#singlephotonsources

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.

New SPDC Source at 900-950nm Improves Single-Photon Generation

Sudden Parametric Down Conversion

Quantum technology has advanced by boosting the possibility of producing crucial single photons by refining a time-multiplexed approach for Spontaneous Parametric Down-Conversion (SPDC). This groundbreaking study examines a quantum SPDC source in the 900–950 nm range under the guidance of V. O. Gotovtsev, I. V. Dyakonov, and O. V. Borzenkova, with help from K. A. Taratorin and T. B. Dugarnimaev. By correctly estimating and modeling heralding efficiency and purity, this study shows how to build brighter and more reliable single-photon sources for future quantum applications.

The Single-Photon Source Foundational Challenge

Single photons are essential for many quantum technologies, but generating them is challenging. A higher-energy pump photon spontaneously divides into two lower-energy photons, called twin photons with associated characteristics, causing SPDC, or parametric fluorescence. This method is used in quantum optics, metrology, computing, cryptography, and fundamental physics rule testing.

SPDC's inefficiency hinders many quantum information and cryptography applications. Probabilistic photon pair creation uses Poisson distribution. Only a small percentage (10−5 to 10−12) of the pump beam is used to make a duplicate pair of down-converted photons, leaving the remainder unmodified. Increasing pump power to boost output also increases the adverse possibility of creating multiple pairs.

Due to this inefficiency, reliable, heralded single-photon sources that signify a single photon when its double is detected are necessary.

Time-multiplexing boosts efficiency

Researchers developed and refined time-multiplexing to overcome poor single photon generation probability. This approach makes photon pairs using SPDC pumped by laser pulses. The idler photon, the entangled companion photon, starts signal photon storage in optical memory, which is the fundamental innovation.

Once the idler photon is recognized, the signal photon is steered into an optical memory cell and stored until the pulse series ends. By increasing the chance to capture a single photon inside a temporal window, this method increases detection likelihood.

The experiment employed a femtosecond laser and an advanced field-programmable gate array to precisely store and release photons. Researchers carefully calculated the likelihood of acquiring a single photon after multiplexing by considering photon pair formation rates and detecting system efficiency. Despite their achievement, the team noted that the optical memory cell's actuation speed limitation required careful multiplexing probability computation. Future optical memory cell advances should focus on speed and capacity.

Domain Engineering Ensures Extra Pure

High purity and generation probability are needed to maximize single-photon source dependability. Purity dictates photon quantum state definition for quantum computing and communication.

Through careful investigation and optimization, the team produced high-purity entangled photon pairs, focusing on SPDC photon pair features. Domain engineering was used to shape the generation nonlinear crystal's interior structure. This engineering targets a consistent, single-mode output that promotes purity by regulating down-conversion.

Domain engineering eliminated spectrum correlations between produced photons, which impair purity, and the phase-matching function should be Gaussian. Selecting the crystal length and using a narrow-bandwidth laser increased purity. Researchers used computer simulations to compare their unique domain structure to traditional designs and found a 10% purity gain.

The optimization process also optimized the heralding probability, given as the photon pairs' joint spectral amplitude. The JSA fully describes the two-photon state and idler-signal spectral correlations. Periodically poled laser-pumped potassium titanyl phosphate (PPKTP) crystals were used in experiments. Experimental results confirmed the model's prediction that beam confinement and phase matching conditions must be precisely controlled to maximize photon purity and heralding probability.

Robust Quantum Systems Path

This work is promising since it correctly simulates and computes source parameters and estimates the chance of single-photon production following time multiplexing. Photon storage and release are finely controlled in the unique method to maximize single-photon generation efficiency.