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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

A novel compact and efficient source of photonic entanglement

Fabian Laudenbach, Sebastian Kalista, Michael Hentschel, Philip John Walther, Hannes Hübel

Published Jun 1, 2017• 0 citations
Publisher

Abstract

Summary form only given. Up to this point, photon-entanglement sources suffered from the compromise of being either compact or efficient, but not both at the same time [1-6]. Our new concept of creating entanglement [7] combines most of the advantages of the various existing schemes. The setup offers high coupling efficiencies due to its collinear design and is still as compact and simplistic as a BBO source [1], requiring only one crystal with uniform poling periodicity, pumped from only one direction.Periodic poling of a nonlinear crystal allows for a positive energy transfer from the pump (p) field to signal (s) and idler (i). The required poling periodicity to support a given process kp - ks - kt requires an alternation of the effective nonlinear coefficient deff in integer multiples of a certain grating length, the poling periodicity A = 2πμ/Δk, where m is an odd integer and Δk = kp - ks - kt is the momentum-difference vector. Note that Δk can be both positive and negative. Our source relies on the fact that one periodically poled crystal with grating period A, pumped with a given laser frequency, can provide phase-matching for two downconversion processes simultaneously: one with a positive momentum-difference vector Δk and one whose Δk is of same magnitude but opposite sign. Careful design of the experimental setup allows for generation of two orthogonally polarised pairs which are interchangeable in wavelength. This results in a polarisation- and frequency-entangled state.

DOI
10.1109/cleoe-eqec.2017.8087345

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Summary form only given. Up to this point, photon-entanglement sources suffered from the compromise of being either compact or efficient, but not both at the same time [1-6]. Our new concept of creating entanglement [7] combines most of the advantages of the various existing schemes. The setup offers high coupling efficiencies due to its collinear design and is still as compact and simplistic as a BBO source [1], requiring only one crystal with uniform poling periodicity, pumped from only one direction.Periodic poling of a nonlinear crystal allows for a positive energy transfer from the pump (p) field to signal (s) and idler (i). The required poling periodicity to support a given process kp - ks - kt requires an alternation of the effective nonlinear coefficient deff in integer multiples of a certain grating length, the poling periodicity A = 2πμ/Δk, where m is an odd integer and Δk = kp - ks - kt is the momentum-difference vector. Note that Δk can be both positive and negative. Our source relies on the fact that one periodically poled crystal with grating period A, pumped with a given laser frequency, can provide phase-matching for two downconversion processes simultaneously: one with a positive momentum-difference vector Δk and one whose Δk is of same magnitude but opposite sign. Careful design of the experimental setup allows for generation of two orthogonally polarised pairs which are interchangeable in wavelength. This results in a polarisation- and frequency-entangled state.
We demonstrate a new generation mechanism for polarisation- and colour-entangled photon pairs. In our approach we tailor the phase-matching of a periodically poled KTP crystal such that two downconversion processes take place simultaneously. Relying on this effect, our source emits entangled bipartite photon states, emerging intrinsically from a single, unidirectionally pumped crystal with uniform poling period. Its property of being maximally compact and luminous at the same time makes our source unique compared to existing photon-entanglement sources and is therefore of high practical significance in quantum information experiments.
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.
A periodically poled lithium niobate (PPLN) crystal has been used as an efficient source of non-collinearly generated polarization-entangled photon pairs at 810 and 1550 nm. The PPLN crystal was endowed with a specially designed poling pattern and the entangled photons were generated via the nonlinear optical process of spontaneous parametric down conversion (SPDC). A novel design based on overlapping two concurrent type-I quasi-phase-matching structures in a single PPLN crystals produced correlated pairs of alternatively polarized photons in largely separated spectral regions. The phase of the resulting two-photon state is directly linked to parameters of the nonlinear grating. Continuous tunability of the generated Bell state, from Phi(+) to Phi(-), has been demonstrated by translating a slightly wedged crystal perpendicular to the pump beam.
Entangled photon pair sources are of great interest for quantum communication and computation protocols [1]. To date, the brightest sources are based on SPDC in nonlinear materials, with periodically poled crystals leading the field. Their nonnalized detected pair rate has reached a regime of millions of pairs per second per mW of input power in the visible spectral region [2], At these rates, quantum communication protocols are limited not anymore by the source brightness, but by the performance of single-photon detectors. Therefore, the focus of improving entangled photon pair sources lies on their practicality with respect to their size, robustness and ease of use.