We study the foundations and applications of superconducting quantum circuits. The latter include the astonishing demonstration of textbook quantum mechanics as well as quantum information processing (QIP) and quantum simulation. Our research does not only address the foundations of quantum information systems and superconducting quantum technology, but also key fundamental questions regarding quantum coherence, quantum dynamics, and decoherence mechanisms in solid state quantum systems. Furthermore, it requires extremely sensitive measurements at millikelvin temperatures.

At the Walther-Meissner-Institute, we explore two coupling stategies for the realisation of the nano-electromechanical interaction:

1. a capacitive coupling scheme, where the displacement of the nano-string resonator changes the overall capacitance of the superconducting electronic circuit, and
2. an inductive coupling scheme, where we utilize the tunable inductance of a superconducting interference device to realize this purpose.

With these integrated nano-electromechanical devices, we have demonstrated force sensitivities down to aN/sqrt(Hz) and coupling rates in the tens of kHz range. We utilize these platforms to investigate and understand the interaction itself, study the mechanical properties of the materials involved and realize literal quantum mechanical states.

We exploit the unique quantum properties of microwaves to develop exciting application scenarios such as quantum radar-type sensing, secure information transfer, and gate teleportation between distant superconducting qubits. We have pioneered this research field and are one of the leading groups worldwide. In a long-term perspective, we envision local area networks of superconducting quantum computers and a full-fledged quantum internet.

Microwaves in the frequency range of 1-10 GHz are the natural frequency scale of many well-known applications such as radar, mobile communication, or classical information processing. Consequently, microwaves are also promising candidates for modern quantum technology applications. The latter include quantum radar, networked superconducting quantum computing, and free-space quantum communication on a range also relevant for 5G. One of the key advantages of quantum microwaves compared to other frequency regimes is the fundamental technology match between the circuits used to generate, manipulate, and detect quantum microwaves and those used by several big IT companies and young start-ups for superconducting quantum information processing. In particular, our microwave approach removed one major roadblock on the path to high-fidelity gates between remote quantum circuits, namely the inefficient and demanding frequency conversion from microwave to optics and back.

Our research focuses on the continuous-variable regime, which offers advantages in terms of robustness and technological implementation. Since today’s road to quantum information processing is typically based on a digital approach, we put special emphasis on investigating hybrid approaches between discrete-variable and continuous-variable circuits and signals.

Microwave quantum networks are one technological cornerstone of many quantum microwave applications. For superconducting circuit architectures and quantum microwaves, such networks must currently operate at millikelvin temperatures. Since superconducting quantum information processors come with a proper cryostat, a so-called dilution refrigerator, anyways, care has to be taken only with respect to the proper design of a connection between such cryostats. At WMI, we have set up such a point-to-point connection [link Installation video] between two dilution refrigerators as a testbed for quantum communication and cryptography protocols. Equipped with low-loss superconducting transmission line cables, our installation can be viewed as a quantum local area network (Q-LAN) cable [link installation video] . The transmission losses of few dB/km are comparable to those in standard optical fibers. Through this Q-LAN cable, we plan to run microwave quantum communication and cryptography protocols. Furthermore, we aim to investigate the expected tolerance of our Q-LAN cable to higher temperatures experimentally. In a long-term perspective, we envision a quantum local area network with high connectivity between multiple nodes.

Our long-term vision is to develop distributed quantum computing & communication based on microwave quantum local area networks (QLANs). Microwaves are the natural frequency regime of several quantum computing platforms (superconducting circuits, NV centers, quantum dots). Therefore, microwaves are the natural frequency range for quantum communication between such platforms. In particular, no frequency conversion is required which usually is inefficient and related to significant losses. Moreover, microwaves can be distributed via superconducting cables with surprisingly small losses, eventually allowing for quantum communication and cryptography applications.

An important near-term goal is the demonstration of a QLAN via quantum teleportation and to develop a roadmap to real-life applications for the second/third phase of the European Quantum Technology Flagship. An important enabling technology for achieving the goal is the development of a microwave QLAN cable connecting the millikevin stages of two dilution refrigerators. This technology is developed by WMI together with its industrial partner Oxford Instruments within the European Quantum Technology Flagship project QMiCS.  The resulting “enabling” commercial products are beneficial for quantum technologies at microwave frequencies in general.

Quantum microwaves promise a fundamentally increased sensitivity over classical sensing protocols. As in most sensing applications, the quantum advantage is typically restricted to the low-energy regime with signal energies on the order of a single photon.

The secure transmission of classical information is of tremendous importance in our society, e.g., when exchanging information on finances, health, or private issues. However, the advent of quantum computing poses a serious threat to classical encryption methods. One possible solution is the use of quantum cryptography methods, either based on actual or virtual entanglement between the communicating parties. Once a Q-LAN between quantum computers is realized, the exploration of such protocols is a natural task. Among various cryptography protocols, quantum key distribution (QKD) has the most obvious application potential. Ideally, QKD exploits quantum resources for the intrinsically secure exchange of a classical key for encoding a classical message (text, music, video etc.) between partners. At WMI, we use and extend the well-established toolkit of quantum microwave technology to implement QKD schemes. Potential advantages of microwaves are high secret key rates due to large absolute bandwidths and the potential for short-distance free-space implementations. The latter promise compatibility with the existing classical microwave infrastructure and ranges comparable to that of current 5G.

The ordering of spins in magnetic materials is determined by the subtle interplay between various interactions ranging from exchange and dipolar interactions to more exotic ones such as the Dzyaloshinskii-Moriya interaction (DMI). These interactions may result in simple parallel and anti-parallel spin configurations such as in ferromagnetic and antiferromagnetic materials, but also in more complex spin configurations such as magnetic skyrmions and other topological spin textures. In our research, we aim at identifying the static and dynamic properties of spin structures in magnetic materials and clarifying the underlying physical mechanisms in both bulk materials and magnetic heterostructures. This is of key relevance for their applications in magnetic data storage or in spintronics devices.

In spintronics  – one of the emerging fields for the next-generation nanoelectronic devices – the transport of spin-polarized charge carriers or even the transport of pure angular momentum (pure spin currents) without any charge transport is in the focus of present research. We particularly focus on pure spin currents in electrically insulating magnetic materials carried by quantized spin waves (magnons), as devices based on such currents may have reduced power consumption and allow for an increase in memory and processing capabilities.

We fabricate insulating and electrically conducting magnetic materials as well as complex heterostructures consisting of magnetic and non-magnetic materials. The combination of magnetically ordered materials with non-magnetic metals with strong spin-orbit interactions allows us to generate and detect pure spin currents by the direct and inverse spin Hall effect (SHE). Such structures lead to important discoveries of the spin Hall magnetoresistance (SMR) and the spin Nernst effect (SNE) at WMI. Recently, we also could demonstrate spin transport in a magnetic insulator with zero effective damping and the magnonic analogue of the electronic Hanle effect and the Datta-Das transistor.

The breaking of inversion symmetry at interfaces in heterostructures of magnetic and non-magnetic materials with strong spin-orbit coupling allows one to stabilize skyrmion states in a wide temperature and magnetic field range. We systematically study metallic multilayers nanostructures patterned into these layers to evaluate their suitability for hosting ferromagnetic and antiferromagnetic skyrmions.

Antiferromagnetic Spintronics

Antiferromagnetic materials promise improved performance for spintronic applications, as they are robust against external magnetic field perturbations and allow for faster magnetization dynamics compared to ferromagnets. The direct observation of the antiferromagnetic state, however, is challenging due to the absence of a macroscopic magnetization. The spin Hall magnetoresistance (SMR) effect, however, is a versatile tool to probe the antiferromagnetic spin structure via simple electrical transport experiments. We investigate ...

• the spin Hall magnetoresistance (SMR) effect in antiferromagnetic insulators,
• the antiferromagnetic spin texture and its evolution as a function of the external magnetic field, and
• the spin-transport in antiferromagnetic insulators via the the magnon-mediated magnetoresistance (MMR).

Spin Transport and Magnon-Mediated Magnetoresistance

Pure spin currents represent the chargeless transport of angular momentum. They give rise to novel interface effects like the recently discovered spin Hall magnetoresistance. We investigate ...

• the spin transport and the spin Hall magnetoresistance (SMR) effect in materials wit long-range magnetic order,
• non-local effects and magnon-mediated magnetoresistance (MMR), and
• the topological Hall effect (THE) and related phenomena in materials with large spin-orbit coupling.

At the Walther-Meissner-Institute we pioneered research in the direction of strong magnon-photon interaction, which is key to investigate magnons on the quantum level, but also gives deep insight in the hybridization of magonic and phonic states, an aspect which we have intensely researched by combining this hybrid system with electical readout techniques. 

Spin ensembles based on paramagnetic centers are complementary to their exchange coupled counterpart. Although, the coupling is less intense, they still can be operated in the strong coupling regime. Due to their extreme coherence times, these spin systems are discussed for quantum memory or quantum transduction applications. 

In addition, we study magnon-phonon hybrids based on acoustic resonators and magnetic thin films, as well as coupled nano-string resonator networks. 

We study the dynamics of both paramagnetic and exchange-coupled spin systems by broadband magnetic resonance spectroscopy. We aim at extracting fundamental material parameters like magnetic anisotropy, exchange coupling and damping of spin dynamics in a wide range of materials, ranging from simple ferromagnets to topologically nontrivial magnets with complex spin textures. A particular focus of our research is the study and application of spin-orbit torques.

Like magnetism and superconductivity, those quantum phases are not only of high interest from the fundamental science perspective, but also have potential applications in quantum science and technology. We fabricate thin film and multilayer quantum materials, including magnetically ordered insulators (e.g. Y₃Fe₅O₁₂ or α-Fe₂O₃), spin-orbit driven materials (e.g. Sr₂IrO₄), or Dzyaloshinskii-Moriya-active interfaces...