Author: Yoad Michael

Photonics plays a significant role in the emerging quantum information industry, and entangled photon pairs are the workhorse of many key applications in quantum optics, where each application utilizes a certain property of the quantum state.

The development of new effective sources of entangled photon pairs (EPS) is crucial for practical applications in the ever-growing field of quantum technologies, spanning from fundamental research in quantum information science, quantum communications, cryptography, sensing, imaging, and more.

There are several types of entangled photon sources which are based on different quantum states and one important source is based on polarization-entangled photon pairs, which are composed of a superposition of perpendicularly polarized signal and idler.

Unlike squeezed light, which is in principle achieved by a single-pass in a nonlinear gain medium (such as a PPKTP Crystal), polarization-entanglement requires the addition and subtraction of photons, for example by passing through more than one crystal. A recent review paper by the group of Alexander Ling [1] surveys the evolution of sources of polarization-entangled photon pairs throughout the years, as well as their possible applications in quantum communications and cryptography.

Many of the different sources shown in this review paper rely on the PPKTP Crystals that we produce at Raicol – mainly type-0 and type-II phase-matched crystals. However, the sources differ from one another in the way the crystal is utilized.

For example, in a recent quantum cryptography experiment by the University of Science and Technology of China (USTC) [2], a polarization-entangled photon source, based on type-II PPKTP Crystal placed inside a Sagnac interferometer, was used to demonstrate quantum cryptography over a distance of 1,120km. Similarly, a different bell state can also be generated using the same Sagnac configuration, with a type-0 crystal placed instead of the type-II.

Raicol Quantum
Polarization-entanglement from a Sagnac interferometer, adapted from Ref [1].

A different approach for a source of polarization entanglement, utilized by the group of Ling [3], is birefringent splitting and recombination of photon pairs. In this scheme, photon pairs are generated from two spatially-separated pump beams (where one pair experiences polarization rotation), and the beams are recombined using a birefringent material. Compensation crystals are also used to match the temporal delay between the perpendicular pairs.

Birefringent splitting and recombination of beams, adapted from Ref [1].
Birefringent splitting and recombination of beams, adapted from Ref [1].

A third approach, which is currently being explored in Raicol, is the generation of polarization entanglement through domain engineering of the nonlinear crystal, as was shown by Kuo [4]. In this work, the researchers have designed a nonlinear crystal that can perform two simultaneous type-II processes, and showed the polarization-dependent coincidence between the two photons.

Domain-engineered source of orthogonal pairs, adapted from Ref [1].
Domain-engineered source of orthogonal pairs, adapted from Ref [1].

All of these novel methods are part of a general effort to improve sources of polarization-entangled photons, which were initially demonstrated at the intersection of two polarization cones in a type-II BBO Crystal [5]. This process relies on a non-colinear interaction, therefore rendering only a small portion of the down-converted light to be used.

As the world leader in Quasi-phase matching solutions, Raicol Quantum is proud to be part of the next quantum revolution with our advanced periodically polled crystals. For more information:


[1] A. Anwar et al. “Entangled photon-pair sources based on three-wave mixing in bulk crystals”, Review of Scientific Instruments 92, 041101 (2021).

[2] J. Yin et al. “Entanglement-based secure quantum cryptography over 1,120 kilometres”, Nature 582, 501-505 (2020).

[3] A. Lohrmann et al. “Broadband pumped polarization entangled photon-pair source in a linear beam displacement interferometer”, Appl. Phys. Lett. 116, 021101 (2020).

[4] P.S Kuo et al. “Demonstration of a polarization-entangled photon-pair source based on phase-modulated PPLN”, OSA Continuum 3, 2, 295-304 (2020).

[5] P.G Kwiat et al. “New High-Intensity Source of Polarization-Entangled Photon Pairs”, Phys. Rev. Lett. 75, 4337 (1995).

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Author: Yoad Michael

PPKTP Crystal  is a quasi-phased-matched crystal that converts light of one wavelength into a different wavelength. The original purpose of this crystal was humble: It was designed to be an efficient frequency converter for laser system by second harmonic generation. However, with recent advances in quantum optics, the process of spontaneous parametric down conversion (SPDC) is now the dominating application of PPKTP.

SPDC is a process in which a strong pump beam is converted into correlated photon pairs, coined Signal and Idler. These correlations are the basis for various sources of quantum light, such as heralded single photons, time-energy or polarization entangled photon pairs, and squeezed light.

While it is convenient to consider only the quantum properties these light sources, the nonlinear properties of the interaction play a very significant role. For example, in PPKTP, the phase matching can be fully controlled such that: the signal and idler have the same polarization (type-0) or orthogonal polarization (type-2), same wavelength (degenerate) or separate wavelengths (nondegenerate), emitted along the direction of the pump beam (collinear) or at some angle (noncollinear). The type-0/type-2 is determined by the polling period and is something that needs to be determined at the manufacture stage, while degeneracy and collinearity can be fine-tuned by temperature (to avoid working at bizarre temperatures, it is advised to specify these parameters beforehand and adjust the polling period).

We can dive deeper into these differences. Polarization is not the only optical property that changes between type-0 and type-2 interactions; Spectral bandwidth, pair rate, and tolerance to temperature are also different. This is best illustrated by the wonderful work of the Ursin group [1], in which they compared type-0 and type-2 crystals as sources of polarization entangled photon pairs. As shown in Figure. 1, Type-0 is significantly broader than Type-2 and can be made nondegenerate by temperature tuning. In addition, the authors reported that the pair generation (per nm) of type-0 is about 10 times higher than that of type-2.

PPKTP Crystal
Figure 1: Comparing the spectral bandwidth and degeneracy of Type-0 and Type-2 ppKTP crystals. Taken from Ref. [1] with permission from the authors.

To make things a little more complicated, the dispersion and the length of the KTP Crystal also affects the spectral bandwidth and pair rate. Longer crystals generate more pairs at the cost of reduced spectral bandwidth, and signal/idler photons are much broader at telecom wavelengths (~1550) than they are at NIR (~810).

All these examples show that PPKTP is an extremely versatile component, and it is therefore important to first consider the needs of each application before choosing the right crystal. Below we present a few recent utilizations of PPKTP for various applications and provide our recommended crystal. We are proud to say that all this knowledge came from the brilliant researchers that use our PPKTP Crystals and shared their findings with the scientific community. Feel free to contact us if you think we are missing a key application or research work.

Boson Sampling and quantum interference

In Boson Sampling, quantum light is usually placed at the input of a large interferometer that includes multiple splitting and re-combining of beams. Boson Sampling relies on quantum interference (Hong-Ou-Mandel effect) and therefore benefits from high spectral purity. Special efforts were made by the Fedrizzi group for creating aperiodically polled crystals for high spectral purity at 1550nm [2], and a similar design was implemented by USTC’s photonic quantum supremacy experiment [3]. Recent quantum computing efforts by Xanadu [4] and QuiX [5] utilized PPKTP at the same wavelength region, due to both higher purity at these wavelengths and compatibility with peripheral platforms such as Silicon Nitride. Raicol has developed (through a collaboration with Prof. Ady Arie) a method for the design and manufacture of high spectral purity APKTP Crystals for Boson sampling and quantum interference close to the group velocity matching point of 1550nm.

Recommended Crystal: Type-2 APKTP or PPKTP at 775->1550. APKTP offers higher spectral purity while PPKTP offers higher pair rate.

Quantum Key Distribution

PPKTP plays a role in entanglement-based QKD as a source of polarization-entangled photon pairs. In this field there are many available options depending on whether the system is designed for free-space or fiber. In general, detector efficiency and the availability of 405nm lasers usually pushes these applications towards entanglement at 810nm [6, 7]. Type-2 crystals are easier to use because of their narrow linewidth, easy separation of the signal and idler with a polarizing beam splitter, and robustness to temperature, while type-0 crystals are broader and offer higher pair rate, making them good candidates for multiplexed QKD [8].

Recommended Crystal: Type-0 or Type-2 PPKTP at 405->810. Type-0 offers higher pair rate and spectral bandwidth, while Type-2 offers ease of usability.

Squeezed Light

Squeezed light usually utilizes the crystals in the strong pumping regime (unlike heralded single photons or polarization entanglement), and benefits from a strong nonlinear response, therefore making type-0 crystals the favorable option. Examples include the Furusawa group with 9dB of squeezing at 860nm [9], The Schnabel group has demonstrated 15dB and 13dB of squeezing at both 1064 and 1550nm [10, 11], and the Bowen group using the former for a demonstration of squeezing-enhanced microscopy [12]. Squeezed light can be generated anywhere from 780nm (390nm pump) to 3.8 microns and is a function of the exact specific application.

When choosing a crystal for squeezed light applications, the researcher should first decide if the squeezing is going to be generated in single-pass or in a cavity. For the former, a standard crystal suffices, while for an optimal parametric oscillator, monolithic or hemi-monolithic options are preferred [13].

Recommended Crystal: Type-0 ppKTP, optional hemi or fully monolithic.

Imaging With Undetected Photons

Imaging with undetected photons usually utilizes type-0 crystals with varying degeneracy. For fundamental research it is convenient to be able to detect both photons [14], while the Ramelow group generated a signal in the visible and idler at the mid-IR for microscopy applications [15].

Recommended Crystal: Type-0 PPKTP, with a period that is designed for nondegeneracy. No better example than Ramelow’s 660->800+3800.

We will try to update this list periodically, so visit this page every now and then!



[1] Steinlechner et al. “Efficient heralding of polarization-entangled photons from type-0 and type-II spontaneous parametric downconversion in periodically poled KTiOPO4”, JOSA B 31, 9, 2068-2076 (2014).

[2] Graffitti et al. “Independent high-purity photons created in domain-engineered crystals”, Optica 5, 5, 514-517 (2018).

[3] Zhong et al. “Quantum computational advantage using photons”, Science 370, 6523, 1460-1463 (2020).

[4] Madsen et al. “Quantum computational advantage with a programmable photonic processor”, Nature 606, 75–81 (2022).

[5] Taballione et al. “20-Mode Universal Quantum Photonic Processor”, arXiv:2203.01801.

[6] Yin et al. “Entanglement-based secure quantum cryptography over 1,120 kilometres”, Nature 582, 501–505 (2020).

[7] Mishra et al. “BBM92 quantum key distribution over a free space dusty channel of 200 meters”, Journal of Optics, 24, 7 (2022).

[8] Brambila et al. “Ultrabright Polarization-Entangled Photon Pair Source for Frequency-Multiplexed Quantum Communication in Free-Space”, arXiv:2205.10214.

[9] Takeno et al. “Observation of -9 dB quadrature squeezing with improvement of phase stability in homodyne measurement”, Optics Express 15, 7, 4321-4327 (2007).

[10] Vahlbruch et al. “Detection of 15 dB Squeezed States of Light and their Application for the Absolute Calibration of Photoelectric Quantum Efficiency”, Physical Review Letters 117, 110801 (2016).

[11] Schönbeck et al. “13 dB squeezed vacuum states at 1550 nm from 12 mW external pump power at 775 nm”, Optics Letters 43, 1, 110-113 (2018).

[12] Casacio et al. “Quantum-enhanced nonlinear microscopy”, Nature 594, 201–206 (2021).

[13] Ast et al. “High-bandwidth squeezed light at 1550 nm from a compact monolithic PPKTP cavity”, Optics Express 21, 11, 13572-13579 (2013).

[14] Gilaberte Basset et al. “Video-Rate Imaging with Undetected Photons”, Laser & Photonics Reviews 15, 6 (2021).

[15] Kviatkovsky et al. “Microscopy with undetected photons in the mid-infrared”, Science Advances 6, 42 (2020).

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In nonlinear crystals, a special nonlinear process occurs. A pump photon is spontaneously converted into two complementary photons with lower energy. In this process both energy and momentum are conserved, i.e.   and .

In PPKTP Crystal or APKTP Crystal the  momentum mismatch of the process is compensated by the crystal reciprocal vector which is determined by the spatial frequency .

Vacuum energy and momentum fluctuations give rise to the spontaneous creation and annihilation of many random photons.

But the output photons of the SPDC (spontaneous parametric down-conversion nonlinear process) are the ones that conserve momentum and energy.

In type 1 SPDC  all the photon pairs which conserve momentum are positioned on a ring shaped structure:

In type 1 SPDC  all the photon pairs which conserve momentum are positioned on a ring shaped structure:

See a movie

Changing the temperature or wavelength of the crystal will change the phase matching condition thus radius of the rings will be changed as well as seen in the movie.

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A growing number of companies are switching to industrial RTP crystals for electro-optic Pockels cells

In the past year, there has been a growing trend for industrial companies to switch their traditional Pockels cell (PC) components, mostly KD*P, to industrial RTP Crystal.

The excellent EO properties of RTP, its simple setup and configuration, along with its attractive pricing, makes it a great alternative for current PC solutions. Industrial RTP enables increased ROI for a project, by saving time and money for the laser production line and across the product’s lifetime.


About RTP

RTP belongs to the KTP Crystal family. The outstanding electro-optical properties of RTP, together with its high damage threshold, enable its use in high-end laser applications, it is ideal for use in applications that require advanced characteristics, such as being non-hygroscopic, having high thermal stability, and for higher-repetition rates.

Raicol’s RTP PC is widely recognized for the following features:

  • Higher laser damage threshold
  • Non-hygroscopic material
  • Low absorption losses
  • No acoustic ringing (up to at least 200kHz)
  • Stability over a wide temperature range (10ºC –50ºC)


While providing many advantages over existing solutions, the advanced properties of RTP came with a price tag that prevented its widespread use for industrial applications.


Raicol’s new industrial RTP

Raicol’s new iRTP PC is the first product that brings the advantages of RTP to the EO mass market. The modified iRTP PC version of Raicol’s RTP is especially designed for the needs of the industrial laser market. Raicol’s iRTP PC is a standard off-the-shelf RTP PC that offers high performance EO cells at the price level of standard industry PCs.


iRTP vs KD*P Benefits

  • Improved thermal stability – iRTP has thermal stability over a wide range of temperatures, eliminating the need for thermal stabilization, thus eliminating the need for an oven or thermal control systems.
  • Reduce laser initiation time – RTP’s high thermal stability reduces laser stabilization and start up times, as well as the overall system initiation time.
  • Simple alignment – iRTP requires only 1D alignment to reach an optimum extinction ratio, in comparison to KD*P, which requires 3D axis control-significantly complexifying the alignment process.
  • Environmental stability- iRTP is a non-hygroscopic material with a temperature compensation design that allows it to function in non-controlled environments with a variety of temperatures and humidity levels.
  • Mechanical robustness and stability – iRTP’s requirement for only 1D alignment means that the iRTP package and mount has a simple and more mechanically stable design. The working parameters do not change throughout the laser’s lifetime, or during temperature changes, hence, requires little to no calibration over time.
  • Small size – The size and footprint of iRTP is much smaller compared to similar KD*P, due to a reduction in assembly, alignment, and temperature control components.
  • High repetition rate – iRTP supports a repetition rate up to 200 kHz.
  • Standard Pockels cell assembly – iRTP is an off-the-shelf product with standard industry EO cell specifications.
  • High damage threshold – iRTPs high damage threshold enables its use for high power lasers.

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