Unlocking quantum communication potential with reliable and custom optical components

# #

Exail delivers two key solutions for implementation into ThinkQuantum’s Quantum Key Distribution (QKD) systems, both for the already mature terrestrial market and the in-development space market. It has helped the company to be responsive to the increasing demand for QKD solutions. Exail expertise in the development of solutions tailored to customer needs and the reliability of its photonics components are acknowledged by a promising startup in the quantum communication field.


The booming of an Italian expertise in quantum communication

Founded only very recently, in 2021, the startup ThinkQuantum stands on the shoulders of a pioneer research group at the University of Padova (Italy). The research group* of its co-founder, Prof. Villoresi, started to work on the exploitation of single-photon sources in space at the beginning of the 2000s in partnership with the Agenzia Spaziale Italiana. They made a first experimental demonstration from a satellite as soon as 2008. Prof. Villoresi research group’s expertise in the development of QKD setups, both fibered and in free-space, expanded until 2015 with the demonstration of a 143 km-long QKD link between two Canarian islands. It was followed by a demonstration of long-lasting daylight QKD link performed in an urban environment in 2016.

From 2019 to 2023, the team of Prof. Villoresi was the unique Italian partner in the OPENQKD program, the first large European Commission project dedicated to QKD (for which Exail was also involved). They worked on the prototype of a more robust QKD system for fiber links that have been tested in real-world scenarios by exploiting the deployed network of the University of Padova, also with the coexistence of classical and quantum communication over the same fiber. A partnership between the University of Padova and the Italian company Officina Stellare Spa, a telescope manufacturer specialized in laser communication, helped industrialize the QKD system through the creation of the spin-off ThinkQuantum.

Today, ThinkQuantum delivers to the market QUKY, its fiber-based Quantum Key Distribution platform, including QUKY-TX as emitter (Alice) and QUKY-RX as receiver (Bob). The solution integrates Exail high-quality reference of LiNbO3-based electro-optic modulators. “Our requirements for this product were the low optical loss, low insertion loss, and low Vπ, but even more important was the overall performance repeatability reached by the different components delivered. We received high-quality and high-consistency products from Exail.” Notes Costantino Agnesi, Scientist & Product Developer at ThinkQuantum.

ThinkQuantum QUKY integrates Exail LiNbO3 electro-optic modulators


Enhancing the time-to-market of a complex quantum technology

In quantum mechanics, an observer affects the system that he wishes to observe and automatically changes its behavior. The amount of correlation between a signal sent from a QKD transmitter and a QKD receiver can be quantified mathematically, as well as the quantity of information that a potential eavesdropper would extract. A quantum communication system based on the distribution of a quantum key has to be designed in a certain way to be fully secure against any eavesdropper. In a discrete-variable QKD system, such as the one developed by ThinkQuantum, the security of communication relies on the way the photons are encoded. To reach a high level of security, a lot of attention must be paid to the quality of the laser-based photonics setup operated.

A common approach in QKD is to use four different lasers to encode the quantum bits of information into the different polarization states of the lasers. But each of the four lasers owns specific characteristics: each of them has a specific temporal response, a specific spectral response, and they age differently.” Explains Costantino Agnesi. These characteristics can be considered as security flaws from an attacker perspective, called “side-channel attacks”. Indeed, it will be possible to extract information from the degrees of freedom of the transmitted photons, and that information is related to the distributed key.

Closing any of these possible security flaws is important. To do so, ThinkQuantum uses only one laser and a technology developed in-house called “iPOGNAC encoder”, which allows the generation of high-quality polarization states. Using a single laser allows closing any side-channel attack. “The technology is thus secure, but also very stable over days, with record low quantum bit error rates, and with a high-quality in the quantum state generation. It means we can produce a great number of keys before having to correct any potential error.” Adds Costantino Agnesi. That is where Exail modulators are used, to modulate the polarization state of the single laser and thus encode the information.

Exail has helped ThinkQuantum to be responsive in the market while the demand for their QKD system increased tremendously and unpredictably.

Costantino Agnesi ThinkQuantum
“With Exail as a supplier of robust and reliable components, we could respond to important demand with good lead time while maintaining a very high level of quality, with repeatable performance. That was not obvious as we often see that quality is sometimes a parameter that drops when startups must start mass production of their device for the first time.“

The reliability of Exail’s lithium niobate (LiNbO3)-based modulators comes from the company’s core expertise in fiber-optic gyroscopes, some of which have clocked more than 6 million hours in orbit on more than 30 satellites without incident.

Unlocking QKD potential in space with Exail space-grade solutions

In addition to the mature terrestrial QKD and QRNG (Quantum Random Number Generation) solutions, ThinkQuantum also offers its expertise in the development of a satellite-based source (transmitter) and the matching ground-based station (receiver part) for a space-QKD channel. “The space market for QKD is still at a very early stage. We are involved in it through some development projects, including public tenders (i.e. QUDICE project).” Highlights Costantino Agnesi.

One big challenge in QKD from a satellite, usually on a Low Earth Orbit (LEO), to a ground-based station is the narrow window of time (10-15 min) during which a transmission can be established to transmit a quantum key. “In this situation, you don’t want to lose time doing useless procedures such as alignment of the reference frame between the laser source and the receiver. With our high-quality polarization states generation technique, the alignment is automatic.” adds Costantino.


Exail micro-optics technology was acknowledged by a Telcordia norm to be compatible with telecom applications. Its exceptional alignment stability over an extended operating temperature range, and its small footprint, make it a suitable technology for space applications.


Exail is a great partner for our development in space-QKD. First of all, because you are a European company, this is key as geopolitics is an important aspect when working for the European Space Agency and for other projects funded by the European Commission. Also, working with a company that has experience in the space market, with your already space-qualified optical components, is a huge added value.” Says Costantino Agnesi. He also sees the recently launched COTS (Commercial-On-The-Shelf) product line of Exail space-grade modulators as very relevant for companies working on the development of applications for the New Space market. “This is an interesting solution for commercial telecom companies working on space-QKD, as they always look for the best but also the most cost-effective solutions.”

Some interesting developments are under progress with ThinkQuantum for the space-QKD, based on Exail expertise to miniaturize and spatialize complex optical technology into compact integrated micro-optic benches. The first prototype of a micro-optic device was delivered and is currently being finely tuned and simplified to match the requirements of ThinkQuantum space project.

Costantino Agnesi ThinkQuantum
“The micro-optic assembly you deliver at Exail is an advanced technology that is important to face the challenge of miniaturization for space applications. Footprint is a very important hurdle in space, compared with the very stable and much less hostile environment of terrestrial telecom QKD.“

* QuantumFuture research group at the Department of Information Engineering of the University of Padova