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.
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.
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.
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.
Efficient generation and detection of indistinguishable twin photons are at the core of quantum information and communications technology (Q-ICT). These photons are conventionally generated by spontaneous parametric down conversion (SPDC), which is a probabilistic process, and hence occurs at a limited rate, which restricts wider applications of Q-ICT. To increase the rate, one had to excite SPDC by higher pump power, while it inevitably produced more unwanted multi-photon components, harmfully degrading quantum interference visibility. Here we solve this problem by using recently developed 10 GHz repetition-rate-tunable comb laser, combined with a group-velocity-matched nonlinear crystal, and superconducting nanowire single photon detectors. They operate at telecom wavelengths more efficiently with less noises than conventional schemes, those typically operate at visible and near infrared wavelengths generated by a 76 MHz Ti Sapphire laser and detected by Si detectors. We could show high interference visibilities, which are free from the pump-power induced degradation. Our laser, nonlinear crystal, and detectors constitute a powerful tool box, which will pave a way to implementing quantum photonics circuits with variety of good and low-cost telecom components, and will eventually realize scalable Q-ICT in optical infra-structures.
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.
Efficient Generation of Entangled Photon-Pairs The first experiments with correlated photons have been performed in the context of EPR-Bell experiments on the realistic and local properties of quantum mechanics. The source used there produced pairs of polarization entangled photons from a 2-photon decay of Calcium atoms. The technical requirements of these experiments were high (vacuum systems, stronge dye-lasers, etc.) whereas the efficiency of the source was quite low. An important step forward was the introduction of spontaneous parametric down conversion (SPDC), which has become the most common source in quantum optics for generating correlated or entangled photon pairs. In this process photons of an intense pump laser convert to photon pairs in an optical nonlinear crystal. Conservation of energy and momentum leads to strong correlations between the generated photons. With this kind of two-photon source it was possible to realize or improve many experiments on the foundations of quantum mechanics addressing the EPR-Paradoxon and in the new field of quantum information. But again, more advanced experiments and applications suffer from the limited ef- ficiency of the fluorescence process. Many photon pairs are lost by spatial and spectral filtering, which is necessary to achieve polarization entanglement and long coherence times. Different techniques have been implemented to increase the number of photon pairs using two-crystal arrangements, focusing techniques or periodically poled crystals. Most of these methods have the disadvantage that no entangled photons have been observed. It is the aim of this work to increase the yield and to improve the mode definition of entangled photon pairs generated by resonant enhancement of the pump mode and the fluorescence modes. As a first step a linear cavity for the pump mode was realized. Since the conversion probability is proportional to the pump power it was possible to increase the photon pair count rate by factor of 7 over the previous source. Besides the possibility of further improvement on already established pair correlation experiments, such an enhancement allows to build a compact source for photon pairs, in which an expensive argon-ion laser is replaced by a cheap diode laser. Among other applications such sources are of strong interest for quantum cryptography. 3 In many quantum information experiments optical fibers are use to carry the photons over long distance. Therefore, light from the parametric down-conversion source has to be efficiently coupled into fibers. In the second part we report on a new method to optimize collection efficiency by matching the angular distribution of the parametric fluorescence to the spatial mode of an optical fiber. By using this technique, we detected 366500 polarization-entangled photon pairs per second in the near-infrared region in single-mode optical fibers for 465 mW pump power (at 351.1 nm) with a 2 mm BBOcrystal. The entanglement of the photon pairs was verified by measuring polarization correlations of more than 96% in a HV-basis and in a ±45◦-basis. To our knowledge, such enormous count rates of highly entangled photon pairs have not been reached yet with any other technique. In the third part of this thesis we investigated the process of parametric downconversion in a cavity which is resonant to certain longitudinal down-conversion modes only. The idea of placing the parametric down-conversion source inside a cavity is not new. Such a device is usually referred to as a single or double resonant optical parametric oscillator (OPO) and is mainly used to generate squeezed quantum states. In that kind of application the system is operating close to but still under the threshold of oscillation. In our application the situation is quite different. The system is operating far below threshold so that mainly spontaneous emission occurs. In that mode correlations between single photons can still be observed. But bouncing the light back and forth inside the cavity increases the interaction length and hence enhances the signal levels of the down-conversion fields. Further, by resonating two certain modes only, the bandwidth is reduced by orders of magnitude and the coherence time is found to be inverse proportional to the bandwidth. A similar experiment has already been realized with a type-I parametric down-converter in the resonator. We have tried to realize a compact double resonant OPO far below threshold with a type-II parametric down-converter in a high-finesse cavity to realize a bright source of entangled photon pairs with extremely narrow bandwidth.
The generation of high purity single photons and entangled photon pairs is a crucial first step in many quantum optics experiments, including those that require quantum teleportation [1], entanglement swapping [2], and the generation of multi-photon entanglement [3]. The most widely used and established approach for creating photon pairs is spontaneous parametric down conversion (SPDC) - where pump photons undergo a non-linear optical process to create a down-converted photon pair [4]. Traditionally, the generation of high-purity heralded single photons required severe spectral filtering, introducing additional loss. However, through careful engineering of spectral correlations of the down-converted photon pairs one can circumvent the need for filtering and avoid the accompanying reduction in heralding efficiency.