The theory section of QUANTOP supports ongoing experiments and sets new goals for future projects. Central topics include:
- quantum limits for sensing
- quantum backaction evasion
- quantum noise evasion in gravitational wave detectors
- macroscopic entanglement generation and quantum teleportation
- quantum transduction
Quantum mechanics dictates that it is impossible to simultaneously have exact knowledge of two physical observables represented by non-commuting operators, e.g., the position and momentum of a microscopic particle or even a macroscopic mechanical object. This circumstance is enforced by the presence of quantum noise, in particular manifesting itself as quantum backaction (QBA) noise on a material object when probed, e.g., by light or microwaves. Quantum noise results in fundamental sensitivity limits on a variety of sensing tasks. These limits can be attained by exploiting the concept of a negative-mass reference frame . In this way, the QBA on a macroscopic mechanical oscillator can be evaded using a polarized spin ensemble as the negative-mass auxiliary .
The ability to reduce (or, ideally, completely evade) the quantum noise associated with the probing of a system is key to quantum sensing tasks such as force and position sensing beyond the standard quantum limit , and essential quantum networking protocols such as entanglement generation between remote systems . The latter was realized using the macroscopic spin-optomechanical system in QUANTOP’s Hybrid lab .
The research has particular focus is on the advantages (and challenges) of using hybrid quantum systems, e.g., spin-optomechanical systems where an atomic spin ensemble is combined with either tabletop optomechanical systems (as realized in QUANTOP’s Hybrid lab) or a large-scale gravitational wave detector such as LIGO (proof-of-principle experiment underway in QUANTOP’s Gravitational Wave lab [correct denomination?]). As another example, electro-optomechanical systems offer the capability of quantum transduction  of, e.g., microwave signals to the optical domain via an intermediary mechanical mode . Sensitive optical detection of classical radio-frequency signals using an electro-optomechanical system was demonstrated in QUANTOP’s lab . Quantum transduction is an essential ingredient in various hybrid quantum network protocols , which require bridging the gap between the disparate operating frequencies of its various components.
- Oliver Sandberg (PhD student)
- Emil Zeuthen (Assistant Professor)
- Polzik, E.S. and Hammerer, K., Trajectories without quantum uncertainties. Annalen der Physik 527: A15-A20 (2015). https://doi.org/10.1002/andp.201400099
- Møller et al., Quantum back-action-evading measurement of motion in a negative mass reference frame. Nature 547, 191–195 (2017). https://doi.org/10.1038/nature22980
- Zeuthen et al., Gravitational wave detection beyond the standard quantum limit using a negative-mass spin system and virtual rigidity. Rev. D 100, 062004 (2019). https://doi.org/10.1103/PhysRevD.100.062004
- Xuang et al., Unconditional steady-state entanglement in macroscopic hybrid systems by coherent noise cancellation. Rev. Lett. 121, 103602 (2018). https://doi.org/10.1103/PhysRevLett.121.103602
- Thomas et al, Entanglement between Distant Macroscopic Mechanical and Spin Systems. arXiv:2003.11310 [quant-ph]; to appear in Nature Physics. https://arxiv.org/abs/2003.11310
- Zeuthen et al., Electrooptomechanical equivalent circuits for quantum transduction. Rev. Applied 10, 044036 (2018). https://doi.org/10.1103/PhysRevApplied.10.044036
- Wu et al., Microwave-to-Optical Transduction Using a Mechanical Supermode for Coupling Piezoelectric and Optomechanical Resonators. Rev. Applied 13, 014027 (2020). https://doi.org/10.1103/PhysRevApplied.13.014027
- Bagci et al., Optical detection of radio waves through a nanomechanical transducer. Nature 507, 81–85 (2014). https://doi.org/10.1038/nature13029
- Zeuthen et al., Figures of merit for quantum transducers. Quantum Sci. Technol. 5 034009 (2020). https://doi.org/10.1088/2058-9565/ab8962