The efforts of this research have been directed towards improving both the heralding efficiency and heralding rates of a high performance fiber-coupled single photon source based on an ultrafast pulsed beam-like Type-II parametric down conversion (PDC) source. Entangled photons generated from PDC are inherently broadband (many frequencies) and exit the nonlinear crystal at varying angles (different directions). This makes coupling the photons into single-mode fibers, a desirable feature for quantum computing, difficult. In response to these difficulties we have explored applying multi-layered thin film broadband anti-reflection coatings onto the ends of single-mode fibers using the technique of RF Planar Magnetron Sputtering in an effort to increase the coupling efficiency of the beam-like down converted photons. In addition, we have explored theoretically the broadband nature of novel phase matching geometries to increase heralding rates and minimize photon losses. With the ability to tailor coupling fibers to various down conversion sources we hope to increase both the heralding efficiency and heralding rate to levels needed for a broad range of future experiments.
The efforts of this research have been directed towards improving both the heralding efficiency and heralding rates of a high performance fiber-coupled single photon source based on an ultrafast pulsed beam-like Type-II parametric down conversion (PDC) source. Entangled photons generated from PDC are inherently broadband (many frequencies) and exit the nonlinear crystal at varying angles (different directions). This makes coupling the photons into single-mode fibers, a desirable feature for quantum computing, difficult. In response to these difficulties we have explored applying multi-layered thin film broadband anti-reflection coatings onto the ends of single-mode fibers using the technique of RF Planar Magnetron Sputtering in an effort to increase the coupling efficiency of the beam-like down converted photons. In addition, we have explored theoretically the broadband nature of novel phase matching geometries to increase heralding rates and minimize photon losses. With the ability to tailor coupling fibers to various down conversion sources we hope to increase both the heralding efficiency and heralding rate to levels needed for a broad range of future experiments.
We describe an experiment in which photon pairs from a pulsed parametric down-conversion (PDC) source were coupled into single-mode fibers with heralding efficiencies as high as 70%. Heralding efficiency or mode preparation efficiency is defined as the probability of finding a photon in a fiber in a definite state, given the detection of its twin. Heralding efficiencies were obtained for a range of down-conversion beam-size configurations. Analysis of spatial and spectral mode selection, and their mutual correlation, provides a practical guide for engineering PDC-produced single photons in a definite mode and spectral emission band. The spectrum of the heralded photons were measured for each beam configuration, to determine the interplay between transverse momentum and spectral entanglement on the preparation efficiency.
Summary form only given. Recent progress in quantum information processing has highlighted the benefits of integrated optic devices for quantum applications (e.g.[l]). The miniaturization of systems with increased complexity drastically reduces the required space and paves the way for future commercialization.In the past five years, several groups have demonstrated promising approaches to achieve quantum interference and photonic gates on-chip (e.g.[2]). However, in all of these experiments the preparation of the photon pairs has been performed outside the integrated devices employing bulk crystal parametric down-conversion (PDC) sources, while the efficient coupling between these sources and the integrated circuit remains as a bottleneck for the design of systems with increasing complexity. On the other hand remarkable efforts have been devoted to the development of integrated PDC sources for photon pair generation inside channel waveguides [3]. The main benefits of guided-wave PDC processes include high conversion efficiencies and spatial mode control. We combine the prospects of an efficient type-I PDC source based on Titanium-indiffused periodically poled Z-cut Lithium Niobate waveguides (Ti:PPLN) with a passive wavelength division demultiplexing coupler on-chip, shown in Figure 1. The coupler allows for excellent spatial separation generated photon pairs. The non-degenerate decay of pump photons at 532 nm into signal and idler photons provides us with a photon pair at wavelengths of 1575 nm and 800 nm, respectively. Thus low-loss fiber transmission of the prepared states at 1575 nm can be accomplished on the one hand, while on the other hand efficient heralding of conditioned states can be implemented with off-the-shelf silicon-based detectors. The quality and efficiency of our source was improved by the deposition of endface-coatings. This allows for the efficient launching of pump light into the waveguide and it simultaneously provides a close-toperfect transmission of signal and idler at the output of our source, reflecting the pump light by more than 99%. In conditioned coincidence measurements in the picosecond pulsed regime we achieved heralding of single photons with almost no influence of background radiation, expressed by high coincidences-toaccidentals ratios up to CAR > 7400. The preparation exceeded 60% of efficiency in the low pump power regime, accompanied with ultra-low second-order correlation functions of g(2) (0) <; 4.10-3. These figures of merit in conjunction with the integration of active and passive elements make our source a candidate for a basic building block in more complex and miniaturized quantum network realizations in the future.
We present a pulsed and integrated, highly non-degenerate parametric down-conversion (PDC) source of heralded single photons at telecom wavelengths, paired with heralding photons around 800 nm. The active PDC section is combined with a passive, integrated wavelength division demultiplexer on-chip, which allows for the spatial separation of signal and idler photons with efficiencies of more than 96.5%, as well as with multi-band reflection and anti-reflection coatings which facilitate low incoupling losses and a pump suppression at the output of the device of more than 99%. Our device is capable of preparing single photons with efficiencies of 60% with a coincidences-to-accidentals ratio exceeding 7400. Likewise, it shows practically no significant background noise compared to continuous wave realizations. For low pump powers, we measure a conditioned second-order correlation function of g (2) = 3.8 × 10 −3 , which proves almost pure single-photon generation. In addition, our source can feature a high brightness of 〈 n pulse 〉 = 0.24 generated photon pairs per pump pulse at pump power levels below 100 μ W. The high quality of the pulsed PDC process in conjunction with the integration of highly efficient passive elements makes our device a promising candidate for future quantum networking applications, where an efficient miniaturization plays a crucial role.
We report on the development and use of a high heralding-efficiency, single-mode-fiber coupled telecom-band source of entangled photons for quantum technology applications. The source development efforts consisted of theoretical and experimental efforts and we demonstrated a correlated-mode coupling efficiency of 97% ± 2%, the highest efficiency yet achieved for this type of system. We then incorporated these beneficial source development techniques in a Sagnac configured telecom-band entangled photon source that generates photon pairs entangled in both time/energy and polarization degrees of freedom. We made use of these highly desirable entangled states to investigate several promising quantum technologies.
Summary form only given. Parametric downconversion (PDC) allows the generation of highly coherent, spectrally pure single photons for quantum photonics. In a typical heralded PDC source, the signal photon emerges in a spectrally mixed state due to strong (anti)correlation in the joint spectrum of PDC photon pairs.One method for reducing spectral correlations is spectral filtering, but this compromises the heralding efficiency and the brightness of the source. A more sophisticated method for increasing the heralded photon purity is group-velocity matching, which works by erasing part of the timing information between the pump and PDC photons. What group-velocity matching cannot address, however, are the remaining correlations in the joint spectrum that come from the sinc-shaped phase-matching function (PMF) typical to standard PDC crystals. Further source engineering is therefore required to remove these correlations. Several studies have introduced domain-engineering techniques for shaping the phase matching function (PMF) in periodically poled crystals to improve single photon purities [1-4]. We present a new technique for tailoring crystal nonlinearities which combines the advantages of a deterministic method for domain engineering [4] and an annealing method developed for classical applications in higher-harmonic generation [5]. In our method, we first choose each domain orientation such that the generated field amplitude along the crystal matches the target field amplitude obtained by integrating the desired nonlinearity profile. Using a one-block structure instead of a two-blocks structure (as in [4]) results in a higher flexibility of the algorithm in tracking the target function and yields to more accurate results. We then use an annealing algorithm to slightly vary the width of each domain, further improving the effective PMF. Our technique outperforms its predecessors, particularly for short crystals, leading to high single photon purities, as shown in Fig.1. We experimentally characterise our method through a high-precision measurement of multiphoton interference between two heralded photons generated by two independent sources.We also notice that by considering a complex target field amplitude function it is possible to generalise the algorithm in [5] to any constant domain width (not necessarily equal to the coherence length), leading to a better approximation of the desired phase matching function, especially in the very short crystal regime. Moreover, tracking both the real and imaginary components of the target function allows us to approximate more exotic cases such as chirped nonlinearity profiles. Since scalable quantum photonics will require integrated architectures in which nonlinear processes are typically employed for single-photon generation, it will be interesting to explore new techniques similar to in-fibre Bragg grating structures which might yield quasi-periodic four-wave mixing and apply our algorithm to these new platforms.
We report on a systematic experimental study of heralding efficiency and generation rate of telecom-band infrared photon pairs generated by spontaneous parametric down-conversion and coupled to single mode optical fibers. We define the correlated-mode coupling efficiency--an inherent source efficiency--and explain its relation to heralding efficiency. For our experiment, we developed a reconfigurable computer controlled pump-beam and collection-mode optical apparatus which we used to measure the generation rate and correlated-mode coupling efficiency. The use of low-noise, high-efficiency superconducting nanowire single-photon detectors in this setup allowed us to explore focus configurations with low overall photon flux. The measured data agree well with theory and we demonstrated a correlated-mode coupling efficiency of 97±2%, which is the highest efficiency yet achieved for this type of system. These results confirm theoretical treatments and demonstrate that very high overall heralding efficiencies can, in principle, be achieved in quantum optical systems. It is expected that these results and techniques will be widely incorporated into future systems that require, or benefit from, a high heralding efficiency.