Deterministic Single Photon Sources in the Telecom C-Band
Researchers Solve Telecom C-Band Single-Photon Indistinguishability 90% Issues
Deterministic Single-photon sources
The University of Stuttgart and Julius-Maximilians-Universität Würzburg have built deterministic single photon sources, advancing quantum communications. For the first time, a solid-state emitter at the essential telecommunications C-band has exceeded a 91.7% photon indistinguishability barrier, closing the performance gap between on-demand sources and their probabilistic equivalents.
The Ideal Photon Search
“Quantum Internet” demands the ability to produce 100% identical, “indistinguishable” photons on demand. Long-distance quantum networking protocols and quantum logic gates use two-photon interference, which requires this characteristic.
Researchers have generally had two source options. Probabilistic spontaneous parametric down-conversion (SPDC) sources emit high-quality, indistinguishable photons at random times. This randomness makes scaling systems with dozens of photons, such photonic quantum computing, nearly hard.
However, semiconductor quantum dots (QDs) are deterministic “artificial atoms” that release one photon every laser pulse. Despite their success at shorter wavelengths (780 nm to 960 nm), the scientific community has struggled to obtain great indistinguishability in the telecom C-band (about 1550 nm).
Starting the Revolution
The Nico Hauser, Matthias Bayerbach, and Stefanie Barz study team used a specific device architecture to overcome these challenges. Indium arsenide (InAs) quantum dot in InAlGaAs cladding is the source. A circular Bragg grating (CBG) resonator with the quantum dot increased photon output and reduced environmental decoherence. In Nature Communications, the researchers noted that optimum growth reduces dephasing time while incorporation into the CBG resonator reduces excitonic durations. Ternary digital alloying improved crystal quality and structural refinement.
Meeting 90% Benchmark
The study's most notable result is 91.7 ± 0.2% two-photon interference visibility. This value, much higher than past estimates of 72% for comparable devices, sets a new bar for C-band spectral spectrum indistinguishability.
To reach this result, the researchers meticulously examined four laser excitation systems:
Excitation over the band gap: 800 nm pumping. Incoherent excitation with LA-phonon assistance: Blue-tuned from resonance.
Pumping 1404.2 nm is resonance #1.
Pumping 1498.2 nm is resonance #2.
The LA-phonon-assisted excitation approach achieved the maximum indistinguishability and lowest multi-photon emission probability with a second-order autocorrelation value (g(2)(τ=0)) of 0.017 ± 0.001. The researchers attributed this success to the shortest excitonic lifespan under this stimulation mode, which decreases noise exposure.
Moving toward real-world implementation
Telecom C-band selection is not random. This wavelength range is the norm for silicon-based integrated photonics and modern fiber-optic networks since it loses the least signal. To enable scalable quantum technologies, the team showed that deterministic sources can match probabilistic SPDC sources in photon quality.
These devices must overcome difficulties before being deployed in commercial networks. Despite good photon quality, the current setup's efficiency is just 0.5% (adjusted to 2.1% with setup losses). The researchers found that blinking—when the quantum dot randomly stops emitting—is the greatest constraint with a blinking-related efficiency of 26.9%.
Significant Turning Point
The study is a “key milestone” that makes solid-state emitters suitable for photonic quantum networking despite efficiency issues. The authors noted that our findings are crucial to scalable quantum dots-based photonic quantum devices that integrate on-demand operation with exceptional photon quality.
The DFG, Carl Zeiss Foundation, and others will fund coupling efficiency and photon extraction improvements. They assure that quantum communication will use the internet's fiber-optic base.