temporal correlation or poor focus, dragging quality down. A collaborative team of PhD students in Basel and Paderborn tackled this by using biexciton decay within semiconductor quantum dots integrated into a specialized optical cavity.
A quantum dot acts as an artificial atom inside a semiconductor, generating individual light particles. By placing an indium gallium arsenide quantum dot inside an open optical microcavity—featuring a bottom mirror built into the semiconductor and a curved upper mirror allowing light escape—the research team tuned the resonance by adjusting mirror spacing. Study Lead Author Timon Baltisberger from the University of Basel explained the mechanism.
The so-called ‘biexciton cascade’ in a semiconductor quantum dot emits photons at the push of a button, which are of great interest for modern applications. This is a process in which a quantum dot is doubly excited and the excitation then decays. This generates two photons, one after the other.
Optical Cavity Boosts Photon Coherence and Visibility Scores
Controlling the biexciton decay via the optical cavity accelerated the first emission, boosting coherence. The biexciton-to-exciton transition photon reached a raw interference visibility score of 90% with a two-percentage-point uncertainty. The second photon reached 80% with a six-percentage-point uncertainty under the same cavity settings. Without the cavity effect, typical theoretical visibility sits near 60%.
When researchers corrected for imperfect single-photon purity, the visibility estimates climbed further. The first photon reached 94%, while the second rose to 82%. Yet, the two emissions did not achieve identical scores. Spectral filters successfully isolated the first or second emission for measurement, testing interference between successive photons separated by 13.1 nanoseconds.
Crystal Lattice Vibrations Limit Photon Purity
Further investigation into underlying physical mechanisms, supported by Dr. Arne Ludwig at Ruhr University Bochum, revealed that resonator optimization can maximize photon purity. However, a physical ceiling remains. Dr. Klaus Jöns from Paderborn University noted that the purity is ultimately limited only by vibrations in the semiconductor’s crystal lattice, known as phonons.
We have found that the purity of the photons generated can also be optimized using the resonator and is limited only by vibrations in the semiconductor’s crystal lattice (phonons). This phenomenon, known as ‘cavity feeding’, must be taken into account in future designs and can then be systematically minimized even further.
Addressing these crystal vibrations and improving the collection efficiency of the second photon remain essential steps for transforming this setup into a practical entangled pair source. The study itself was published in Physical Review Letters under DOI:10.1103/t8sk-b2w4.
