Which specific project activities are you involved in?
Our group is one of the five theory teams within PASQuanS, and we focus on developing new ways to control and engineer the behaviour of the PASQuanS experimental platforms, together with potential applications ranging from basic science to end-user problems. We make detailed models of the quantum simulators and analyse their potential for demonstrating a practical quantum advantage for different application areas. I am also the theory coordinator within the Executive Team of the project.

Which results have already been achieved on your end and what will be the next milestones?
We have developed a number of theoretical building blocks that will help extend the the experimental platforms to new application areas, and allow for better benchmarking of their capabilities. For example, we have developed new control techniques for neutral atoms in Rydberg arrays, showing how highly entangled states can be prepared in few steps by shuffling the atoms in optical tweezers, as is possible in PASQuanS platforms. We have also separately worked with our experimental colleagues in Heidelberg to measure and benchmark entanglement for indistinguishable particles. The next milestones on our side will be particularly exciting as well look at more detailed quantification of where these platforms exhibit a practical quantum advantage for relevant problems, as well as developing further means to clearly verify this in ongoing experiments. As the project develops through this final year, we will see important steps in basic science coming together more clearly with the technological impacts of our work, helping us to identify new areas in which these technologies are relevant to computational challenges beyond basic science.

For you personally, what has been most fascinating about the project so far and how do you think PASQuanS will impact future research and developments in this field?
The great thing about being part of PASQuanS is that it brings together world-leading teams from theory and experiment in academia, as well as world-leading industrial technology and end-user partners to work towards a common goal of advancing these technologies. Part of the reason for the rapid development in our field has been the regular sharing ideas between leading academic groups from different countries and different technical backgrounds. PASQuanS extends this further by bringing on board the interaction with industry, not only bringing technical expertise, but also introducing us to a range of end-user problems that we as physicists seldom encounter. In this sense, I think and hope that the lasting impact of PASQuanS reaches beyond the development in science and technology, in setting up a common language and exchange between industry and academia in this area.

What are the biggest challenges for quantum researchers and engineers – and how do you think projects like PASQuanS can help to overcome them?
There are a lot of technical steps to in develop and improve these technologies, but perhaps the biggest challenges are (1) in identifying the areas where quantum simulation can have the largest impact, and (2) developing a common language and dialogue between scientists, engineers, and broader industry players with expertise on the computational challenges in their sector. We are very excited about the potential for quantum simulation and quantum computing, but both will require a lot of small steps, and expertise from many sides before we reach the point where – beyond basic science – we can solve a range of computational problems faster or more precisely than we can with classical computers. It requires both technological input on developing the platforms, and – more challenging still – identifying the most promising use cases, and determining how we can apply quantum devices to solve them. The starting point of a common language between the developers of the platforms and the end users is already often difficult to establish.

Projects like PASQuanS directly address these key challenges, by bringing together leading experts from this area, and allow a collaborative project to draw on international expertise to work on a common set of technological challenges. The focus around specific platforms and challenges is the ideal environment to develop new ways to work together, and the learn from different sides. The project is at the same time the ideal learning environment for postdocs and PhD students, who will be the next generation of leaders in academia and industry.

You work in both quantum computing and simulation. Which one is your favourite and how can you benefit from your cross-linked expertise in PASQuanS?
Because we work a lot on the architecture of experimental platforms, our theoretical work in quantum computing and simulation are strongly interlinked. One particularly interesting development, is the way that these areas are moving towards each other to identify near-term applications. In quantum computing we often talk about variational algorithms for near-term applications on Noisy, Intermediate-Scale Quantum (NISQ) Devices. In a sense, many interesting applications of Quantum Simulation might come from taking a similar approach, but making use of the full analogue controllability of an experimental platform, rather than restricting the set of operations as is done in digital quantum computing (to set the stage for fault-tolerance, which we don’t have in on NISQ devices).

Which of the five theoretical teams within PASQuanS do you work with and how is it linked to the experimental side of the project?
We benefit greatly from regular exchanges with all of the theory partners on PASQuanS – for example, we have ongoing discussions with Padua on numerical methods for 2D systems, Julich on quantum control and controllability, ATOS on emulation and end-user applications, Berlin on benchmarking techniques, and with Innsbruck on verification of quantum advantage for specific applications. Several of these are likely to lead to joint publications, and all give important additions to our understanding, and how we orient the research we do. Our work focusses on the architectures of the experimental partners – we already have a joint paper with Heidelberg, and have ongoing discussions with each of the other partners.

In your own words, what is the “Quantum Advantage” and where do we stand on our way to “Quantum Supremacy”?
Achieving a practical quantum advantage is one of the big aims in quantum computing and simulation. For me, it entails (1) being able to show that our quantum devices are capable of solving a computational problem better or faster than any known classical algorithm, and which is relevant beyond just testing the quantum device, and (2) being able to verify that the solution is reliable. “Quantum Supremacy” is a much-debated term, which perhaps doesn’t properly reflect its own meaning, and implies that a quantum device is able to solve some problem (which might not be relevant as a computational challenge beyond testing the device) better or faster than is possible with a classical algorithm. With analogue quantum simulation platforms and for problems relevant to science, we can already demonstrably reach regimes that are beyond the capabilities of existing classical simulation, and we’re close to being able to quantitatively verify that this is the case. Demonstrating a practical quantum advantage for problems beyond basic science is a very important next step. It will require the breadth of expertise that PASQuanS has brought together – across theory and experiment in academia, with industrial technology and end-use partners to take the critical next steps towards this challenge.

Which specific project activities are you involved in?
I am a researcher at CNRS (the French national organisation for scientific research), working at the Institut d’Optique (Palaiseau, France). Together with Prof. Dr. I. Bloch from the Max Planck Institute for Quantum Optics, I coordinate the PASQuanS project. This implies in particular management tasks and reporting, as well as ensuring a smooth flow of information between the partners and the various Flagship projects. Besides, together with my colleague Thierry Lahaye and a team of post-docs and Phd students at the Institut d’Optique, we develop a quantum simulator based on arrays of atoms trapped in optical tweezers, that are made to interact by exciting them with laser beams. Our main tasks within PASQuanS consist in developing new technologies to trap the atoms, to improve the performances of each manipulation, as well as demonstrating that we can use this machine to solve open scientific questions related to magnetic properties of materials, out-of-equilibrium dynamics of quantum systems, etc., that are very hard to solve by any known numerical methods.

Which results have already been achieved on your end and what will be the next milestones?
Apart from important technological developments (new trapping techniques, laser systems with improved stability ), we have recently demonstrated the quantum simulation of a two-dimensional antiferromagnetic material using up to 200 individual atoms. With the help of theory colleagues, we could compare our results with the predictions of some of the most advanced numerical methods available. These methods only work up to about 100 atoms, but showed that we had a good understanding of our platform and of the role of the residual imperfections. The next milestones would be to use our platform to investigate open questions related, for example, to an effect called “geometric frustration” when the atoms are placed on a triangular array as encountered in some magnetic compounds: the theoretical description of this system is very challenging and quantum simulation is a promising approach to tackle it. Before doing so however, we need to suppress some of the residual imperfections to be able to observe this phenomenon. We are also working towards increasing the number of atoms up to around 1,000 by trapping them in a cryogenic environment with the help of the company MyCryoFirm, which is a also partner in the project. These are our goals before the end of PASQuanS.

For you personally, what has been most fascinating about the project so far and how do you think PASQuanS will impact your future career?
I am very impressed by the experimental improvements that the various groups of PASQuanS have achieved. For example, for us, trapping and controlling 200 atoms as we have shown in 2020 was not something I was fully anticipating even 5 years ago, as it seemed quite out of reach back them. But our very driven students and post-docs made that work! At the end of the day, one has to remember the daily challenge that we face: we observe and control individual atoms that, when I was a student 25 years ago, I was told could not be seen! And these amazing developments are of course also happening in the other experimental groups of PASQuanS, and in many around the world.

The second thing that impresses me is the creativity of the theorists to come up with new approaches. For example, characterising the quantum correlations in our many-body systems is extremely challenging, but many theory colleagues from the consortium developed original methods that can be tried in the lab.

Last but not least, for me this was the first time that I have interacted strongly with people from the industry. I discovered their genuine interest for quantum technologies and the huge range of potential applications, from optimisation of processes, to quantum materials, low energy consumption, etc. For instance, we have started a collaboration with researchers at eDF (the main French electricity supplier) on the optimisation of the charging of a fleet of electrical vehicles, that may be solved efficiently on a quantum simulator. This exchange would not have happened without PASQuanS, as I would not have spontaneously thought of contacting them. This is just the beginning of a closer collaboration between academia and industry, and we are expecting a huge development in the years to come. In this respect, an important development for us at the Institut d’Optique was the creation of a start-up company (pasqal.io), now a full partner of PASQuanS, to further develop pre- industrial quantum simulators based on the technology we have demonstrated in our research laboratory. This start-up will be much better equipped than us as an academic group to meet the needs of the industry and to work with companies in the future.

How can quantum technologies impact our daily lives: Where do we stand now and what will be its future role in business and society?
This is a tough question. Quantum technologies are developing at the same time as the potential markets and their impact are not yet visible. Quantum technologies are still in their infancy, we are currently at the stage of proof-of-principle demonstrations to confirm their potential.

The promises are huge though and the fields of potential application manifold: more energy-efficient ways to perform computations, new approaches to hard optimisation problems such as finding the best way to load airplanes, trucks, the stabilisation of national or supranational electricity grids, the charging of fleets of electrical vehicles, the spatial arrangement of antennas for cell phones, or the development of new materials or new drugs, better solar cells. All these developments will have an impact on our societies when they are mature enough. An important aspect for the industry is also for them to learn about quantum technologies, their potential and what we can do today in order to invest wisely: knowing about the quantum is a competitive advantage. Quantum technologies will also create new ecosystems, with new job opportunities. As we develop this “quantum industry”, we will need to develop dedicated curricula to train students and teach them the required know-how. This is already an ongoing process at the European level reflected in the definition and set-up of a core cursus to educate “quantum engineers”.

Now, if the question is, whether we will all have a quantum laptop in our houses, I have to admit that I don’t know. But if we ever do, this is not in the near-future.

Compared with other research endeavours and EU projects in the field of quantum simulation, what’s unique about the aims and scope of PASQuanS?
PASQuanS gathers a unique combination of platforms and of skills with the consortium consisting of experimentalists and theorists from academia, partners from the industry who act as enablers , and potential end-users. Hence, we already have the full value chain represented in our consortium: from the fundamental demonstrations in the labs to the industry-driven applications via the suppliers of commercial technologies.

Scientifically, PASQuanS relies on neutral atoms in optical lattices or in arrays of optical tweezers, as well as on trapped ions. These are today the platforms with probably the highest potential for scaling up the number of qubits. Besides they are to a large extent programmable: one can engineer the interactions between the qubits and their connectivity by “simply” changing the parameters of the machine. Neutral atoms in optical lattices are very interesting as they allow the direct simulation of fermionic particles such as electrons in materials. No other platform can do that. Importantly, all of the PASQuanS platforms operate near or even deeply in the quantum advantage regime, and have already shown that they could challenge existing theory and numerical methods.

Besides, PASQuanS has a very strong theory component with experts in quantum information, condensed matter physics, quantum optics, etc. They all have strong links with the experimentalists, having worked with them for many years. In this way, ideas can be rapidly implemented on the various platforms of the consortium. Most of their effort is devoted to answering the crucial question of how to characterise large-scale quantum systems conceptually, and to develop experimentally feasible methods to do so, but also to assess, for specific problems, a potential quantum advantage (which often starts by assessing what can be done classically, an already surprisingly hard question).

PASQuanS targets applications in material science, quantum chemistry, high-energy physics and optimisation. Which one of them do you expect to benefit from new simulation technologies first and why?
At the moment the most promising, near-term applications seem to relate to combinatorial optimisation problems that are encountered almost everywhere in industry and in finance. They could be solved by the quantum simulators developed within PASQuanS with a few more experimental improvements. However, today, we do not think that quantum simulation will offer a universal gain with respect to classical methods whatever the optimisation problem: we rather expect a “quantum advantage” for specific problems. This would already be an important development. Furthermore, even if quantum simulators would not present an advantage for some problems, it is likely that they will solve them with considerably less amount of energy than using high-performance-computing facilities. The quantum simulation approach would therefore have an important environmental impact.

Other potential applications often mentioned are quantum chemistry and the design of new materials as well as of new drugs. These are certainly very exciting perspectives, and the community is trying to assess what it would require in terms of hardware and resources. But one has to keep in mind that these are quite long-term goals.

Which specific project activities are you involved in?
I am an associate professor at Sorbonne University and, together with Michel Brune and Jean-Michel Raimond, I am leading the research activities of the PASQuanS team in Kastler Brossel Laboratory, in Collège de France, Paris. We are working towards the realization of a quantum simulator using giant atoms of a very special kind, so-called “circular Rydberg atoms”. These atoms interact very strongly and live quite long, at least from an experimentalist point of view, namely few hundredths of a second. This combination of strong interaction and long lifetimes is ideal for the study of solid-state matter within a quantum simulator.

Which results have already been achieved on your end and what will be the next milestones?
Compared to other experimental platforms in PASQuanS, our quantum simulator is still at an early stage of development. Circular Rydberg atoms are more challenging to prepare and use than other (Rydberg) atoms used in the other platforms.

The atoms used in a quantum simulator need to be maintained at very specific positions, typically using laser beams, namely to be laser-trapped. Very recently, we have demonstrated in our cryogenic environment, running at -270°C, the first laser-trapping of circular Rydberg atoms. This is a decisive step towards the realization of a quantum simulator with circular Rydberg atoms.

In this work, the circular Rydberg atoms were trapped but unorganized, as the particle of a gas would do in a closed box. Our next goal is to laser trap several circular Rydberg atoms in so-called optical tweezers, so that the atoms are pin-pointed at very precise positions with a carefully tuned distance between them. This will immediately open proof-of-principle tests of our novel quantum simulator.

For you personally, what has been most fascinating about the project so far and how do you think PASQuanS will impact your future career?
Slightly more than 10 years ago, when I started my research career, scientists were still learning how to manipulate single atoms, single photons, how to make them interact together. The foundations of quantum mechanics were being tested, not to prove it wrong or true, but rather to demonstrate the high level of manipulation of those fragile quantum systems that was being reached. The results of the experiments were somehow known beforehand but the experiments themselves were for long believed to be too difficult to be realized.

This specific project, the studies performed within PASQuanS and the development of quantum technologies in general are fascinating as they reveal that we have bridged a gap: we can now use, and not only manipulate, those fragile quantum systems (atoms, ions, photons…) to actually do things, to solve problems, to study systems that are otherwise too complicated to apprehend. In other words, to now run experiments of which the outcomes are unknown, even by supercomputers. This will certainly modify the way quantum scientists will do research in the future, at the frontier between fundamental physics and technology.

Within PASQuanS, there are five experimental groups. Which one(s) are you involved in and how do they interrelate with the theoretical teams and the other experimental groups?
I am part of the experimental group of Laboratoire Kastler Brossel (CNRS, ENS, Sorbonne Université, Collège de France). Our (future) platform is very similar to the one originally developed in the experimental group of Institut d’Optique, where non-circular Rydberg atoms are used. While we need to adapt their set-up to the specificities of our experiment, and in particular make it compatible with cryogenic temperatures, we are in close contact with them in order to efficiently tackle problems they may have faced as well or that we both would simultaneously face. PASQuanS offers an excellent forum for this exchange. PASQuanS also provides a great opportunity to interact and collaborate with theoretical teams. Of course, since our platform is still in the development stage, actual protocols or ideas that they may think of cannot be directly implemented in our setup and exchanges may not be as intense as with other experimental groups with running simulators. This motivates us all the more to fully and rapidly finish the construction of our own quantum simulator.

PASQuanS targets applications in material science, quantum chemistry, high-energy physics and optimisation. Which one of them do you expect to benefit from new simulation technologies first and why?
Many of the most advanced quantum simulators have been originally designed to study condensed matter systems. Many questions are left open in these systems and quantum simulators are believed to be good candidates to provide answers. Some experiments already run in regimes where supercomputers cannot compete anymore. Material science may therefore be one of the first to benefit from quantum simulators.

It is to be noted too that quantum simulators are almost per essence made to solve optimisation problems. The latter have indeed a very natural counterpart in the quantum world: finding the configuration of a system that requires minimal energy. Quantum simulators can efficiently answer those questions, leading to a growing interest in such an application. Optimisation in certainly another good candidate for being the first to benefit from quantum simulation technologies.

If you had to explain Rydberg atoms to a non-scientist in very few sentences, what would your explanation be?
By shining light of a carefully tuned colour on an atom, one can “excite” an atom, or more precisely one of the electrons that orbit its nucleus. In such an excited state, this electron is brought further away to the nucleus than he would normally be. When this new distance to the nucleus is very large, so large that the excited electron cannot distinguish the other non-excited electrons from the nucleus of the atom anymore, one says that the excited electron is in a Rydberg level, or that the atom is a Rydberg atom.

Rydberg atoms are gigantic atoms, thousand times bigger than “standard” non-excited atoms. Rydberg atoms can be as large as a bacterium. Such an extreme atom naturally comes with exaggerated properties. Most importantly, being so large, a Rydberg atom acts as a giant antenna that can feel the presence of another distant Rydberg atom, several microns away, while “standard” atoms generally need to collide to interact. The strong interaction between Rydberg atoms is the key property that make them so valuable for quantum simulations.

Which specific project activities are you involved in?
ParityQC, a spin-off from the University of Innsbruck, will be responsible for delivering the operating system for the efficient encoding in PASQuanS’ Rydberg atom experiments. We specifically focus on solving hard optimization problems and within the project we will map the identified problems of interest on quantum processors in the most efficient way.

Which results have already been achieved on your end and what will be the next milestones?
ParityQC had developed a novel encoding of the four-colouring problem in Rydberg atoms which was published together with the groups in Paris and EDF in EPJ Quantum Technology. We were able to show that in a two-layer system, the architecture by ParityQC allows for a more efficient encoding compared to standard encodings. This is directly relevant for “smart charging”. This is the optimization problem of efficiently charging autonomous cars. We hope that we are able to contribute with this use-case to a wider recognition of the PASQuanS experimental efforts.

For you personally, what has been most fascinating about the project so far and how do you think PASQuanS will impact future research and developments in this field?
Clearly the enormous control that experiments have over their individual qubits. For example, placing individual atoms in the form of an Eiffel Tower would have been unthinkable just a few years ago.

You are the founders of the start-up ParityQC which has already won several prizes. What is the company’s vision and what makes it unique?
ParityQC is a quantum architecture company which focuses on solving hard industry-relevant problems on quantum computers. With our architecture we influence the whole quantum stack and therefore are able to unify the developments on the hardware and software side. Our philosophy is that the simultaneous development of both sides is essential to achieve a perfectly aligned quantum computer and achieve value creation in the short-term. We are working closely together with hardware developers around the world to develop scalable and fully programmable quantum systems.

ParityQC is not the first company you founded. Which role did the PASQuanS project play here and how was this different from other foundations you have been part of?
Participating in PASQuanS is a great opportunity of ParityQC. The main difference of PASQuanS compared to other collaborations is the concentration of distinguished experts in the field. The partners of PASQuanS are all world-wide leaders in quantum computing and quantum simulation. This is an ideal situation for a start-up company to gain international recognition in atom-based quantum technologies.

Looking at it the other way around, how will ParityQC contribute to the work of PASQuanS?
ParityQC builds upon years of research by Wolfgang Lechner and his research group at the University of Innsbruck where he was previously a Postdoc in Peter Zollers group. Our main goal is to develop blueprints for quantum computers to solve useful problems as early as possible. With our ParityOS operating system we are able to efficiently encode optimization problems with large connectivity, constraints and higher-order interactions to a simple geometry of Rydberg atoms. The PASQuanS consortium consists of Europe’s most advanced research groups and companies in this field and we are very glad to contribute our part in this project.

What would you recommend to other founders and young entrepreneurs regarding participating in EU-funded research projects like PASQuanS?
If you get the chance to work with renowned experts in your field, EU-funded research projects are a perfect way to collaborate. They allow you to participate in more research focused fields, which need a longer time to market. Especially in the quantum computing field, research collaborations are essential, which is why projects like PASQuanS are very valuable not only for young entrepreneurs and spin-off founders, but for the quantum computing ecosystem as a whole.

Which specific project activities are you involved in?
I am one of the coordinators of the PASQuanS project and one of the scientific directors at the Max Planck Institute for Quantum Optics in Garching, Germany. In addition, I hold a chair for Experimental Quantum Optics at the Ludwig-Maximilians-Universität in Munich. My team is working on quantum simulations using ultracold neutral atoms in optical lattices. We are trying to advance this platform to emulate the behaviour of complex condensed matter systems using highly controllable model systems.

Which results have already been achieved on your end and what will be the next milestones?
Our team realised “Quantum Gas Microscopes”, which allow to observe individual atoms trapped in artificial crystals of light (so called optical lattices). The atoms in the light crystal mimic the behaviour of electrons in real solid. One amazing aspect is that we can control and detect each individual atom in the system with a resolution down to single lattice sites. This enables us to explore the phases and dynamical properties of such artificial materials in completely new ways and learn about their working principles. The idea here is that this knowledge can, via theory, be transferred to real materials and help develop better classical numerical methods for which our systems can serve as benchmarks.

For you personally, what has been most fascinating about the project so far and how do you think PASQuanS will impact future research and developments in this field?
It has been amazing to see how industry and research have come together to address some of the most outstanding challenges in quantum simulations. As the field very much depends on technological developments in supporting systems, such as lasers, control electronic etc., the joint effort enabled by PASQuanS has significantly contributed to the advancement of the field. Plus, the three platforms pursued in PASQuanS also very nicely complement each other.

PASQuanS was launched in 2018 as one of 20 projects funded in the ramp-up phase of the EU Quantum Flagship initiative. Who came up with the very first ideas for the proposal and how did they evolve?
Together with my colleagues Antoine Browaeys, Peter Zoller and Andrew Daley, we discussed first ideas to form a joint European effort to advance quantum simulations and find new application areas relevant to end users. We then rapidly brought together complementary team members from theory, experiment, and industry to bring this effort to life.

The number “five” seems to have a special meaning in the organisational structure of PASQuanS: Could you explain why?
What a coincidence… I think it nicely symbolizes the balance between experiment, theory and industry across Europe!

Quantum simulation has the potential to solve crucial issues in multiple industrial fields. What are the current shortcomings and how do the programmable platforms developed in PASQuanS address these?
In order to make our simulations even more relevant to material science, we have to be able to enhance their programmability, i.e. we have to be able to realise more complex lattice structures akin to the ones we find in real materials. And all this is supposed to happen in a highly flexible way at the push of a button. In addition, we would like to enhance the initial state preparation, i.e. cool the system down to lower temperatures, such that we can access lower temperature phases and potentially discover new ones. This will also be one of the major development lines for the major development lines for an envisaged follow-up project.

In January 2020, you have initiated the first European end user workshop on “Applications of Quantum Simulation”, and associated end users of PASQuanS include large industry players such as Airbus, Bayer, Siemens or Bosch. What have been the main outcomes of the workshop and what are you doing to bring the PASQuanS project results closer to industrial application?
This exchange between industry and science has been very important to identify possible applications for industry and inform our industrial partners in an objective way about the novel capabilities afforded by quantum simulators. All this has been done through intense, fact-based information and discussion without some of the unfortunate ‘hype’ one often finds around quantum technologies.

Which specific project activities are you involved in?
The question how to realise and to assess the power of (programmable) quantum simulators is very close to my heart. Quantum simulators promise to allow for new insights into strongly correlated quantum matter (and presumably also for industrially relevant use cases).

But before this aim can be fully achieved, we have to find ways to clearly define what the quantum simulator is actually doing precisely. Therefore, ideas of benchmarking, certification and the seemingly innocent question of read-out are at the centre of our activities. Only if we have predictive power, quantum simulators can be seen as genuine quantum technological devices. And this is far from trivial: After all, quantum simulators outperform classical simulation methods, so oftentimes, we cannot simply check whether the quantum simulation has been right.

Some of the questions we tackle within PASQuanS are very hands-on, practically minded, and provide tools relevant for the consortium as a whole. Here, we see ourselves as service providers for the entire network. Other questions are rather visionary aiming to give the project a medium-term scope and perspective.

Which results have already been achieved on your end and what will be the next milestones?
The project has so far been enormously productive for us. We have gained a much deeper understanding of how we can benchmark and verify quantum simulators, a topic that is very important for PASQuanS. Plus, we have helped develop quantum simulations of non-equilibrium phenomena presumably beyond the classical realm, and developed new classical tensor network methods to identify applications and to be used in future benchmarking. Maybe most importantly, we have identified quantum simulation architectures with a provable quantum advantage over classical supercomputers, and ones that can at the same time be verified. This has been one of the core tasks.

However, perhaps it is the unexpected applications, those we did not foresee at the beginning, that are the most exciting. These new directions render such a project particularly interesting. For example, that quantum devices have a provable scope for being better in certain learning tasks than classical ones. Or we have seen that there is an intermediate realm between analog and digital quantum simulation with applications in quantum chemistry - unexpected, but spot on to the aims of PASQuanS.

For you personally, what has been most fascinating about the project so far and how do you think PASQuanS will impact your future career?
The interactions with others have been tremendously stimulating. This interaction is of key importance, as it measures theoretical ideas against what is possible and important in practice.

Within PASQuanS, there are five theoretical teams. How are they divided and what are the common areas they work on?
This division of labour works very well and we contribute complementing insights. And even in case of overlap, e.g., when developing methods of verification in Berlin and Innsbruck, the endeavours are highly mutually stimulating and beneficial. We focus more on benchmarking, while Innsbruck brings in new ideas related to self-verification. As for tensor networks, we bring in unique expertise in developing methods for higher dimensional quantum systems. Overall, the project has been going extremely well also in this respect.

PASQuanS targets applications in material science, quantum chemistry, high-energy physics and optimisation. Which one of them do you expect to benefit from new simulation technologies first and why?
These are ambitious aims. To start with, we should identify quantum simulators as quantum technological devices, with clear predictive power and which have been properly benchmarked. Then, there are already lots of interesting applications for analog quantum simulation. In the long term, and with regard to the next steps, other applications will emerge. High energy physics is probably next, as the problems at stake have already been cast into a form amenable to programmable quantum simulators.

Moreover, there comes quantum chemistry: even though there is still too big a gap between basic variational eigensolvers and programmable quantum simulators to tackle interacting problems in quantum chemistry. We have worked hard to bring the effort down. Maybe the last application will be to find better approximations in combinatorical optimization problems. Here, we should be aware of the fact that it is all about approximation levels. Nobody expects quantum devices to efficiently solve NP-hard problems.

Everyone is talking about the “Second Quantum Revolution” – with research results of the last centuries now turning into a new generation of quantum technologies and, ultimately, products on the market. In your opinion, which fields of application have been the most promising so far and what can be achieved in the next decade?
I see what might make it appealing to use powerful buzz words such as ”revolution”, and this can in instances provide some stimulating rhetoric. But we should not forget that our future prospects and advances have only little in common with a revolution. It is true, quantum devices offer an enormous potential for tackling exciting problems beyond the reach of supercomputers. But this will be a long and winding journey. It will require more expertise, stamina, ideas, and funding. And PASQuanS sets, to my understanding, precisely the right aims.

It does not promise hyped applications in the financial world, does not speak of quantum advantages in risk portfolio management. Instead, it sets out to bring analog quantum simulators to a new level, to understand them as devices with enormous predictive power, and to add elements of programming to them. For this, we have assembled a world-class team. And then, if these steps have been achieved, we go for the next steps. And if the applications laid out materialize, yes, then we are in some sense about to have a revolution.

Which specific project activities are you involved in?
I am a theoretical physicist working in atomic physics and quantum optics. PASQuanS pursues building scalable quantum simulators with atomic platforms, and identifying and implementing applications of quantum simulation. I have always understood myself as somebody who, as a theorist, is close to experiment, trying to think of novel fundamental physics and technological applications we can do in light of experimental progress in the lab. In the early days, our main focus was to devise new concepts and proposals for quantum hardware, as are today in the focus of PASQuanS.

The emphasis has now shifted towards programming quantum simulators in our labs, which are scalable, although non-universal quantum, devices, both from a basic science and application point of view. This applies in particular also to our Innsbruck quantum environment and experiments on trapped ion quantum simulators.

Which results have already been achieved on your end and what will be the next milestones?
We have written a number of publications, which try to look ahead and identify, what is possible with near future atomic quantum simulators, in particular in light of the increasing programmability of these quantum devices. These ideas range from variational quantum simulation, where quantum circuits are programmed on atomic simulators from solving the quantum many-body problem, to applications like optimal quantum sensors, e.g., as better atomic clocks; or novel measurement protocols like our randomized measurement toolbox, which allows us to get much deeper insight into the role of entanglement in quantum many-body physics.

In the early days it often took many years to realise such ideas in the lab. However, the remarkable experimental progress in control of engineered atomic quantum systems has led to a much shorter cycle of theoretical proposal and experimental realisation. This points to the accelerating progress in building atomic quantum simulators in the lab.

For you personally, what has been most fascinating about the project so far and how do you think PASQuanS will impact future research and developments in this field?
There are too many fascinating projects and too many frontiers to elaborate here in detail, but a highlight has been the local and non-local collaborations with the experimental PASQuanS partners, and to see that in trapped ion experiments our new physics idea works in an amazing way. PASQuanS represents a group of international leaders and top researchers working on a plurality of atomic platforms. This has been an extremely stimulating scientific environment, and should be considered a highlight and success story of the EU Quantum Flagship.

Quantum technologies are a field of strong international competition. Where does Europe stand and how can the translation of basic research into applications be successful?
Europe is, and has been extremely strong in quantum science. Many of the fundamental ideas on the theory side, and many of the first and seminal experiments have been done in Europe. Clearly, Europe is at the forefront of quantum science, and atomic quantum simulation in particular. But it is a challenge for Europe to translate this scientific leadership into a technological leadership. While in the US big tech companies get involved with serious financial commitments, it is part of European culture to act much more cautiously.

You have a strong personal research focus on quantum optics and quantum information. If you had to explain both terms to a non-scientific audience in a few sentences – how would you describe them?
The roots of quantum information science reach back to the early 1990’s. These were the days when first quantum algorithms and applications were discovered where quantum computers and quantum simulators would provide a “quantum advantage” in solving problems not only of interest in physics, but in a much broader context with impact on society. In the 1990’s the central question was: “How to build a quantum computer or simulator?” And atomic physics provided an excellent starting point to answer these questions. Atomic physics had learned how to prepare, trap and laser cool single atoms, and arrays of atoms, and the tools like manipulation with laser light were developed.

Historically, many of these tools in atomic physics were driven by the field of high-precision measurement, one prominent example given by the (optical) atomic clock. Thus, for us theoretical physicists working in atomic physics and quantum optics it soon became obvious that we could develop the tools and concepts to quantum computers and simulators with atomic platforms. Based on the amazing experimental progress, today these ideas are reality in the lab, and PASQuanS is a prime example for this atomic physics and quantum optics success story.

Together with Ignacio Cirac, you have developed a model of a quantum computer as early as in 1995. If you had been asked about your expectations for the year 2020 back then, what would you have said? Have your expectations come true and, if not, what do you think has hampered the expected achievements in the meantime?
Let me first say, it is amazing for me to see that the original idea from 1995 has resulted in small-scale functioning quantum computers existing in our laboratories today, successfully competing with other platforms. And even companies have been built around these ideas. Maybe it was obvious to us as theorists that this should work, but in reality, this turned out to be a long road of experimental ingenuity, hard work and engineering, and credit should go to my experimental friends for actually achieving this goal. While small-scale quantum computers are today a reality, the scalable, fault tolerant quantum computer we are dreaming of is still far in the future.

But there is significant new physics and relevant applications which we can actually “do” in the coming years with intermediate-scale quantum devices already existing in our labs today. I want to add here that Ignacio Cirac and I have been very lucky that many of these early day ideas and concepts actually made it into the lab: this includes quantum simulators for Hubbard models with optical lattices, or the dipole blockade behind the Rydberg quantum simulators, as we see today in PASQuanS.

Everyone is talking about the “Second Quantum Revolution” – with research results of the last centuries now turning into a new generation of quantum technologies and, ultimately, products on the market. In your opinion, which fields of application have been the most promising so far and what can be achieved in the next decade?
The next decade will still see analogue quantum machines, as we have in PASQuanS. But the cross-over from analogue to digital – the latter meaning “universally programmable and fault tolerant” – will be a continuous transition. Analogue quantum simulators of today have the unique feature of being scalable to large atom numbers, i.e. to a regime beyond the capabilities of classical computations. In the next decade we will progress to more and more digital versions, where we programme increasingly complex quantum circuits from the basic building blocks provided by analogue quantum simulators. While all of us can list a number of applications for these “noisy intermediate scale quantum devices” addressing interesting and fundamental questions in many-body physics, real-world applications like quantum chemistry or the quantum computer are probably still far in the future.

Our goal must also be to find applications with impact beyond those quantum physicists are interested in. One example, which we have been working on, partly also with experimentalists, is to have quantum computers or simulators generate large-scale optimal quantum states to be used in quantum sensing. Here the programmability of simulators, for example as low-depth variational quantum circuits, and their scalability to generate highly entangled states of many atoms, finds application in building optimal quantum sensors, beyond what can be achieved with uncorrelated atoms.

Which specific project activities are you involved in?
I work in the theory team at the University of Strathclyde and my role is to investigate new ways to control quantum simulators that are used in the experimental groups within PASQuanS. Quantum systems are unavoidably coupled to their surroundings and can dissipate energy into them or establish correlations with the environment. This interaction with the surroundings includes the coupling with the experimental apparatus that allows us to make measurements on the system, which ultimately disturb it. In my project I describe how quantum systems behave when we start making measurements and when we put information, recovered from the measurement, back into the system. This allows us to study new ways to control the system’s behaviour by engineering measurement and feedback systems. Investigating this is useful in the context of quantum simulations and we hope that it will open up new opportunities to use such platforms to solve new problems relevant to applications within the PASQuanS project.

Which results have already been achieved on your end and what will be the next milestones?
We made good progress in terms of engineering schemes of measurement and feedback on quantum systems to obtain states with reduced uncertainties. This can be useful to increase the efficiency of quantum simulators, as well as to achieve more precise measurements in the context of quantum metrology and quantum sensing. This way we can overcome the limitations imposed by any classical protocols in different technological applications, ranging from higher precision atomic clocks to imaging to gravitational waves detection. The next milestone would be to identify key elements to integrate these protocols into the experimental apparatus used by other groups in PASQuanS.

For you personally, what has been most fascinating about the project so far and how do you think PASQuanS will impact your future career?
The most fascinating thing about the project is that I gained more insights about how what I do is not strictly theoretical, but eventually leads to solving experimental problems by introducing or rethinking some methodologies. Despite the fact that I am not sure about the direction I will take in my future career, I am really excited about all the opportunities that are opening up in industry. Also, the fact that PASQuanS offers a bridge between academic and industrial partners opens new prospects across a broad range of fields for the future.

The Quantum Flagship Initiative will help to bring quantum technologies from the lab to the market. Which role do the PASQuanS platforms play here and where do we stand?
Following up from the previous questions, I find it really exciting that PASQuanS is bringing together both theoretical and experimental groups in academia with industry, as well as setting the ground for delivering products in the market for end-users.

What we are witnessing is that we are at the stage where the boundaries between different fields are getting less sharp. As an example, I work with ultracold atoms and combine elements of quantum simulations and quantum sensing. The applications of quantum sensors are so broad that they range from imaging, which is of interest not only to physicists but also to biologists and chemists, to the detection of gravitational waves, which involves astrophysicists and engineers.

Bringing people together in projects like PASQuanS is therefore one of the necessary steps towards the long-term goal of having more quantum technology in the market, and ready for end-users.

How did you get to join the PASQuanS consortium and how do young researchers in particular benefit from the involvement in EU research projects?
I joined the PASQuanS consortium thanks to Prof. Andrew Daley who offered me some exciting projects to work on. Being involved in EU research projects is beneficial for young researchers because people from different countries and universities usually have different backgrounds, and this diversity is key to bringing new elements and ideas to projects.

More broadly speaking, participating in EU research projects is beneficial for all researchers, for the reasons I mentioned in the answer to the previous question: different countries have different strengths in various fields, and as quantum technologies grow, people from a broad range of expertise have to work together to bring their knowledge and strengths to other areas. The more people from different countries are part of a network with a common goal, the greater the chance that we will create something that is world leading.

One thing we’d like to do within PASQuanS objectives is to bring the general public closer to understanding quantum simulators and quantum technologies. How relevant do you think this is at this stage, and what can we do in terms of science communication?
I have personally been involved in several outreach activities where I try to bring people of any age closer to understanding quantum physics. What I see is that there is still a gap in the general knowledge, as some people seem to be intimidated by quantum physics, thinking that it is something out of their reach and relevant only to physicists. As the target of our research in quantum simulators moves to the market and end-users, however, a broader knowledge in quantum physics from the general public would be required. I think it is becoming even more important now to advocate for more outreach activities and to introduce young people to concepts of quantum physics already in schools.

Which specific project activities are you involved in?
Within PASQuanS, I am involved in the work package dedicated to the development of applications of programmable analogue simulators. The scope of these applications ranges from physics to chemistry to combinatorial optimisation. The goal is to investigate and quantify the potential of the various quantum hardware platforms of PASQuanS – whether Rydberg atoms, ultracold atoms or trapped ions – for solving challenging problems in these fields.

Which results have already been achieved on your end and what will be the next milestones?
Our team at Atos focuses more specifically on the solution of combinatorial optimisation problems with Rydberg platforms. We developed high-performance classical simulation tools to simulate Rydberg platforms and their imperfections, and used them to quantify the success probability of an algorithm for solving a combinatorial optimisation problem called the “Unit-Disk Maximum Independent Set” problem, a difficult computational problem. Our simulations allowed us to define the conditions, in terms of number of Rydberg atoms and noise levels, that should be attained in order for this type of quantum hardware to outperform advanced classical computers in solving this problem.

For you personally, what has been most fascinating about the project so far and how do you think PASQuanS will impact your future career?
A key aspect of PASQuanS is that it brings together the most advanced experimental analogue quantum processors, theory groups and industrial partners. It thus allows for very enriching exchanges between very different fields of expertise. I find it very stimulating, and important for the future development of the field. Being part of it offers me the opportunity to broaden my views in all those fields.

How can the transition of quantum technologies to industrial applications be successful and in which way does PASQuanS help to achieve this aim?
The development of industrial applications based on quantum technologies calls for a genuine dialogue between hardware makers and industrial end-users. Their ways of thinking and the technical constraints they have to deal with are usually very different. Plus, the challenges they face on a daily basis usually leave them little time for exchange with the “outside world”. PASQuanS provides a platform for exchange that can help bridge these worlds.

Atos BULL is the largest industry partner involved in the project. How would you describe the role of (European) research projects like PASQuanS for your company?
The goal of Atos BULL is to provide quantum-accelerated high-performance computing systems to its customers. This requires the identification of quantum computing platforms, whether gate-based (digital) or not (analogue), that can accelerate the solution of some of the most challenging computational problems. The goals of the PASQuanS project are completely in line with Atos’s endeavour.

PASQuanS targets applications in material science, quantum chemistry, high-energy physics and optimisation. Which additional industries could benefit from programmable quantum simulators and what is their market potential?
In principle, all industries that are faced with solving complex and challenging computational problems could potentially benefit from quantum acceleration. One could think of weather forecasting, finance or machine learning, for instance. Yet, at this relatively early stage of development of quantum technologies, whether a quantum advantage can be obtained for these fields crucially depends on the characteristics of the hardware as well as on the programming paradigms and software stacks that allow for an optimal usage of this hardware. Careful and case-by-case investigations must be conducted in order to reply to this question in a quantitative manner, namely to put actual numbers behind the promises of quantum computing.