### TESTING and exploiting macroscopic quantum physics

My research investigates how quantum physics can be tested and exploited with nanoparticles and large molecules. I pursue this within four sub-fields:

- Rotational optomechanics with aspherical nanoparticles
- Levitated quantum electromechanics
- Decoherence, friction, and diffusion of quantum rigid rotors
- Molecular matter-wave interferometry

See here for a Perspective on quantum rotations of nanoparticles and here for a review on quantum experiments with nanoscale particles.

### Rotational optomechanics

The non-linearity of free rigid body rotations gives rise to pronounced quantum interference effects, with no analogues in the body’s free centre-of-mass motion. Optically or electrically trapping and manipulating aspherical nanoparticles thus provides an attracitive platform for tests of quantum physics and for sensing at the quantum limit. Together with our experimental collaborators, we work on techniques to control and observe the mechanical rotation of nanoscale dielectrics, and develop schemes to witness *orientational quantum revivals* and the quantum version of the *tennis-racket effect*.

**Key publications:**

**Quantum rotations of nanoparticles**

B. A. Stickler, K. Hornberger, and M. S. Kim

Nat. Rev. Phys. (2021) [arXiv: 2102.00992 (2021)]**Cooling nanorotors by elliptic coherent scattering**

J. Schäfer, H. Rudolph, K. Hornberger, and B. A. Stickler

Phys. Rev. Lett.**126**, 163603 (2021)**Quantum persistent tennis racket dynamics of nanorotors**

Y. Ma, K. Khosla, B. A. Stickler, and M. S. Kim

Phys. Rev. Lett.**125**, 053604 (2020)**Probing macroscopic quantum superpositions with nanoscale rotors**

B. A. Stickler, B. Papendell, S. Kuhn, B. Schrinski, J. Millen, M. Arndt, and K. Hornberger

New J. Phys.**20**, 122001 (2018)**Optically driven ultra-stable nanomechanical rotor**

S. Kuhn, B. A. Stickler, A. Kosloff, F. Patolsky, K. Hornberger, M. Arndt, and J. Millen

Nat. Commun.**8**, 1670 (2017)

### Levitated quantum electromechanics

Electric traps are ideally suited for stably levitating nano- to microscale dielectrics in ultrahigh vacuum, providing an attractive platform for sensing and tests of for fundamental physics. The levitated nanoparticle can be connected to an electrical circuit via the endcap electrodes, through which its motion can be cooled, monitored, and manipulated. In a recent work we demonstrated how superconducting qubits can be used to generate and readout quantum superpositions of the motional quantum state of a highly charged dielectric.

**Key publications:**

**Quantum electromechanics with levitated nanoparticles**

L. Martinetz, K. Hornberger, J. Millen, M. S. Kim, and B. A. Stickler

npj Quantum Inf.**6**, 101 (2020)**Levitated electromechanics: all-electrical cooling of charged nano- and micro-particles**

D. Goldwater, B. A. Stickler, L. Martinetz, T. Northup, K. Hornberger, and J. Millen

Quant. Sci. Techn.**4**, 024003 (2019)

### Orientational decoherence of quantum rigid rotors

A nanoscale rigid rotor revolving in a homogeneous background gas experiences random collisions with the surrounding gas atoms. These collisions lead to a gradual loss of orientational coherence, and thus classicalize the quantum state of the rotor. In several recent works, we developed the theory of environmental decoherence, friction, and thermalization of arbitrarily shaped quantum rigid rotors. Application of the derived equations to a recent experiment with nitrogen superrotors yields excellent agreement.

**Key publications:**

**Rotational alignment decay and decoherence of molecular superrotors**

B. A. Stickler, F. T. Ghahramani, and K. Hornberger

Phys. Rev. Lett.**121**, 243402 (2018)**Rotational friction and diffusion of quantum rotors**

B. A. Stickler, B. Schrinski, and K. Hornberger

Phys. Rev. Lett.**121**, 040401 (2018)**Spatio-orientational decoherence of nanoparticles**

B. A. Stickler, B. Papendell, and K. Hornberger

Phys. Rev. A**94**, 033828 (2016)

### Molecular matter-wave interference

The interference pattern of large molecules crucially depends on how the particles interact with the diffraction grating. This provides an attractive way to access otherwise elusive molecular properties, such as their optical polarizabilities or absorption cross sections. On the other hand, their interaction with the grating can be used to control the quantum state of the molecules. In recent works we demonstrated Bragg diffraction of large molecules, showed how a combination of molecular diffraction and spatial filtering serves to separate different conformers, and investigated the role of molecular rotations in molecule interference.

**Key publications:**

**Bragg diffraction of large organic molecules**

C. Brand, F. Kialka, S. Troyer, C. Knobloch, K. Simonovic, B. A. Stickler, K. Hornberger, and M. Arndt

Phys. Rev. Lett.**125**, 033604 (2020) [Editor’s Suggestion, featured in Physics]**Conformer-selection by matter-wave interference**

C. Brand, B. A. Stickler, C. Knobloch, A. Shayeghi, K. Hornberger, and M. Arndt

Phys. Rev. Lett.**121**, 173002 (2018)**Molecular rotations in matter wave interferometry**

B. A. Stickler and K. Hornberger

Phys. Rev. A**92**, 023619 (2015)

### Key collaborators

- Markus Arndt, University of Vienna
- Klaus Hornberger, University of Duisburg-Essen
- James Millen, King’s College London