“Though this work is currently carried out in parallel, there will certainly be meeting points in the future: the architecture of quantum processors will likely be distributed in networks and optical quantum communications will be required to link the different processors together whatever their nature.”​

Optical quantum communications will also be necessary to provide the increased security required once quantum computers become a reality. 

“Quantum computers threaten the security of current communication systems, in particular classical encryption protocols for the transfer of sensitive information between locations,” explains Olivier. “Quantum cryptography ensures ultra-secure data transmission through advanced quantum-encryption protocols whereby the cryptographic keys needed to encrypt and decrypt data are encoded and transmitted using photonic qubits in the form of single photons.”

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​​State-of-the-art integrated components for the generation, manipulation and detection of photonic qubits are the essential building blocks needed to produce advanced silicon photonics technology for these quantum applications. Heralded single-photon source technology via a nonlinear process of entangled photon pair generation has been developed at CEA-Leti, enabling the achievement of a high generation rate of photon pairs. 

“The generation rate needs to be as high as possible for quantum communication or computing applications,” explains Olivier. ​

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A generation rate in the MHz range, i.e. a few million photon pairs per second, was demonstrated in 2019. A more advanced architecture design enabled a significant improvement of the source technology leading to the achievement of a GHz generation rate on-chip in 2023. The optimization of these GHz sources continues; first to ensure high efficiency coupling of the emitted photon pairs into fibers for the transmission of quantum encryption keys. The second development involves harnessing the multiple degrees of freedom of photons to encode several qubits per photon pair – referred to as hyper-entanglement – for more efficient quantum communication schemes.​​​​​

Development has also focused on ultrasensitive detectors, capable of detecting the arrival of a single photon. 

“The performance of these detectors is quantified in terms of detection efficiency and noise level,” explains Olivier. “Photonic quantum computing requires detectors with the highest efficiency and lowest noise possible. Today, the integrated superconducting nanowire single-photon detectors we’ve developed have an ultra-high detection efficiency of 80%, together with an ultra-low noise level of less than 100 dark counts per second.” ​

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The downside, however, is the required operating temperature of 2.5K (-270°C). “For less demanding applications, such as quantum communication over limited distances, we’ve developed mercury cadmium telluride-based avalanche photodetectors.” With a much higher operating temperature of 77K (-190°C), this technology is particularly interesting for applications such as Earth- satellite quantum communication, where the weight of the detector installed on the satellite is critical. 

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“Though not at the same level of performance today, these detectors are nevertheless capable of detecting single photons with a high detection rate of 500 MHz and a detection efficiency of about 40%,” explains Olivier. 

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Optimization is underway to increase the detection rate up to the GHz range, improve detection efficiency and reduce noise, thus enabling quantum computing applications to be targeted.​​

“We can then create tailor-made cutting-edge photonic chips to support state-of-the-art quantum applications.”

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These cutting-edge on-chip components represent the fundamental blocks required to build application-specific quantum photonic integrated circuits. 

“Depending on the specific performance requirements and constraints of each application, we choose the most appropriate components and fine-tune their performance,” explains Alexei Tchelnokov, Chief Scientist for the Optics and Photonics Department. “Combined with high-performance routing components, we can then create tailor-made cutting-edge photonic chips to support state-of-the-art quantum applications, such as integrated transmitter and receiver circuits for quantum communication systems or integrated photonic processors for photonic quantum computing. We also develop the fabrication processes to ensure they can be carried out with standard microelectronics equipment, thus remaining CMOS compatible.”​

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​​The expertise of the Integrated Quantum Silicon Photonics group at CEA-Leti is recognized at both the national and international level through their involvement in OQULUS, the photonic quantum computing project of the French Quantum Strategy, in addition to five European projects. 

“The role of CEA-Leti across these different projects is to provide the quantum photonic chip hardware necessary for the assembly of photonic quantum communication and quantum computing demonstrators” explains Olivier.​