Research in the group is focused at the study of mesoscopic phenomena in condensed matter physics. Our research areas span over superconductivity (with emphasis on high temperature superconductors), low temperature physics, quantum electronics and spintronics, and nanotechnology.

# Research subject Condensed Matter and Quantum Optics

The research of Condensed Matter and Quantum Optics is both experimental and theoretical.

The research focuses on the fundamental aspects of quantum phenomena in light and matter, and has several potential applications in quantum information technology, quantum electronics and nanotechnology. Theoretical work also span broader areas of physics.

## Related research subject

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## Researchers

### Research groups

The Bergholtz group explores the world of quantum and complex systems — what it is and what it could be — from the perspective of mathematics and theoretical physics.

We are interested in various aspects of theoretical quantum mechanics. Currently, our main focus is on quantum simulations using quantum optical systems and the theory of open quantum systems.

The Wilczek group explores several frontiers of quantum theory where new or hitherto impractical theoretical concepts are making contact with experimenters’ increasing ability to explore and control the quantum world.

Our research is focused on thermodynamic, transport, and structural characterization of materials with novel electronic properties at low temperatures.

Semiconductor devices and non-linear processes can generate a wide spectrum of quantum states of light that can be employed in tasks of communication, simulation, and sensing. We use best tools and methods available to modern science to generate quantum light, harness its unique properties, and bring it closer to real-world application.

Due to a significant technical progress and precise engineering, we are nowadays able to manipulate individual quantum systems, like single atoms, with a precision that has been unthinkable a few decades ago.

## Research projects

The EU-funded BRISQ project will combine trapped ion technology and Rydberg interaction to build a highly promising physical platform to host a scalable quantum computer.

While entanglement is the primary threat of quantum mechanics, it is also the reason why simulating quantum systems by classical means is challenging. We explore complex quantum systems and explore methods for their simulation.

This project focuses on frontiers of topological matter such as non-Abelian anyons from twists and defects in Moiré heterostructures and topological phenomena in open dissipative systems.

We will investigate the unique properties of quasicrystals that can be linked to their specific structures. Of particular interest are magnetic and electronic properties that have recently been discovered in several Tsai-type quasicrystals.

This project brings together several frontiers of basic science - from materials science and quantum optics to the theory of topological phases and modern mathematics - with a potential to provide a basis of future technology.

Topological semimetals are a class of materials in which electrons obtain exotic propoerties that are not found in elementary particles in nature. We examine Coulomb interaction in these.

Trapped ions are a versatile tool for a wide range of research fields in classical and quantum applications.

Non equilibrium systems abound in nature on all scales but general principles that guide their working are not known. This project concerns investigating an aspect that all non-equilibrium systems have in common- entropy generation.

Quantum materials are materials with strong electronic correlation and emergent orders. In this research project we will experimentally study quantum materials in systems with strongly anisotropic properties.

Quantum information processing is a rapidly developing field of research with a large variety of applications ranging from powerful quantum algorithms to simulations of complex quantum systems.

In this theoretical project, we intend to investigate what happens when we assemble exotic topological components. More specifically, we want to investigate four types of "sculptured topological heterostructures" as briefly described below.

We study topologically ordered systems. One of their characterizing features is quantum number fractionalization, e.g. quasiparticle excitations can carry charge that is a fraction of the elementary electron charge.

Topological phases of matter are amongst the most interesting phases that are being studied, both theoretically as well as in experiments. In this project, we focus on the various types of phase transitions that topological phases can undergo.

An often forgotten energy consumer is the internet. Data centres, where information is stored in the form of magnetic bits, waste most of their energy as heat. Improve the control over magnetic information is crucial for our society is to prosper.

WACQT is a national research programme, coordinated from Chalmers, that aims to take Swedish research and industry to the forefront of quantum technology.

## Courses and programmes

At the Department of Physics at Stockholm University, we offer high-quality education with a strong connection to frontier research. We promise exciting studies in tight student groups and with active researchers as your teachers.

## Doctoral studies

At Fysikum there are four postgraduate subjects: physics, theoretical physics, chemical physics and medical radiation physics.

## News

# Non-Hermitian extended midgap states and bound states in the continuum

Maria Zelenayová, PhD student at Fysikum, and her supervisor Professor Emil Bergholtz looked into localization phenomena in non-Hermitian systems that challenge traditional beliefs about states within the energy continuum and those in the gap. The understanding of non-Hermitian extended band gaps and bound states holds significant implications across various domains. In quantum mechanics, this opens up new possibilities for controlling and manipulating the properties of materials. In technology, these insights can be utilized to develop more efficient semiconductors and other advanced materials.

# Stockholm Researchers Provide clues to solve cosmic conundrum

Why is the vast supergalactic plane teeing with only one type of galaxies? This old cosmic puzzle may now have been solved. Jens Jasche, associate professor at Fysikum, is a member of the research group of these results.

# Single-Shot Measurements of Phonon Number States Using the Autler-Townes Effect

Efficient and reliable detection of motional states is an essential component of many ion-trap experiments. In this recent work a new technique for measuring both Fock and thermal states, based on the Autler-Townes splitting, is demonstrated. The experimental work has been carried out in the group of Markus Hennrich at Fysikum, and was performed in collaboration with the groups of Celso Villas-Boas and Romain Bachelard in São Carlos, Brazil.

# The Nobel Prize in Chemistry 2023 - Quantum dots

Moungi G. Bawendi, Louis E. Brus and Alexei I. Ekimov were awarded the Nobel Prize in Chemistry 2023 for the discovery and development of quantum dots. These tiny quantum systems have unique properties and many applications: they emit light used in television screens and LED lamps, they catalyse chemical reactions, and their emission marks the borders of tumor tissue for a surgeon. Ana Predojević, is associate professor at Fysikum: "We use the best tools and methods available to modern science to generate quantum light, harness its unique properties, and bring it closer to real-world application."

# Particles protected by Braids and Knots

We present a class of systems in which a particle - antiparticle pair cannot annihilate each other after they have moved along a loop, and instead form a new type of composite particle. This occurs in so-called non-Hermitian systems; classical metamaterials or "open" quantum systems that are coupled to the rest of the universe. Lukas Königis a PhD Student at Fysikum and is part of the research group Quantum and Complex Systems.

# Quantum metric unveils defect freezing in non-Hermitian systems

The fundamental postulates of quantum mechanics rest upon the fact that the Hamiltonian, which quantifies the energy of the system, is given by a Hermitian operator. This means that the complex conjugate and transpose of the Hamiltonian should be equal to itself. Almost a century after the formulation of quantum mechanics, researchers are now trying to break free from this constraint — non-Hermitian Hamiltonians, which do not satisfy the aforementioned property, are now a new frontier of exploration in both theoretical and experimental physics. Paolo Molignini is part of the project and is a Postdoctor in Condensed Matter and Quantum Optics at Fysikum.

# Frank's curiosity has earned him the Nobel Prize and several fruitful collaborations

Professor Frank Wilczek's was interested in mathematics and big numbers even as a child. His father was an electrical technician who fixed radios and tvs when they were broken. Frank was inspired and liked to take apart and put together things to see how they worked. This was the beginning of a fantastic career in Physics. In 2004, Wilczek received the highest honor in science — the Nobel Prize in Physics — for his discovery of asymptotic freedom and the development of the theory of quantum chromodynamics (QCD). Since 2016 he has been working at the Department of Physics.

# Word and bit line operation of a 1x1 μm2 superconducting vortex-based memory

The third industrial “digital” revolution is on the way. The volume of digital information, the amount of large computation facilities and the related energy consumption are growing in an explosive manner. The notorious “memory wall” problem reveals ineffectiveness of the standard processor-centered computation for processing of big data volumes and actuates the necessity of both hardware and software development for new types of in-memory computation. Simultaneously, silicon-based electronics is approaching its limits. The large resistivity of nano-scale semiconductor transistors and interconnections impedes operation speed and causes severe heat management problems. Sustainability of the present digitalization trend requires radical innovative solutions.

# Workshop about exploring simulations of gravity and quantum gravity

In July Fysikum hosted a workshop in Stockholm, jointly organized by Igor Pikovski from Fysikum and Rivka Bekenstein from the Hebrew University of Jerusalem. Its focus was the study of quantum gravity in the lab. 20 participants spent 3 days at Näsby Slott to discuss quantum simulations and quantum aspects of gravity.

# Unlocking the Quantum Enigma: Deep Learning to Quantify Entanglement from Incomplete Measurements

Quantum technologies including quantum computing and quantum communication rely on a very intriguing natural phenomenon – entanglement. Using modern methods of machine learning enables quantification of this important and fragile feature of the quantum world.

# New quantum communication infrastructure

Mohamed Bourenanne is part of the National Quantum Communication Infrastructure in Sweden (NQCIS) project, which received SEK 30 million from Vinnova and is coordinated by KTH. His research group works in the field of quantum communication and their laboratory is located at AlbaNova. - All eavesdropping attempts can be revealed thanks to the basic theories of quantum mechanics, thus increasing information security.

# Quantum mechanics predicts effects that are counter-intuitive

Quantum mechanics predicts effects that are counter-intuitive and impossible to reconcile with our daily experience. One of the most intriguing examples of these predictions is that a pair of particles can be entangled. When two particles are entangled we cannot describe the properties of one of them without referring to the other, they behave as a single entity.

# The Future is Quantum: 20 - 22 February 2023

An emerging field of physics and engineering is quantum technology, encompassing technologies that rely on the properties of quantum mechanics. Quantum computing being one example of these technologies, representing a paradigm shift for computing technology, since it can outperform much more than existing computers. On February 21 at 13:00, in the: Svedberg salen (FD5), AlbaNova, Professor Akira Furusawa from University of Tokyo, RIKEN Center for Quantum Computing will have a presentation with the title THE FUTURE IS QUANTUM - The development of Quantum Computing.

# Frank Wilczek on the 2022 Physics prize

Frank Wilczek is professor of theoretical physics at MIT and The Department of Physics at Stockholm university and a Nobel laureate. He held the Nobel prize in physics in 2004. Hear him comment on the Physics Prize of 2022, and giving advice to this year´s laureates arriving in Stockholm in December.

# Entangled photons are a key enabler of quantum technology

Extremely efficient entangled photon pair sources can be achieved by employing a simple but effective approach – placing quantum dots in broadband pillar microcavities. Professor Ana Predojevic, member of Quantum Photonics group at Fysikum, tells us more.

## Departments and centres

Research in the subject is mainly carried out at Fysikum in the division of Condensed Matter and Quantum Optics.

Department of Physics