Projects

Quantum computers are perhaps the most anticipated of all potential quantum technologies. They provide a quantum advantage that makes it possible to solve problems that lie beyond the reach of classical devices. Yet, the fundamental question “Which physical property lies at the basis of the computation power of a quantum computer?” does not have a clear-cut answer.

In NoRdiC we aim to answer this question for continuous variable platforms, where quantum information is encoded in continuous degrees of freedom. Based on results from computational complexity theory, we formulate the research hypothesis that non-Gaussian quantum correlations play a crucial role in achieving a quantum advantage in such platforms. 

To understand the role of this phenomenon in quantum computational protocols, it is essential to explore its fundamental physics. Therefore, the first goal of NoRdic is to develop a theoretical framework to study these quantum correlations and understand their properties in small-scale systems. These small-scale systems serve as building blocks to create the large systems required for quantum protocols. The next step in NoRdiC is to find coarse-grained signatures that provide the means to identify the presence of non-Gaussian quantum correlations in these large systems. To do so, we exploit the idea that, even though the full multimode quantum state is inaccessible, we can extract information of all the two-mode subsystems. This allows us to construct networks, where every node corresponds to a mode, and a connection in the network represents the correlation between these modes. Our aim is to find signatures of non-gaussian entanglement in these network. 

Once the fundamental physics of non-Gaussian quantum correlations is unveiled, we can investigate their role in quantum protocols. As a first step, we aim to formally prove that such quantum correlations are necessary to implement of protocol that is hard to simulate with classical resources. Then, we aim to develop protocols that use non-Gaussian quantum correlations to solve graph-theory problems. These problems are narrowly related with the network-based analysis that was carried out to find signature of non-Gaussian quantum correlations. Hence, the same techniques that unveil the fundamental physics of the large non-Gaussian states, will now be fine-tuned to implement computational protocols. Thus, NoRdiC should make it possible to demonstrate a practical quantum advantage on continuous variable platforms.

Photons have infinite decoherence time, an unparalleled asset compared to matter-based approaches to quantum computations. Photonic quantum computing also has excellent prospects for scalability since the platform heavily relies on the well-established semiconductor industry. The photonic platform is the only one with superconducting qubits that has demonstrated a computational quantum advantage with sampling algorithms very much connected to a photonic interfering point of view.

Photons are thus true contenders in the quantum computing race as evidenced by i) the landmark quantum computational advantage demonstrations, ii) the profile and amount of private investment gone into North American (PsiQuantum and Xanadu) and European (Quandela, QUIX, Q’ANT) startups iii) State investment in Europe, China and North America.

In this race there is head-start in France mirrored in OQuLus ambitions and consortium.  Very few countries indeed could propose a national project like OQuLus gathering key expertises with groups at the forefront of the international community. OQuLus is thus truly strategic for the French effort in Quantum Computing. It encompasses very diverse expertise across semiconductor physics, integrated optics, quantum information, to quantum computer science, theory and experiment, digital - discrete variable - and analog - continuous variable - encoding.

In OQuLus, French experts in quantum photonics and technology gather to build two optical noisy intermediate-scale quantum computers along two approaches: 

Within the discrete variable framework, we develop an 8-qubit prototype based on bright quantum dot sources of single and entangled photons coupled to ultra-low loss silicon nitride reconfigurable computing circuits. We also generate photonic clusters and demonstrate first steps of measurement-based computing. We further work on the improvement of key building blocks for next generation processors featuring a higher number of qubits: extension of quantum dot spin coherence time for longer cluster states, generation of highly indistinguishable photons from distinct quantum dot devices, fast reconfigurable processing circuits for feedforward, waveguide-integrated superconducting single photon detectors and deterministic photon-photon gates. 

In continuous variables, we follow a measurement-based approach by using time-frequency modes to create cluster states of between 10 (cavity-based) and 10000 (single pass with temporal multiplexing) nodes, which we combine with mode-selective photon addition/subtraction to implement non-Gaussian operations. 

Aware that high potential disruptive DV sources and CV gates should benefit from OQuLus dynamics of pushing each module and addressing all the issues of combining them in functioning devices, we also set goals for alternative approaches: color centers in silicon coupled to SOI waveguides and color centers in nanodiamond coupled to SiN waveguides, ultracompact III-V on SOI heralded single photon sources and large clusters. 

The project is providing the hardware to reach a full stack computer. Each advanced internationally recognized quantum light resource is developed in OQuLus together with very deep and practical technological development dedicated to the resource: SiN or Si platform but also optimized growth and processing and piezoelectric substrates, mesoscopic detectors and spin-based or Rydberg-based gates. In that sense, OQuLus potentiates the funded efforts in PEPR Algorithmiques and is a necessary soil for PEPR NISQ2LSQ.

OQuLus is complemented by a roadmap of models to account for implementation specificities and a companion theory approach, at the interface of CV and DV, knitting closely experiments, technological and components specifications up to the software level. The ambitious goal of demonstrating functioning NISQ (Noisy intermediate scale quantum) machines that will run tailored and cutting-edge protocols for the flexible plaforms shapes the whole project.

Quantum devices offer great promise for computation, cryptography, communication, and sensing. Alternative approaches to quantum information processing in which bosonic modes are the carriers of information have attracted increasing attention, because they offer a hardware-efficient path to fault-tolerance and scalability thanks to their inherently large Hilbert space. However, this poses the problem of providing rigorous guarantees of the correct functioning of these promising bosonic architectures, a task known as quantum verification. To date, this verification is performed by general-purpose tomographic techniques, which rapidly become intractable for large quantum systems. Thus, other methods are needed as quantum devices are scaled up to achieve real-world advantages. VeriQuB will introduce a new approach to the verification of quantum computing architectures with bosons based on continuous-variable measurements. VeriQuB’s technological toolbox will comprise two main elements. (i) We will experimentally demonstrate the verification of multi- mode bosonic systems for optical and superconducting architectures well beyond the state-of-the-art, and provide the first demonstration of verified quantum computational speedup. (ii) We will develop a theory framework that defines the fundamental advantages of our contribution, putting special emphasis on identifying and verifying resourceful bosonic quantum devices. The VeriQuB consortium comprises world leading scientific partners who are ideally positioned to achieve the ambitious vision of this project and build a state- of-the-art verification technology toolbox, enabling bosonic quantum computing architectures to scale up, and positioning Europe as a leader in this domain.