I study quantum optics and its applications in quantum information processing. Quantum physics is a peculiar and counterintuitive field of physics that describes the behaviour of individual atoms or larger systems cooled to extremely low temperatures; the subfield of quantum optics focuses on the interaction of these systems with light. Like many other researchers the world over, I am trying to understand how we can use the unusual features of these systems to build better, more efficient computers, communicate more securely, or measure properties of various objects with better precision.

I am a theoretical physicist. I don’t spend my days in a laboratory building experiments and collecting data. Instead, I try to come up with ideas for new experiments others can do or, occasionally, understand what they measured. My work therefore involves a lot of mathematical calculations as I try to understand how a certain system can be described and how it behaves. To make sure that my ideas are feasible, I often run numerical simulations on my computer to see how well an experiment with realistic parameters would work.

As our knowledge progresses, the experiments scientists can perform and their description become more and more complex. We know that while a quantum system might be a good choice for, say, quantum computing, it wouldn’t be suitable for sending quantum information between two places. Experiments are thus starting to focus on combining several quantum systems into larger systems. Such hybrid quantum systems can use the advantages of their building blocks and form devices capable of more than their subsystems alone.

Hybrid quantum systems pose many challenges to a theoretical physicist. A deep understanding of various quantum systems is necessary to find ways to build new hybrid quantum devices that are both feasible and useful. On the other hand, one needs to develop new mathematical tools that capture the main features of the resulting hybrid systems; the complexity does not allow one to describe and understand hybrid quantum systems in full detail.

## Systems and tools I work with

#### Cavity optomechanics

Interaction between light and mechanical oscillators—small drums or tiny strings—offers a powerful tool to control and read out mechanical motion. Measuring the motion can give us useful information about external influences acting on the oscillator, such as electric or magnetic fields and even passing gravitational waves. We can also use optomechanics for testing the predictions of quantum physics at large scales or for building networks of quantum computers.

#### Circuit quantum electrodynamics

Superconductors enable electrical current to pass through without any resistance. Electrical circuits for information processing in these materials make it possible to preserve the quantum nature of the current and are thus suitable for encoding and processing quantum information. I am particularly interested in understanding how such quantum computing chips can be connected to light and form quantum communication networks.

#### Quantum measurement and feedback

Quantum systems interact with their environment which usually leads to them behaving classically. Nevertheless, when we observe the environment, we can learn about our quantum system’s behaviour and use this information to preserve the system’s quantum nature or to perform useful operations.

#### Gaussian states and operations

Mathematical description of quantum systems poses many challenges; the size of the mathematical objects describing the system (the wavefunction or density operator) grows exponentially with the size of the system. In some cases, however, the description can be simplified owing to the internal structure of the system and its dynamics. One such class of systems is characterized by having Gaussian phase-space distribution and can cover, among other things, a broad range of operations performed on light or mechanical oscillators.

## Projects I have worked on

#### Cavity optomechanics with hybrid mirrors

I analyse hybrid optomechanical systems formed by dielectric membranes doped with two-level quantum emitters or patterned with photonic crystal structures. Such hybrid mirrors strongly reflect light around a particular wavelength, leading to a modified response to light. I am trying to find out what interesting applications such devices might have. On the one hand, some existing optomechanical experiments (such as optomechanical cooling) can be improved with this approach; there are also novel effects that I want to understand.

#### Optomechanical transduction

Mechanical oscillators can couple to a broad range of external forces and are thus suitable for conversion of signals between different carriers. One particular—and important—example is the conversion between microwaves and light which can be used improve detection of weak microwave signals or for conversion of quantum signals between superconducting quantum computers (operating at microwave frequencies) and light (suitable for long-distance quantum communication).

#### Gaussian entanglement of light

Gaussian states of light are an important subclass of all quantum states of light owing to their easy creation and manipulation. Most importantly, entangled Gaussian states can be prepared deterministically, unlike entangled states based on single photons. An important topic of research is finding states best suited for a specific task (such as quantum teleportation) or developing efficient protocols involving non-Gaussian operations (which are necessary, for instance, for entanglement concentration and distillation).