A nonlinear process based on backward quasi-phase matching (BQPM) can be used to realize mirrorless optical parametric oscillation, the generation of paired photons with a separable joint spectral amplitude and narrow wavelength bandwidth, and the preparation of counterpropagating polarization-entangled photons, which shows distinct advantages over some applications based on forward quasi-phase matching. In this work, three types of BQPM in a bulk periodically poled potassium titanyl phosphate crystal with a single period are theoretically analyzed. Experimentally, the harmonic wave generated by second-harmonic generation in type 0 and type I exhibits a narrow bandwidth of 15.5 GHz. Furthermore, photon pairs generated by spontaneous parametric downconversion in all types of BQPM (type 0, type I, and type II) at 7th order are observed and characterized. Their coincidence-to-accidental ratios are all greater than 5 × 10 3 in the pump power range from 10 mW to 500 mW. This research lays the foundation for further applications of BQPM in nonlinear optics, quantum optics, and quantum information processing.
A nonlinear process based on backward quasi-phase matching (BQPM) can be used to realize mirrorless optical parametric oscillation, the generation of paired photons with a separable joint spectral amplitude and narrow wavelength bandwidth, and the preparation of counterpropagating polarization-entangled photons, which shows distinct advantages over some applications based on forward quasi-phase matching. In this work, three types of BQPM in a bulk periodically poled potassium titanyl phosphate crystal with a single period are theoretically analyzed. Experimentally, the harmonic wave generated by second-harmonic generation in type 0 and type I exhibits a narrow bandwidth of 15.5 GHz. Furthermore, photon pairs generated by spontaneous parametric downconversion in all types of BQPM (type 0, type I, and type II) at 7th order are observed and characterized. Their coincidence-to-accidental ratios are all greater than 5 × 10 3 in the pump power range from 10 mW to 500 mW. This research lays the foundation for further applications of BQPM in nonlinear optics, quantum optics, and quantum information processing.
Entanglement enables many promising applications in quantum technology. Devising alternative generation methods and harnessing entanglement are prerequisites for practical applications. Here, we realize a distinct polarization-entangled source by simultaneously achieving type-0 and type-I backward quasiphase matching (BQPM) through spontaneous parametric down-conversion in a single bulk crystal, which is different from all previous entangled-source configurations. Pumping the crystal with a single polarized beam generates a nonmaximally polarization-entangled state, which can be further projected to a maximal Bell state with a pair of Brewster windows. Hong-Ou-Mandel interference experiments are done on polarization-degenerate photon pairs for both type-0 and type-I BQPM processes for the first time. The emitted photons in both processes have a bandwidth as narrow as 15.7 GHz. The high quality of this source is characterized by various methods. The rather simple configuration, narrow bandwidth, and high entanglement quality make the source very promising for many quantum information tasks.
Entangled photon pairs are crucial resources for quantum information processing protocols.Via the process of spontaneous parametric downconversion (SPDC), we can generate these photon pairs using bulk nonlinear crystals.Traditionally, the crystal is designed to satisfy a specific type of phase-matching condition.Here, we report controllable transitions among different types of phase matching in a single periodically poled potassium titanyl phosphate crystal.By carefully selecting pump conditions, we can satisfy different phase-matching conditions.This allows us to observe first-order Type-II, fifth-order Type-I, third-order Type-0, and fifth-order Type-II SPDCs.The temperature-dependent spectra of our source were also analyzed in detail.Finally, we discussed the possibility of observing more than nine SPDCs in this crystal.Our work not only deepens the understanding of the physics behind phase-matching conditions, but also offers the potential for a highly versatile entangled biphoton source for quantum information research.
We theoretically investigate the properties of extended phase-matching (EPM) for spontaneous parametric down-conversion (SPDC) in four kinds of potassium titanyl phosphate isomorphs ( i.e. , KTiOPO 4 , KTiOAsO 4 , RbTiOPO 4 , and RbTiOAsO 4 crystals). The technique is based on Type II 2nd-order nonlinear optic interaction in periodically poled ferroelectric domain structures, where a single photon with a frequency of 2 ω generates a pair of photons with frequencies of ω that are orthogonally polarized with respect to each other. Under EPM, both quasi-phase matching (QPM) and group velocity (GV) matching between interacting waves are efficiently satisfied for the generation of a pair of polarization-entangled bi-photon states with frequencies of ω . Our simulation results show that the generated photon pairs have broad spectral bandwidths of over 69 - 95 nm in tele-communication bands, which are much broader than those of the typical non-EPM case ( e.g. , ~ sub-nm bandwidth when only the QPM, not the GV matching, is achieved). We will describe the EPM properties of the four kinds of KTP isomorphs in terms of interaction type, GV matching wavelength, domain poling period, and spectral bandwidth. We highlight that the KTP isomorphs have nice potential as suitable components for constituting new quantum information processing systems.
We report the generation of frequency-uncorrelated photon pairs from counter-propagating spontaneous parametric down-conversion in a periodically-poled KTP waveguide. The joint spectral intensity of photon pairs is characterized by measuring the corresponding stimulated process, namely, the difference frequency generation process. The experimental result shows a clear uncorrelated joint spectrum, where the backward-propagating photon has a narrow bandwidth of 7.46 GHz and the forward-propagating one has a bandwidth of 0.23 THz like the pump light. The heralded single-photon purity estimated through Schmidt decomposition is as high as 0.996, showing a perspective for ultra-purity and narrow-band single-photon generation. Such unique feature results from the backward-wave quasi-phase-matching condition and does not has a strict limitation on the material and working wavelength, thus fascinating its application in photonic quantum technologies.
Backward-wave three-wave mixing is a difficult χ (2) interaction to observe, because it requires ultrashort poling periods to achieve phasematching. Having realized sub-micrometer periods in periodically poled KTiOPO 4 (PPKTP), we demonstrate for the first time first-order quasi-phasematched, backward-wave spontaneous parametric downconversion (BW SPDC). We pumped the PPKTP crystal at 800 nm and obtained a forward-wave signal at 1400 nm and a backward-wave idler at 1868 nm. We estimated an internal pair production rate of 4.0 × 10 4 pairs/s/mW. The backward-wave phasematching constraints lead to the unique tuning property that spectral features of the pump are transferred to the forward-wave signal photons, which makes BW SPDC an attractive source of spectrally shaped, heralded single photons. These spectrally shaped photons are useful for quantum computing and quantum interconnects. For the first time, we experimentally show this effect by observing frequency translation between a spectrally shaped pump beam and the BW SPDC signal photons. Due to their unique properties, BW-SPDC-based devices will be important building blocks for quantum information processing.
Backward wave optical parametric oscillators (BWOPO) are a special class of nonlinear optical devices with unique properties, due to the fact, that the pump photon is split in two photons propagating in opposite directions.The counter-propagating nature of the interaction automatically establishes positive feedback, without any external cavity, making the system very robust and insensitive to misalignment.Furthermore, the forward wave inherits the phase modulation of the pump, while the backward generated wave is inherently narrow [1].Although the concept of BWOPO was proposed in the 1960s [2], the fist experimental demonstration had to wait until 2007 [3].This long delay is ascribed to fact that quasi-phase matched (QPM) structures with sub-um periodicities are required to compensate for the large phase-mismatch between the counter-propagating waves.Thanks to the recent progress in periodic poling based on coercive field engineering, several BWOPO interactions with ps and ns pump pulses have been achieved, demonstrating efficient single pass configuration with conversion efficiencies up to 50% at a mJ output level [4].BWOPO operating at degeneracy, especially in the telecommunications band, is of special interest due to the possibility to have phase locking inherent to degenerate parametric processes which can be used for generation of biphoton states, a resource for quantum communications and quantum sensing [5,6].Efficient BWOPO operation in the spectral range of 1.5 µm -1.6 µm, requires first-order QPM, which can be realized in periodic nonlinear structures with a periodicity of ~400 nm, i.e., close to half of the pump wavelength in vacuum.In this work, we take advantage of our coercive-field engineering technique [7] to fabricate 435 nm-period in 1 mm thick Rb-doped KTiOPO4 (PPRKTP) crystals.The resulting domain pattern shows excellent uniformity over the entire crystal aperture and a duty cycle close to 50% (see Fig. 1a)) To the best of our knowledge, this poling period marks the shortest QPM period ever fabricated in ferroelectric crystals of comparable aperture.The crystals are used to demonstrate the first realization of a degenerate BWOPO, generating counter-propagating signal and idler in the spectral band of optical communications.