
Quantum communication
Quantum communication based on quantum key distribution (QKD) is achieved with optical telecommunication links, either via optical fibers or via the propagation of light in vacuum (or in the atmosphere) for satellite links. Exail leverages more than 5 years of expertise in designing and manufacturing reliable optical components for leading Quantum Key Distribution (QKD) players.
Since 2023, Exail is leading the QKISS (Quantum Key Industrial SystemS) project, funded by the European Commission within the EuroQCI initiative (European Quantum Communication Infrastructure), aiming to develop a European ready-to-deploy QKD system.
What is quantum communication based on Quantum Key Distribution?
From classical to quantum secured telecommunications
Modern society relies heavily on private telecommunications. Among the many activities that depend on it are e-banking, e-health, or secure government communications. However, modern encryption techniques used to establish privacy have limitations as they rely most often on the supposition that an eavesdropper has access to a limited computational power. This supposition depends on whether the eavesdropper is an individual or a state agency. Also, his computational power may be much larger in a decade (20-years-old communications are much easier to decrypt nowadays).
Quantum communication, which can be based on post-quantum cryptography (PQC) or on quantum key distribution (QKD), offers a forever privacy guaranteed by the laws of physics. When PQC relies on encryption algorithms, QKD uses photonics sources for the generation and distribution of secure quantum encryption keys.
How to implement QKD with optical solutions?
Thanks to QKD, a spy trying to intercept some information is detected before a message is even sent. And this is achieved simply by adapting the emitter and receiver hardware of an optical link (no need to send guards all along your optical fiber). In practice, QKD is achieved with optical telecommunication links, either via optical fibers or via the propagation of light in vacuum (or in the atmosphere) for satellite links, where Exail’s solutions are used.
There are two different approaches to implement QKD: one focuses on a discrete variable (DV-QKD) and relies on single photons with encoded random data. The other one plays on the wave nature of light with information encoded in the quadrature of its electromagnetic fields, it is the continuous variable (CV-QKD). Coherent homodyne or heterodyne detection is used to continuously retrieve the quadrature value of the signal to read the key into it.
Photonics solutions to implement QKD
Exail provides modulation solutions with matching components for the transmitter side of the communication (Alix). For the receiver side (Bob), Exail’s COH 90° optical hybrid demodulator is an integrated solution, based on the company expertise in micro-optic assemblies, dedicated to demodulation of the information as soon as coherent detection is used. The company also has the capacity to integrate its modulation and demodulation solutions within complete systems, including its narrow linewidth single frequency laser as seeder laser.
An example of CV-QKD setup using Exail’s components is given here:
Amplitude Modulator 1
to generate very short pulse
Amplitude Modulator 1
to generate very short pulse
Phase Modulator
to randomize the optical pulse
Amplitude Modulator 2
to randomize the optical pulse amplitude
90° Optical Hybrid
For coherent detection
LAZ-LAB
Narrow Linewidth Single Frequency Fiber Laser
Exail provides reliable components (modulators, VOA, COH) and sub-systems (LAZ-LAB-NL) to implement CV-QKD, both for the transmitter side and the receiver side. The information is encoded in both the amplitude and the phase of laser pulses using our solutions: two amplitude modulation blocks AM1 and AM2 are cascaded with a phase modulation PM1.
Modulators and matching RF amplifiers for QKD
Using an Arbitrary Waveform Generator (AWG), a first modulation block AM1 is used to generate short optical pulses. Using Exail’s NIR-MX800, MXER1300 and MXER high contrast and wide bandwidth amplitude modulators, very short optical pulses width from 70 ps can be achieved at 850 nm, 1310 nm and 1550 nm respectively. The modulator is combined with the driver DR-VE-10-MO which can be set either as a limiting or linear amplifier for either square or gaussian pulse waveforms. Using Exail’s bias controller MBC-DG-LAB, a high pulse contrast stability is obtained for frequency repetition rates up to several GHz.
An additional modulation block AM2 generates the random amplitude required for each pulse in CVQKD. This is achieved using the MXAN-LN (C-Band) or the MXAN1300 (O-Band) or NIR-MX800 (for 850 nm) and the highly linear DR-VE-10-MO (see left pictures: Multi-level output driver versus electrical input level).
A phase modulator PM1 sets the phase of each pulse. The MPZ-LN-01 (coming with more than 3 GHz electro-optical bandwidth) or the MPZ-LN-10 (typical 16 GHz of bandwidth) is used in combination with the driver DR-AN-10-HO to continuously modulate the phave over the range 0 to 2π. For the O-Band operation, the MPZ-LN-10 is selected to operate at both wavelengths 1310 nm or 1550 nm. For 850 nm, NIR-MPX800-LN-05 (8 GHz bandwidth operation) or the NIR-MPX800-LN-10 (more than 16 GHz bandwidth) are used.
COH 90°, the most advanced demodulation solution for CV-QKD coherent detection stage
CV-QKD uses a coherent detection stage in order to generate the quantum keys, therefore COH (Coherent Hybrid) range of products are widely used for this process.
Exail’s COH 90° optical hybrid is the most advanced solution dedicated to demodulation on the receiver side (Bob) for CV-QKD generation. It is a solution based on a free-space design of a micro-optics assembly, enabling to extract phase, amplitude and polarization by performing interferences between a Signal and a Local Oscillator.
COH 90° optical hybrid is a flexible solution. It relies on Exail core competencies in the design and the assembly of a free-space optical solution adapted to any kind of modulation (QAM-QPSK-DQPSK) and based on telecom technologies. It can feature a phase tunability option in order to perfectly adjust the 90° of the hybrid depending on the wavelength used.


LAZ-LAB-NL, a narrow linewidth laser seeder for integrated QKD solution
Exail’s single frequency fiber lasers (LAZ-LAB-NL-1560) are based on UV Bragg grating technology applied to active rare-earth photosensitive fibers. Integrated into a carefully designed module using in-house ultra-stable pump drivers, this solution delivers a stable single frequency laser line with ultra-low intrinsic noise and linewidth smaller than 0.1 kHz.
Such high-grade and high-performance lasers are ideal laser seeders for QKD integrated setups using Exail’s components.
Variable Optical Attenuator (VOA)
The Polarization Division Multiplexing Emulator (PDME) is a device that enables to control the delay between both polarizations. The PDME can be used to emulate Polarization Division Multiplexing (PDM) signal by launching a polarized signal with a given modulation format. At the output of the device, the modulation format will be emulated on both polarizations.
To have the best emulation, a key point is to have the same power in both arms before they are recombined. Even if the difference is always low it may slightly vary depending on the wavelength used. With the Variable Optical Attenuator (VOA) option, one can easily adjust the power difference between both arms with 0.1dB accuracy and then get a perfect emulation.
A trusted European industry leader in QKD
In January 2023 started the EU-funded project, Quantum Key Industrial SystemS (QKISS - read the press release). It brings together two high-tech industrial groups, Exail and Thales, and two leading academic experts Prof. Philippe Grangier (IOGS, Paris-Saclay Univ.) and Dr. Eleni Diamanti (CNRS, Sorbonne Univ.), with the goal to produce a complete, high-performing, secure and certifiable European Quantum Key Distribution (QKD) system. QKISS includes the manufacturing of opto-electronic components, the development of specialized signal processing and coding algorithms, the full system integration and the field demonstrations. QKISS will also produce field evidence of compatibility with telecom network systems, with the QKD systems functioning together with Mistral encryptors from Thales, adapted to the specific QKD framework. After an industrialization phase, QKISS systems will be available for deployment in the EuroQCI and applications relying on private communications: e-banking, e-health, government communication or the management of critical infrastructure.
Partnering with the best academic and industry player
Exail provides modulation solutions to QKD manufacturers and to research institutions. In addition to the solutions listed above, Exail also offers polarization switches and pulse pickers.
Started in 2019, the OPENQKD project funded by the European Union’s Horizon 2020 program aimed at reinforcing Europe’s global position at the forefront of quantum communication capabilities. The goal was to raise awareness of the maturity of QKD and its seamless integration into existing security and networks for a wide range of use-cases. IN a collaboration with CNRS, Exail has integrated next generation CVQKD systems pushing for higher baud rate with a design compatible with field deployment.
