The first stand-alone quantum light source
Quantum information processing is set to deliver exciting new possibilities in the computing space and France is already at the forefront of this technological revolution. Harnessing the power of quantum mechanics to achieve a giant leap in processing power, France aims to solve today’s hardest problems with the computational solutions of tomorrow.
The goal is to provide a European-based answer to the challenges facing industry and academia worldwide.
In just a few years, it has been possible to do the design and construction of a high-quality generator of optical qubits based on single photons.
This device is at the heart of modern quantum technologies and will help deliver quantum-secure communication networks and scalable quantum processors. This device has already been delivered to first-class industrial and academic players all around the globe.
This pioneering work is the outcome of more than 20 years of ground-breaking research in many fields, from nanotechnology to photonic engineering. The technology holds many advantages over other quantum systems, and will continue innovating to keep competitive edge.
This year the ambition is to provide a fully open-source environment in which companies and researchers alike will learn to program quantum computers.
The new and challenging field of quantum technologies has already shown great promise.
PHOTONIC QUANTUM COMPUTING
Photonics provides one of the most promising pathways to practical quantum computing. The computing platform is modular, upgradable, interconnected : it is scalable by design. It integrates with classical photonic technology, modules are interconnected with optical fibers, and it operates largely at room temperature.
Quantum light sources deliver high quality photons to specially designed quantum processing units in the form of parametrizable chips. Tailored opto-electronics modules facilitate the interconnection of solid-state photon sources with integrated photonic chip processors, or with fiber-based loops in the case of quantum computing with time encoding. On chip or in fiber, information is encoded in photons and then process that information in ways that exploit nonclassical quantum interactions.
The chips may be designed specifically for certain algorithms or informatic tasks, for instance tailored to the needs of clients, or to allow for general computations. Like any of the hardware components in the platform a chip can be substituted for another, or indeed replaced or upgraded as the technology and capabilities evolve.
In quantum computing more generally, one often speaks of qubits, and indeed we can talk about photonic qubits and treat the processors as qubit processors. But photons are versatile and offer a richer variety of other possibilities. It is well-known that in some instances it is highly advantageous to treat photons as elementary computational units in their own right – not least for the accessibility of statistical behaviors that cannot be simulated even by classical supercomputers.
WHY TO BELIEVE IN PHOTONS
Many different technologies propose routes to quantum computing, from matter-based approaches using electrons, atoms or ions, to light-based approaches using photons. Each has its virtues and challenges depending on the applications envisioned and on scalability. One might consider the modularity, connectivity, computational universality, or computational power.
Across all technologies a consensus is forming that the fastest route to scaling up is through the interconnection of intermediate-sized processors, rather than targeting a single giant processor with millions of qubits. Photonics is unique in being the sole approach that at once offers a scalable path to universal quantum computing and natively permits networking, distributed computing, quantum communications, and eventually a quantum internet. An intrinsically modular approach, built upon “flying” photonic qubits, it is already adapted for this transition.
Infrastructurally the platforms don’t require heavy cryogenics and refrigeration, and in large part consist of components that are already widespread in telecommunications industry of today (lasers, optical fibers, integrated optical circuits). This represents a significant advantage for reliability and stability, as well as the integration of quantum computers into standard environments like data centres.
HOW ARE THESE DEVICES DIFFERENT ?
The semiconductor quantum dot – the artificial atom acting as quantum emitter – is included in a micrometre size optical cavity designed and fabricated with high precision and manufacturing quality. The precision with which the quantum dot is placed in the cavity, and the cavity itself, are crucial for obtaining a state-of-the-art performing device.
To achieve < 100 nm alignment precision you have to leverage a unique proprietary technology which allows to carefully identify the position of the quantum emitters, placed randomly inside the semiconductor. You have to perform an optical lithography step at cryogenic temperature that allows to precisely build the optical cavity tailored in size and shape to the chosen emitter. Additionally you have to include electrical contacts to tune the quantum dot emission via the Stark effect by the application of an external bias. This allows to adjust single-photon emission energy and cavity mode, to achieve best emission conditions, and reduce charge noise. As a result, you can efficiently fabricate tens of deterministic photon sources, which present at once a high emission rate, high single-photon purity and indistinguishability. HOW CAN THESE DEVICES BE USED TO BUILD SCALABLE PHOTONIC QUANTUM COMPUTERS ? This unique technology consists of photon sources based on semiconductor nanostructures which have been honed over 20 years of research at France’s National Centre for Scientific Research (CNRS). The source efficiency – characterized by the probability of delivering a photon on demand – is above 50%, a figure that could reach close to 100% and that engineers keep pushing up. Competing approaches employ light sources based on a probabilistic process (frequency conversion) generating photon-pairs with efficiencies around 1% to 3%. To reach the efficiency of a single source, alternative approaches require multiplexing hundreds of inefficient but identical integrated sources, each individually coupled to a highly efficient detectors and a complex routing circuit. In such way the number of components grows exponentially with the number of qubits, while an optimized fabrication of solid-state source would keep the resources limited and scalability within reach. QUANTUM COMMUNICATION Quantum communication networks are under construction around the world, combining existing telecommunication infrastructures with emerging quantum technologies. They will enable exchange of quantum information between multiple users or even quantum devices, like computers and sensors. Crucially quantum networks will also enable security guarantees of the ultimate level, via quantum key distribution (QKD). But quantum networks can only be made possible with quantum light. Quantum light sources, built on ground-breaking solid-state emitters and innovative opto-electronic modules, are pushing the boundaries of current QKD-based quantum communication towards the development of large-scale quantum networks and the quantum internet. Development of sources at a variety of wavelengths (780 nm, 930 nm, 1300 nm, and 1500 nm) to exploit both free-space and fiber-based communication for maximal compatibility with diverse quantum technologies, from memories and clocks to quantum computers and sensors. Semiconductor technology allows to develop deterministic sources of entangled photons and photonic cluster states which will become an increasingly central resource for long-distance quantum communication and measurement-based quantum repeaters.
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