WHITE PAPER: Non-linear Optical Crystals Used for Quantum Technology

Quantum Technology

Quantum Technology (QT) is an exciting area of science which is already making a difference to our lives. The impact of QT will increase in the coming years and non-linear optical (NLO) crystals will have a key role to play in the commercialisation of the technology. QT is expected to have utility across multiple applications in three primary categories:

• Sensing and Timing: The extreme sensitivity of quantum systems to environmental influences can be exploited to measure physical properties with more precision.
• Communications: Attempts to observe a quantum communication channel will irreversibly alter the state of the system in a way that is detectable by the parties exchanging information. A quantum network can distribute entangled photons as ‘keys’ between distant users to ensure that the data is not being intercepted.
• Computing: Using the principles of superposition and entanglement, significant speedup over classical computers is theoretically possible for some problem types. Quantum Computing promises to revolutionise computing especially where large data sets and complex calculations are involved.

Early products are being commercialised today, but the QT industry is still in its infancy and will require new components and systems from a range of providers creating a stable supply chain currently emerging in the field. Photonics will be at the heart of the supply chain and whether its generating entangled Photon pairs, cooling atoms or generating stable quantum states NLO crystals and components will be key enablers.

In comparison to more conventional birefringent phase matching (BPM) used in homogeneous materials, micro-structured QPM materials offer the benefits of simple co-linear optical alignment, non critical angular walk-off, access to the largest non-linear coefficients, and a highly flexible design space.

Types of Non-linear crystals

MgO doped lithium niobate (MgO:LN)has a well-established wafer supply chain due to the widespread use of the material in other components and it can be periodically poled to increase frequency conversion efficiency. It has the highest nonlinear effective nonlinear coefficient among commercial nonlinear materials and a wide transmission range of 380nm to 5um making it extremely well-suited to high-efficiency frequency conversion of CW and pulse sources.

Potassium Titanyl Phosphate (KTP) has a lower nonlinearity than MgO:LN and is used when higher resistance to photorefractive damage is required. KTP can also be periodically poled to allow improved SHG efficiency at shorter wavelengths but has limited use in the Mid-IR due to a narrower transmission window of 350nm to 4um. KTP also suffers from a supply chain that is not as well established as MgO:LN and thus the material quality has greater variability and is more expensive.

Beta Barium Borate (BBO) and Lithium Borate (LBO) crystals have become popular for generation of wavelengths in the near-UV to blue portions of the spectrum, because their transparency at these short wavelengths is better than KTP and MgO:LN. BBO is transparent down to 190nm and LBO is transparent to 155nm. Both crystals are used in a bulk unpoled configuration, and while BBO has a larger nonlinearity and temperature tuning bandwidth than LBO, LBO has a smaller walk off angle, higher damage threshold, and broader angle and wavelength tuning capabilities. Both crystal types are hampered by a much lower nonlinearity than lithium niobate, but the ability to use large crystals with very high input powers allows both crystal types to be used effectively in larger frequency conversion systems.

The choice of NLO crystal for a particular application is driven by the required wavelength, available pump sources and NLO conversion efficiency and if the application requires significant volumes the cost of material and stability of the supply chain. Other considerations include the required output power, linewidth, operating temperature etc.

When considering different crystal materials, lithium niobate (LiNbO3) is a particularly attractive option since it has a very high non-linear coefficient.¹ MgO:LN and KTP are a ferroelectric materials in which the domain structure can be inverted by application of an electric field. By applying a spatially patterned electric field, so called periodic poling, a periodic reversal in the in-built polarization can be produced within the crystal. This then enables Quasi Phase Matching to be used to access the highest (d₃₃) non-linear coefficient. This Technique is not suitable for use with LBO and BBO.

Non-linear frequency conversion processes

NLO crystals can deliver frequency conversion using a number of different mechanisms which make them a very flexible solution for producing narrow linewidth, high beam quality, stable sources of light across their transmission window. This flexibility combined with readily available commercial sources driven by the telecoms industry make them ideal for wavelengths of interest in QT. The diagram below highlights some target wavelengths that can be obtained for atomic transitions of interest.

The processes include:

Second harmonic generation (SHG), or frequency doubling, is the most commonly used second order non-linear process. In SHG, two input pump photons with the same wavelength λ_P are combined through a nonlinear process to generate a third photon at λ_SHG, where, λ_SHG = λ_P/2 (or in terms of frequency f_SHG = 2f_P). NLO SHG crystals can be fabricated with QPM grating periods suitable for a wide range of commercially available pump laser wavelengths from 976nm to 3300nm, allowing generation of frequency doubled light between 488nm and 1550nm.

Sum frequency generation (SFG) combines two input photons at λ_P and λ_S to generate an output photon at λ_SFG , where λ_SFG = (1/ λ_P + 1/ λ_S)-1 (or in terms of frequency f_SHG = f_P + f_S). By combining readily available fixed (e.g. 1550nm) and tunable (e.g. 780/810nm) pump laser sources NLO SFG crystals can provide tunable output light between 500-700nm.

Difference frequency generation (DFG)occurs when two input photons at λ_P and λ_S are incident on the crystal, the presence of the lower frequency signal photon, λ_S, stimulates the pump photon, λ_P, to emit a signal photon λ_S and idler photon at λ_i , where λ_i = (1/ λ_P – 1/ λ_S)-1 (or in terms of frequency f_i = f_P – f_S). In this process, two signal photons and one idler photon exit the crystal resulting in an amplified signal field. This is known as optical parametric amplification (OPA). Furthermore, by placing the nonlinear crystal within an optical resonator, also known as an optical parametric oscillator (OPO), the efficiency can be significantly enhanced. NLO DFG crystals can be designed to work with common fixed and tunable pump wavelengths (e.g. 1064/1550/775nm) to cover a broad, continuous output tuning range from the near-IR to beyond 4.5μm in the mid-IR.

Wavelengths of Interest to Quantum Applications

The special structure of alkali metal atoms is the foundation of precision spectra, laser cooling and trapping of atoms, atom interferometers, atomic frequency standards. Amongst these atoms, Rubidium (Rb), Caesium (Cs), Beryllium (Be), Barium (Ba), Strontium (Sr) have been studied in detail. NLO crystals are most commonly used in quantum optics systems where narrow linewidth lasers are needed to access specific atomic transitions to manipulate and cool atoms and ions where diode lasers do not produce the power, linewidth, beam quality or wavelength required or are not readily available.

NLO crystals are a very attractive option in these systems as they can be precisely designed to give the required output power and wavelength. They also are attractive as they build upon an existing supply chain where pump sources are low cost due to commercially available Telecoms lasers, thus watt level power output at precise wavelengths is readily achievable via NLO crystal wavelength conversion.

The wavelengths generated can either be the target wavelength for the desired atomic transition or an intermediate state which then is further converted by another crystal e.g. the combination of 1051nm and 1550nm in MgO:PPLN gives an output of 626nm which is then doubled to 313nmby BBO.²

Examples of NLO Crystal use in Quantum Applications

Atom Cooling and trapping. Laser cooling and trapping is the technology to get atoms down to near absolute zero, and to confine and support these atoms in the traps. The atoms in their ground state can store the quantum information and long-range interactions between highly excited Rydberg atoms are essential for successful operation of many quantum information protocols in quantum computing.

The high precision and scalable technology offered by atom interferometry enables more sensitive detection of features e.g. smaller size or greater depth. Many atom optics applications favour a high laser power whilst maintaining a narrow linewidth and high spatial beam quality. For example 780nm generation from a 1560nm source (SHG) is required for Magneto optical trapping (MOT) of Rb atoms in applications utilizing cold atom interferometry such as gravimetric sensing and atomic clocks.³

In these application commercial off-the-shelf (COTS) telecoms lasers at 1560nm can be efficiently frequency doubled to 780nm, with conversion efficiencies of up to 70% demonstrated for waveguide solutions⁴. The combination of COTS pump laser components together with a frequency doubling crystal provides cost effective generation of both the 780nm power and narrow linewidth required for supporting Rb atom trapping.

Conclusion

NLO crystal-based laser systems have been used in many quantum applications. MgO:PPLN crystal has the highest effective non-linear coefficient among commercial NLO crystals and is the crystal of choice for applications in the 380nm to 5μm range, however where very high power (e.g. >3W CW at 532nm) or wavelengths outside the optical range are required KTP, BBO and LBO crystals can be used.

Non-linear frequency generation is an efficient way to get desired output wavelengths with low phase noise, high beam quality and narrow linewidth for QT. As a leading supplier of PPLN-based wavelength conversion products Covesion is able to offer advice on customer specific solutions, as well as technical support in their set-up, use and optimization. With an extensive portfolio of COTS products as shown on the diagram below, as well as having custom design and manufacturing capabilities for nonstock items, Covesion is your ideal partner to support the widest range of wavelength conversion applications.

References

1. M. Odstrcil, et al., “Nonlinear ptychographic coherent diffractive imaging,” Optics Express, pp. 20245-20252, 2016.
2. Hsiang-Yu Lo, et. al, “All-solid-state continuous-wave laser systems for ionization, cooling and quantum state manipulation of beryllium ions, “Applied Physics B, vol 114, pp. 17-25, 2014.
3. Diviya Devani, et al., “Gravity sensing: cold atom trap onboard a 6U CubeSat,” CEAS Space Journal, vol. 12, p. 539–549, 2020.
4. Sam A. Berry, et al, “Zn-indiffused diced ridge waveguides in MgO:PPLN generating 1 watt 780 nm SHG at 70% efficiency,” OSA Continuum, vol. 2, no. 12, pp. 3456-3464, 2019.
5. Thomas A. Wright, et al, “Two-Way Photonic Interface for Linking the Sr+ Transition at 422 nm to the Telecommunication,” Phys. Rev. Applied, vol. 10, p. 044012, 2018.