Resources for Flexible Quantum Information Processing

Over the past two decades, quantum technologies for computation, communication and sensing have progressed from theoretical proposals to experimental reality. Although these technologies are widely expected to outperform their classical counterparts in the future, present day stateoftheart setups operating in the quantum domain cannot compete with currently available classical devices such as smart phones or laptop computers. This is, in part, due to the difficulties in controlling and precisely manipulating the required quantum systems such as trapped atoms or quanta of light (socalled photons). Consequently, modern prototypes for quantum computers operate with only a handful of information carrying systems called “qubits”, the quantum equivalents of classical bits.
In this research project, we investigate how the scarce quantum resources such as the available qubits can be used most efficiently to perform desired tasks such as running computations on a quantum computer. In particular, a novel aspect of this theoretical research project will be the focus on developing flexible multipurpose devices for quantumenhanced sensing and computation [Friis et al., New J. Phys. 19, 063044 (2017)]. That is, if resources are very limited and costly, it is desirable to design gadgets that are able to perform several tasks of interest using the same basic resources. The project is in this sense inspired by the technological miniaturization and functional integration that has taken place for modern smart phones. In our quest to develop flexible integrated quantum devices, we will hence study how resources for quantum computation and quantum communication can be converted and used most efficiently to perform also other tasks such as the precise measurement and estimation of unknown parameters. Apart from the conversion of informationtheoretic resources into each other, we will also consider the exchange with physical resources such as the available energy [e.g., conversion of energy into correlations, see Vitagliano, Klöckl, Huber, Friis, arXiv:1803.06884 or energy cost of measurements, see Guryanova, Friis, Huber, Quantum 4, 222 (2020)]. To achieve this, our research of computational and communicational tasks will be embedded in the context of physical theories such as quantum thermodynamics (e.g., to study the exchange and extraction of energy from systems with some temperature) and quantum optics (the interaction of light and matter in the quantum domain). Using these techniques, we will investigate, for example, how energy may be traded for information gain in the estimation of an unknown quantity, and how such tasks may be influenced by practical and fundamental limitations.
In this research project, we investigate how the scarce quantum resources such as the available qubits can be used most efficiently to perform desired tasks such as running computations on a quantum computer. In particular, a novel aspect of this theoretical research project will be the focus on developing flexible multipurpose devices for quantumenhanced sensing and computation [Friis et al., New J. Phys. 19, 063044 (2017)]. That is, if resources are very limited and costly, it is desirable to design gadgets that are able to perform several tasks of interest using the same basic resources. The project is in this sense inspired by the technological miniaturization and functional integration that has taken place for modern smart phones. In our quest to develop flexible integrated quantum devices, we will hence study how resources for quantum computation and quantum communication can be converted and used most efficiently to perform also other tasks such as the precise measurement and estimation of unknown parameters. Apart from the conversion of informationtheoretic resources into each other, we will also consider the exchange with physical resources such as the available energy [e.g., conversion of energy into correlations, see Vitagliano, Klöckl, Huber, Friis, arXiv:1803.06884 or energy cost of measurements, see Guryanova, Friis, Huber, Quantum 4, 222 (2020)]. To achieve this, our research of computational and communicational tasks will be embedded in the context of physical theories such as quantum thermodynamics (e.g., to study the exchange and extraction of energy from systems with some temperature) and quantum optics (the interaction of light and matter in the quantum domain). Using these techniques, we will investigate, for example, how energy may be traded for information gain in the estimation of an unknown quantity, and how such tasks may be influenced by practical and fundamental limitations.
Current Topics of Interest
Quantum Thermodynamics (QT) is concerned with the investigation of phenomena such as thermalization and equilibration, heat engines, and foundations of thermodynamics in the quantum regime.The study of thermodynamics in the quantum domain provides an amazing example for the crossfertilization between different areas of physics, involving classical thermodynamics and solid state physics, as well as quantum information and quantum field theory. For recent reviews see [Goold, Huber, Riera, del Rio, Skrzypczyk, J. Phys. A: Math. Theor. 49, 143001 (2016)], [Vinjanampathy, Anders, Contemp. Phys. 57, 1 (2016)] and [Millen, Xuereb, New J. Phys. 18, 011002 (2016)].
My current reseach interest within QT lies in fundamental and practical restrictions for thermodynamic processes in quantum optics. In particular, I am interested in learning about the usefulness (and restrictiveness) of Gaussian operations for tasks such as energy extraction, storage, and distribution, as well as converting work and correlations into each other. More information can be found here. 
Quantum metrology exploits distinctive quantum features, such as entanglement, to enhance the estimation precision of parameters governing the dynamical evolution of the probe systems beyond that achievable by classical means. This enhancement is manifested in the form of a scaling gap in precision with respect to the available resources (the number of probe systems or the average input energy) between the corresponding optimal quantum and classical strategies, and depends on the specific encoding of the parameter in the Hamiltonian describing the evolution.
Here, I have recently been pursuing the question, whether architectures for measurementbased quantum computation can be employed as flexible metrology devices. In this context, we have recently investigated the possibility of creating a versatile allpurpose device for quantum metrology based on 2D cluster states [Friis, Orsucci, Skotiniotis, Sekatski, Dunjko, Briegel, Dür, New J. Phys. 19, 063044 (2017)]. A presentation of this work can be found here (pdf). 
Another research interest in quantum metrology concerns estimation scenarios where the parameter of interest is encoded in Gaussian transformations, see [Friis, Skotiniotis, Fuentes, Dür, Phys. Rev. A 92, 022106 (2015)], and more generally in the connection between quantum metrology, quantum computing and quantum thermodynamics.
For more information see this page.
For more information see this page.
An interesting connection between quantum computing and artificial intelligence arises from modelling the decisionmaking processes of autonomous learning agents via quantum random walks. The framework for this investigation is the Projective Simulation (PS) model, introduced in [Briegel, De las Cuevas, Sci. Rep. 2, 400 (2012)], and its variant the Reflecting Projective Simulator [Paparo, Dunjko, Makmal, MartínDelgado, Briegel, Phys. Rev. X 4 031002 (2014)]. A key element in the computational architecture that allows the agent to update its decisionmaking process when it learns is coherent controlization, i.e., a method of adding control to a set of a priori unspecified unitaries. We have investigated these methods for systems of trapped ions [Dunjko, Friis, Briegel, New J. Phys. 17, 023006 (2015)] and superconducting qubits [Friis, Melnikov, Kirchmair, Briegel, Sci. Rep. 5, 18036 (2015)]. More information can be found on this website.

Fermions make up essentially all standard matter in the standard model of particle physics, but interestingly their role in quantum information and computation is sometimes regarded as marginal. Nonetheless, growing interest in fermions and their interesting properties is arising through applications in solid state physics. Many theoretical studies rely on mappings between fermions and qubits  the Pauli principle practically limits the occupation of a fermionic mode to at most one particle, effectively turning them into twolevel systems. However, there is an interesting mismatch between fermions and qubits. My research interest is hence to study this difference, to learn in what sense "fermions are not qubits", and to study the quantum correlations in fermionic systems with no fixed particle number. Read about it in [Friis, Lee, Bruschi Phys. Rev. A 87, 022338 (2013)], and in [Friis, Reasonable fermionic quantum information theories require relativity, New J. Phys. 18, 033014 (2016)], or in our recent article on fermionic teleportation [Debarba, Iemini, Giedke, Friis, Phys. Rev. A 101, 052326 (2020)]. Download a presentation [pdf], or find out more on this website.
