Quantum communication

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 quantum physics to securely share random bits between two parties, Alice and Bob. Because any attempt to intercept or measure the quantum signals—like photons—will disturb them and create detectable noise, eavesdropping can be spotted. If no spy is detected, Alice and Bob can use the shared bits as a secret key for encrypted communication over a traditional network, with security guaranteed by the laws of physics, rather than assumptions about computational power.

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 a classical telecommunication link (no need to send guards all along your optical fiber). In practice, QKD is achieved with optical links, either via optical fibers or via the propagation of light in vacuum (or in the atmosphere) for satellite links, where Exail’s solutions can be used as well.

There are two different approaches to implement QKD: one relies on single photons with encoded random data, the discrete variable QKD (DV-QKD). The other one plays on the wave nature of light with information encoded in the quadrature of its electromagnetic fields: the continuous variable QKD (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 offers key components for both DV-QKD and CV-QKD, including electro-optical modulators and free-space micro-optical assemblies

DV-QKD using Exail’s components:

Exail offers LiNbO₃ amplitude, phase, and polarization modulators known for their high performance and stability. Exail’s MINT delay-line interferometers, integrated into compact micro-optical benches, which enable precise interference of time-bin encoded qubits. Additionally, Exail supplies polarizing fibers that maintain polarization quality at both the emitter and receiver ends, ensuring stable quantum state transmission.

Example of CV-QKD setup using Exail’s components:

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.

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

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, 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

Exail electro-optic modulators have been praised by customers over the years for the repeatability of their performance, in particular relatively low bandwidth and high signal quality which are key factors for the implementation of QKD systems, for all types of modulation scheme.

• Phase modulators, both for CV-QKD and DV-QKD, from 1300 to 1550 nm.
• Intensity modulators, Mach-Zehnder modulators both for CV-QKD and DV-QKD.
• Polarization modulators
• Nested Mach-Zehnder (IQ) modulators (Exail MXIQER), for simultaneous integration of the phase and amplitude modulation functions into one unique component
• Matching components: RF amplifier and Modulator Bias Controller (MBC)

Free-space micro-optics assemblies for QKD

Exail offers hermetic sealing and small footprint modules for terrestrial to space QKD systems.

COH 90°: advanced demodulation solution for CV-QKD coherent detection stage

Flexible solution to extract phase, amplitude and polarization by performing interferences between a Signal and a Local Oscillator. It is adapted to any kind of modulation (QAM-QPSK-DQPSK) and based on telecom technologies. 

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MINT delay line interferometer:  DPSK demodulator for DV-QKD with time-bin encoding

Time-bin encoding uses two distinct time slots (early and late) to represent qubit states. The delay-line interferometer introduces a fixed time delay between two paths, allowing the early time-bin from one path to interfere with the late time-bin from the other. This interference enables the receiver to measure superpositions of time-bin states, which is essential for detecting phase-encoded qubits and verifying quantum coherence. 

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MICS: complete range of space-grade DWDM multiplexer / demultiplexer

Custom MUX/DEMUX: useful solution to efficiently separate quantum and classical Telecom signals by wavelength, maximizing bandwidth usage while minimizing crosstalk.

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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.

QKISS has already helped Exail to improve the performance of its optical components for the low-noise requirements of QKD, while addressing sovereignty issues.



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.