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Development of a Pulsed, Diode-Pumped, Fiber-Based, Entangled Photon Source

Andi Shahaj

Published Mar 1, 2024• 0 citations
OpenAlex

Abstract

Entangled photon sources have applications in such fields as quantum communications, quantum sensing and quantum computing. Of particular interest are pulsed entangled photon sources, as they provide an additional degree of freedom – the temporal degree of freedom – to manipulate entanglement properties, hence increasing the information capacity of the entangled photons they produce. An entangled photon source in the telecom wavelength typically requires a 780 nm pump laser. So far, in most cases, pulsed entangled photon sources are pumped by the Ti:Sapph mode-locked laser, which, due to its bulky size and high cost, has limited the widescale adoption of pulsed entangled photon sources. In this thesis, I demonstrate a simple, cost-effective, pulsed laser diode capable of competing with the Ti:Sapph for the purposes of entangled photon generation. The diode and driver are capable of pulsing at up to 60 MHz , with peak power exceeding 1 W and pulse energy in excess of 1 nJ , and with a spectral width narrower than 0.1 nm. I also demonstrate a pulsed, fully fiber-based, entangled photon source, with high entanglement quality (maximum concurrence of 0.985) and a high pair generation rate (0.15 pairs/pulse). The source is to our knowledge the first of its kind.

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Entangled photon sources have applications in such fields as quantum communications, quantum sensing and quantum computing. Of particular interest are pulsed entangled photon sources, as they provide an additional degree of freedom – the temporal degree of freedom – to manipulate entanglement properties, hence increasing the information capacity of the entangled photons they produce. An entangled photon source in the telecom wavelength typically requires a 780 nm pump laser. So far, in most cases, pulsed entangled photon sources are pumped by the Ti:Sapph mode-locked laser, which, due to its bulky size and high cost, has limited the widescale adoption of pulsed entangled photon sources. In this thesis, I demonstrate a simple, cost-effective, pulsed laser diode capable of competing with the Ti:Sapph for the purposes of entangled photon generation. The diode and driver are capable of pulsing at up to 60 MHz , with peak power exceeding 1 W and pulse energy in excess of 1 nJ , and with a spectral width narrower than 0.1 nm. I also demonstrate a pulsed, fully fiber-based, entangled photon source, with high entanglement quality (maximum concurrence of 0.985) and a high pair generation rate (0.15 pairs/pulse). The source is to our knowledge the first of its kind.
Entangled-photon sources are a key element for most quantum-information applications, ranging from quantum communication systems to quantum simulators and full photonic quantum computers. Spontaneous parametric downconversion (SPDC), requiring a pump laser and a nonlinear medium, is up-to-date still the preferred technique to generate such photonic entanglement. To scale the transmission rates or gate operations of the aforementioned applications, high clock rates of the pump lasers are needed. Moreover, photonic quantum-processing algorithms require photons with not only high entanglement visibility but also high spectral indistinguishability and purity. This can either be achieved by narrow spectral filtering, costly in rate, or by suitable choice of the pump pulse length (typically in the ps range). Attempts have been made to increase the rate of conventional fs-lasers by adding multiple spatial-paths [1] but this method cannot be used to go beyond GHz repetition rates. This results in the quest to produce pump lasers with very high repetition rate, ps-pulse width and high enough power to drive the weak SPDC process.
We present an experimental setup for generation of entangled-photon pairs via spontaneous parametric down-conversion, based on the femtosecond-pulsed laser. Our entangled-photon source utilizes a 76-MHz-repetition-rate, 100-fs-pulsewidth, mode-locked, ultrafast femtosecond laser, which can produce, on average, more photon pairs than a cw laser of an equal pump power. The output infrared pump photons (λ = 810 nm) are first up-converted to blue light (λ = 405 nm) and, subsequently, down-converted in a 1.5-mm-thick, type-II BBO crystal via spontaneous down-conversion. The resulting entangled pairs are counted by a pair of high-quantum-efficiency, single-photon, silicon avalanche photodiodes. The total down-conversion efficiency of our system, corresponding criterion of the pump power for real entangled coincident events, has been calculated to be 0.86 × 10-9. Our apparatus is intended as an efficient source/receiver system for the quantum communications and quantum cryptography applications.
Abstract Entanglement is a universal resource in quantum networks, yet entangled photon sources are typically custom-made for a specific use case. Versatility, both in terms of state modulation and tunability of the temporal properties of the photons, is the key to flexible network architectures and cryptographic primitives that go beyond quantum key distribution. Here, we report on a flexible source design that produces high-quality entanglement in continuous-wave and GHz-rate-pulsed operation modes. Utilizing off-the-shelf optical components, our approach uses a fiber-based Sagnac loop to generate polarization-entangled photons at telecom wavelength with high efficiency and fidelities above 0.99. Phase modulation up to GHz before entangled state generation is also possible for fast entangled state switching. We show phase modulation at 100 MHz with an average fidelity of 0.95. Furthermore, the source 60 nm spectral bandwidth is entirely compatible with fully reconfigurable wavelength-multiplexed quantum networks.
Quantum computation and quantum communication require sources of entangled photons, especially from low-cost diode lasers. This study considered a polarization entangled photons pulse source produced via a spontaneous parametric down-conversion process in the type-I phase-matching of two nonlinear crystals, incorporating a continuous wave laser diode operating in pulse mode as a pump. The generated photons were measured based on visibility as a function of the pump frequency and collection apertures. A polarization correlation between photon pairs of approximately 93% and a Bell parameter S value of 2.56 ± 0.05 were obtained using the source described here.
The generation of interband photon pairs with wavelengths near 0.5 and 1.6 μm in a photonic-crystal fiber under low-power cw optical pumping by a diode laser with a central wavelength of 0.8 μm has been experimentally demonstrated. It has been found that the generation rate of entangled photons under cw pumping is comparable with values obtained with pulsed pumping by a femtosecond Ti:sapphire laser if the average cw pump power is an order of magnitude higher than the average pulsed pump power. The reached rates of photon generation are ensured by the used photonic-crystal fiber with a small effective mode area and a special dispersion profile. The reached low noise in the output signal is ensured by the separation of carrier frequencies of generated photons into different spectral bands.
We demonstrate pulsed operation of a bidirectionally pumped polarization Sagnac interferometric down-conversion source and its generation of narrow-band, high-visibility polarization-entangled photons. Driven by a narrow-band, mode-locked pump at 390.35 nm, the phase-stable Sagnac source with a type-II phase-matched periodically poled KTiOPO4{\text{KTiOPO}}_{4}KTiOPO4​ crystal is capable of producing 0.01 entangled pair per pulse in a 0.15-nm bandwidth centered at 780.7 nm with 1 mW of average pump power at a repetition rate of 31.1 MHz. We have achieved a mean photon-pair generation rate of as high as 0.7 pair per pulse, at which multipair events dominate and significantly reduce the two-photon quantum-interference visibility. For low generation probability $\ensuremath{\alpha}$, the reduced visibility $V=1\ensuremath{-}\ensuremath{\alpha}$ is independent of the throughput efficiency and of the polarization analysis basis, which can be utilized to yield an accurate estimate of the generation rate $\ensuremath{\alpha}$. At low $\ensuremath{\alpha}$ we have characterized the source entanglement quality in three different ways: average quantum-interference visibility of 99%, the Clauser-Horne-Shimony-Holt SSS parameter of $2.739\ifmmode\pm\else\textpm\fi{}0.119$, and quantum state tomography with 98.85% singlet-state fidelity. The narrow-band pulsed Sagnac source of entangled photons is suitable for use in quantum information processing applications such as free-space quantum key distribution.