The Quandela journey: from single photons sources to quantum computers

In October 2023, Quandela delivered its first MosaiQ photonic quantum computer to the French company OVHcloud, Europe's leading web hosting provider. The announcement caused quite a stir, but what exactly is a quantum computer, and how does it differ from a conventional computer? What are the potential applications that make its development particularly attractive?

To understand this, we must start with the basic unit of information encoded by today's conventional computers: the bit. This unit stores data in binary form (0 or 1) depending on whether an electric current is flowing through a circuit. A sequence of bits (i.e. a string of 0s and 1s) then provides a given piece of information, which is transformed by logic gates - an electronic circuit composed of transistors that alter the value of each bit - when a computer program is run.

The quantum equivalent of the bit, the qubit, is more complex. It is carried by quantum objects (photons, atoms, superconductors, etc.) and its state is less clear-cut than that of the bit. Like Schrödinger's famous cat, a qubit exists in a superposition of two distinct states (referred to as 0 and 1 by analogy to the bit), until it is observed. At that point, it collapses into a specific state, and the information it carries can be read. This quantum superposition, offering an infinite range of possible values for the qubit, allows it to process information differently and, for certain types of calculation, much more efficiently.

However, a single qubit is barely more interesting than a bit, as the additional information it carries remains inaccessible. Qubits only become really valuable when used together. In such cases, they can be placed into so-called "entangled" states, meaning the state of one qubit is perfectly correlated with the state of another. Entanglement significantly multiplies the possibilities for manipulating information. A computer based on these qubits will therefore, in theory, be able to process more complex calculations and do so faster than conventional computers, thereby providing answers to currently unresolved issues. Such calculations could, for example, provide more reliable weather forecasts than today, or help in modelling new pharmaceutical molecules. With its superior efficiency, quantum computing could also reduce the energy cost of very large calculations.

The dream of the quantum computer

However, the quantum advantage stage, i.e. having a quantum computer that outperforms a classical one, has not yet been achieved. Several challenges remain, notably qubit decoherence. Due to their quantum nature, qubits are highly sensitive to any environmental interaction; temperature, electrical or magnetic fields, or even simple observation can alter their state. The challenge is to create a highly stable environment to minimise errors in the calculations performed by quantum computers. Such an environment must also eventually be able to correct the inevitable errors caused by the environment.

One approach to mitigating error accumulation is using qubits that do not experience decoherence, such as photon-based qubits. Photons offer another key advantage by linking different individual processors. In order to increase data processing power and achieve quantum advantage, the number of qubits in the computer needs to be increased. This requires installing several individual processors, each with a limited number of qubits, but entangled via photons. By entangling the various processors, the quantum states of their qubits become dependant and function as a single, cohesive unit. Using photon-based qubits within each processor therefore presents a real advantage. However, these quantum objects are easily absorbed and consequently lost, making a reliable and efficient single-photon source essential.

The start-up Quandela: a single and pure photon source

In 2008, Pascale Senellart-Mardon developed a technique for manufacturing single-photon sources, capable of controlling the position of these sources. Her team demonstrated the efficiency of these components, which also feature high quantum purity. This property, essential for developing quantum technologies, measures the extent to which the quantum state of the qubit is mixed or pure. The physicist recounts: "As early as 2013, scientists developing quantum technologies started reaching out to us, as they were interested in our components. Then, by 2016, the requests became overwhelming." It was at this point that the idea of creating a start-up emerged, together with Valérian Giesz (now COO) and Niccolo Somaschi (now CEO), who were then PhD and post-doctoral students respectively. Quandela was born in 2017. "The aim was to market these components and turn them into a real product. Today, we are very proud, because most research groups developing photon-based quantum technologies, particularly quantum computers, are equipped with our quantum light sources," says the C2N researcher, co-founder and now the start-up's CSO and scientific advisor.

From components to computers and then software

In 2020, the co-founders decided to broaden the company's ambitions. Previously, Quandela had supported researchers in developing quantum computers, but now they planned to build one themselves. They hired Shane Mansfield, a quantum algorithm specialist, and Jean Senellart, an artificial intelligence expert, bringing together the expertise needed to produce a quantum computer, from the components to software and hardware assembly.

In addition to delivering computers to industrial partners, they established a quantum computer farm. The co-founder proudly points out: "In 2023, we became the first company in Europe to place our 6-qubit quantum computer on the cloud." A cloud accessible to manufacturers and researchers who want to develop quantum programs.

At the same time as it advances its technologies, Quandela makes them user-friendly through its software solutions, teaching quantum computing to entities engaged in complex calculations. Although quantum advantage has not yet been achieved, "it is important to develop everything simultaneously without focusing solely on the technology. We are trying to build an ecosystem by adding new functions on the cloud, new machines and new algorithms," explains Pascale Senellart-Mardon. In particular, the start-up is working with EDF (Electricity of France) on the design of algorithms that will eventually be capable of resolving the issues of detecting cracks in dams. For the C2N researcher, Quandela is on track to achieve quantum utility, where calculations made on quantum machines offer practical advantages over performing the same calculations on supercomputers.

Quandela, today and tomorrow

With around 100 employees, Quandela has become a significant player in the quantum computing world, both nationally and internationally. Since its inception, it has delivered quantum computers to France and Canada. In June 2023, it opened a dedicated assembly and online service facility for quantum computers in Massy (Essonne, France), the first of its kind in Europe. In spring 2024, it inaugurated a cleanroom for the production of single-photon sources. The start-up has also expanded abroad, with a centre in Munich, Germany, and more planned in Korea and Canada. Finally, on 13 November 2024, Quandela established a joint laboratory (LabCom) with C2N to strengthen the synergy between academic and applied research. The aim of this LabCom, named QDLight, is to continue the research into the development of new technologies directly applicable to Quandela's computers.

In March 2024, Quandela's success earned it a place among the five winning start-ups in the PROQCIMA programme run by the French Defence Procurement Agency (Direction générale des armées - DGA). The programme, with a total budget of 500 million euros, is designed to find solutions for achieving quantum advantage. With to these new investments, the company is moving closer to reaching quantum utility and developing a universal quantum computer with error-correction. More recently, in October 2024, Quandela received the "Breakthrough DeepTech Innovation" award at La French Tech's awards ceremony in Germany. An undeniable success.