Hans Hübl

Magnetism, Spintronics and Quantum Information Processing

Walther Meißner Institute

Walther-Meißner Str. 8

85748 Garching

Tel. +49 89 289 14204

hans.huebl[at]wmi.badw.de

Research Website

Description

Research focus: quantum nano-systems, quantum sensing, quantum materials

Transfer of information between various types of harmonic oscillators is of key importance for aspects of quantum information storage and conversion. In particular, the regime where the coupling rate exceeds the individual loss rates is of special interest, as this regime promises an efficient transfer of excitations. Our group investigates linearly and non-linearly coupled harmonic resonators based on superconducting microwave resonators, nano-mechanical string resonators, and spin ensembles.

Coherent control of Spins and Spin Ensembles

Electron spin resonance (ESR) is a key technique for quantum information technologies as it lays the foundation for coherent spin control and the determination of coherence times. Additionally, ESR is a key spectroscopy tool for multiple disciplines like chemistry, biology, and physics.

We presently focus on magnetic resonance techniques at millikelvin temperatures. One aspect is to achieve a large thermal spin polarization and hereby a highly sensitive readout. This, together with a suitable spin density, allows to investigate the coupling between the spin ensemble and the microwave resonator in the strong coupling regime. The investigation of the dynamics of such strongly coupled systems and the understanding how to coherently control the spin ensemble under these conditions are key questions for utilizing these systems for quantum information storage or spectroscopy and sensing applications. We combine conventional ESR with ultra-sensitive microwave detection schemes derived from superconducting circuit QED to further improve the sensitivity of ESR. Long-term research goals in this field are the use of quantum states for sensing spin properties and or magnetic field sensing.

Al-circuit-nanomechanics_2016-DEC-20
Circuit Nanoelectromechanics & Circuit QED

The field of nano-electromechanics aims at studying quantum mechanics in the literal sense. One prerequisite is to cool a vibrational mode to its quantum ground state and then subsequently prepare this mode in a specific quantum state e.g. like a phonon Fock state, squeezed state or cat state. In this context, the integration of nanomechanical elements in superconducting microwave cavities was a big step forward enabling some of the goals stated above. We started with conventional circuit nano-electromechanics, where we demonstrated electromechanically induced transparency and slow light physics in nano-string superconducting microwave resonator hybrid devices. Typically, devices in this field rely on a capacitive coupling between the mechanical element and the microwave resonator where the capacitive participation ratio limits the coupling strength. To overcome this limit, we are presently implement an inductive coupling scheme based on a dc-SQUID with a vibrational element. In parallel, we start combining circuit nano-electromechanics with circuit QED to address the key challenge of preparing phonon Fock states in a nano-string resonator.

Publications

Electrically Induced Angular Momentum Flow between Separated Ferromagnets

R. Schlitz, M. Grammer, T. Wimmer, J. Gückelhorn, L. Flacke, S. T. B. Goennenwein, R. Gross, H. Hübl, A. Kamra, M. Althammer

Physical Review Letters 132 (25), 256701 (2024).

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Converting angular momentum between different degrees of freedom within a magnetic material results from a dynamic interplay between electrons, magnons, and phonons. This interplay is pivotal to implementing spintronic device concepts that rely on spin angular momentum transport. We establish a new concept for long-range angular momentum transport that further allows us to address and isolate the magnonic contribution to angular momentum transport in a nanostructured metallic ferromagnet. To this end, we electrically excite and detect spin transport between two parallel and electrically insulated ferromagnetic metal strips on top of a diamagnetic substrate. Charge-to-spin current conversion within the ferromagnetic strip generates electronic spin angular momentum that is transferred to magnons via electron-magnon coupling. We observe a finite angular momentum flow to the second ferromagnetic strip across a diamagnetic substrate over micron distances, which is electrically detected in the second strip by the inverse charge-to-spin current conversion process. We discuss phononic and dipolar interactions as the likely cause to transfer angular momentum between the two strips. Moreover, our Letter provides the experimental basis to separate the electronic and magnonic spin transport and thereby paves the way towards magnonic device concepts that do not rely on magnetic insulators.

DOI: 10.1103/PhysRevLett.132.256701

Temperature dependence of the magnon-phonon interaction in hybrids of high-overtone bulk acoustic resonators with ferromagnetic thin films

M. Müller, J. Weber, S. T. B. Goennenwein, S. V. Kusminskiy, R. Gross, M. Althammer, H. Hübl

Physical Review Applied 21 (3), 34032 (2024).

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Tailored magnon-phonon hybrid systems, in which high-overtone bulk acoustic resonators couple resonantly to the Kittel mode of a ferromagnetic thin film, are considered optimal for the creation of acoustic phonons with a defined circular polarization. This class of devices is therefore ideal for the investigation of phonon-propagation properties and assessing their capacity to transport angular momentum in the classical, and potentially even in the quantum, regime. Here, we study the coupling between the magnons in a ferromagnetic Co25Fe75 thin film and the transverse acoustic phonons in bulk acoustic wave resonators formed by the sapphire substrate onto which the film is deposited. Using broadband ferromagnetic resonance experiments as a function of temperature, we investigate the strength of the coherent magnon-phonon interaction and the individual damping rates of the magnons and phonons participating in the process. This demonstrates that this coupled magnon-phonon system can reach a cooperativity C approximate to 1 at cryogenic temperatures. Our experiments also showcase the potential of strongly coupled magnon-phonon systems for strain-sensing applications.

DOI: 10.1103/PhysRevApplied.21.034032

Chiral phonons and phononic birefringence in ferromagnetic metal-bulk acoustic resonator hybrids

M. Müller, J. Weber, F. Engelhardt, V. Bittencourt, T. Luschmann, M. Cherkasskii, M. Opel, S. T. B. Goennenwein, S. V. Kusminskiy, S. Geprägs, R. Gross, M. Althammer, H. Hübl

Physical Review B 109 (2), 24430 (2024).

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Magnomechanical devices, in which magnetic excitations couple to mechanical vibrations, have been discussed as efficient and broadband microwave signal transducers in the classical and quantum limit. We experimentally investigate the resonant magnetoelastic coupling between the ferromagnetic resonance modes in metallic Co25Fe75 thin films, featuring ultralow magnetic damping as well as sizable magnetostriction, and standing transverse elastic phonon modes in sapphire, silicon, and gadolinium gallium garnet at cryogenic temperatures. For all substrates, we observe a coherent interaction between the acoustic and magnetic modes. We identify the phonon modes as transverse shear waves propagating with slightly different velocities (Av/v similar or equal to 10(-5)),. i.e., all investigated substrates show potential for phononic birefringence as well as phonon-mediated angular momentum transport. Our magnon-phonon hybrid systems operate in a coupling regime analogous to the Purcell enhanced damping in cavity magnonics.

DOI: 10.1103/PhysRevB.109.024430

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