Coupling single molecule magnets to quantum circuits

In this work we study theoretically the coupling of single molecule magnets (SMMs) to a variety of quantum circuits, including microwave resonators with and without constrictions and flux qubits. The main results of this study is that it is possible to achieve strong and ultrastrong coupling regimes between SMM crystals and the superconducting circuit, with strong hints that such a coupling could also be reached for individual molecules close to constrictions. Building on the resulting coupling strengths and the typical coherence times of these molecules (of the order of microseconds), we conclude that SMMs can be used for coherent storage and manipulation of quantum information, either in the context of quantum computing or in quantum simulations. Throughout the work we also discuss in detail the family of molecules that are most suitable for such operations, based not only on the coupling strength, but also on the typical energy gaps and the simplicity with which they can be tuned and oriented. Finally, we also discuss practical advantages of SMMs, such as the possibility to fabricate the SMMs ensembles on the chip through the deposition of small droplets.Comment: 23 pages, 12 figure

Weak and Strong coupling regimes in plasmonic-QED

We present a quantum theory for the interaction of a two level emitter with surface plasmon polaritons confined in single-mode waveguide resonators. Based on the Green's function approach, we develop the conditions for the weak and strong coupling regimes by taking into account the sources of dissipation and decoherence: radiative and non-radiative decays, internal loss processes in the emitter, as well as propagation and leakage losses of the plasmons in the resonator. The theory is supported by numerical calculations for several quantum emitters, GaAs and CdSe quantum dots and NV centers together with different types of resonators constructed of hybrid, cylindrical or wedge waveguides. We further study the role of temperature and resonator length. Assuming realistic leakage rates, we find the existence of an optimal length at which strong coupling is possible. Our calculations show that the strong coupling regime in plasmonic resonators is accessible within current technology when working at very low temperatures (<4K). In the weak coupling regime our theory accounts for recent experimental results. By further optimization we find highly enhanced spontaneous emission with Purcell factors over 1000 at room temperature for NV-centers. We finally discuss more applications for quantum nonlinear optics and plasmon-plasmon interactions.Comment: published as Phys. Rev. B 87, 115419 (2013

Neutrino Decays over Cosmological Distances and the Implications for Neutrino Telescopes

We discuss decays of ultra-relativistic neutrinos over cosmological distances by solving the decay equation in terms of its redshift dependence. We demonstrate that there are significant conceptual differences compared to more simplified treatments of neutrino decay. For instance, the maximum distance the neutrinos have traveled is limited by the Hubble length, which means that the common belief that longer neutrino lifetimes can be probed by longer distances does not apply. As a consequence, the neutrino lifetime limit from supernova 1987A cannot be exceeded by high-energy astrophysical neutrinos. We discuss the implications for neutrino spectra and flavor ratios from gamma-ray bursts as one example of extragalactic sources, using up-to-date neutrino flux predictions. If the observation of SN 1987A implies that \nu_1 is stable and the other mass eigenstates decay with rates much smaller than their current bounds, the muon track rate can be substantially suppressed compared to the cascade rate in the region IceCube is most sensitive to. In this scenario, no gamma-ray burst neutrinos may be found using muon tracks even with the full scale experiment, whereas reliable information on high-energy astrophysical sources can only be obtained from cascade measurements. As another consequence, the recently observed two cascade event candidates at PeV energies will not be accompanied by corresponding muon tracks.Comment: 20 pages, 6 figures, 1 table. Matches published versio

Cdk1 inactivation terminates mitotic checkpoint surveillance and stabilizes kinetochore attachments in anaphase

Two mechanisms safeguard the bipolar attachment of chromosomes in mitosis. A correction mechanism destabilizes erroneous attachments that do not generate tension across sister kinetochores [1]. In response to unattached kinetochores, the mitotic checkpoint delays anaphase onset by inhibiting the anaphase-promoting complex/cyclosome (APC/CCdc20) [2]. Upon satisfaction of both pathways, the APC/CCdc20 elicits the degradation of securin and cyclin B [3]. This liberates separase triggering sister chromatid disjunction and inactivates cyclin-dependent kinase 1 (Cdk1) causing mitotic exit. How eukaryotic cells avoid the engagement of attachment monitoring mechanisms when sister chromatids split and tension is lost at anaphase is poorly understood [4]. Here we show that Cdk1 inactivation disables mitotic checkpoint surveillance at anaphase onset in human cells. Preventing cyclin B1 proteolysis at the time of sister chromatid disjunction destabilizes kinetochore-microtubule attachments and triggers the engagement of the mitotic checkpoint. As a consequence, mitotic checkpoint proteins accumulate at anaphase kinetochores, the APC/CCdc20 is inhibited, and securin reaccumulates. Conversely, acute pharmacological inhibition of Cdk1 abrogates the engagement and maintenance of the mitotic checkpoint upon microtubule depolymerization. We propose that the simultaneous destruction of securin and cyclin B elicited by the APC/CCdc20 couples chromosome segregation to the dissolution of attachment monitoring mechanisms during mitotic exit

Cavity-enhanced Raman microscopy of individual carbon nanotubes

Raman spectroscopy reveals chemically specific information and provides label-free insight into the molecular world. However, the signals are intrinsically weak and call for enhancement techniques. Here, we demonstrate Purcell enhancement of Raman scattering in a tunable high-finesse microcavity, and utilize it for molecular diagnostics by combined Raman and absorption imaging. Studying individual single-wall carbon nanotubes, we identify crucial structural parameters such as nanotube radius, electronic structure and extinction cross-section. We observe a 320-times enhanced Raman scattering spectral density and an effective Purcell factor of 6.2, together with a collection efficiency of 60%. Potential for significantly higher enhancement, quantitative signals, inherent spectral filtering and absence of intrinsic background in cavity-vacuum stimulated Raman scattering render the technique a promising tool for molecular imaging. Furthermore, cavity-enhanced Raman transitions involving localized excitons could potentially be used for gaining quantum control over nanomechanical motion and open a route for molecular cavity optomechanics

Transverse-mode coupling and diffraction loss in tunable Fabry-Pé rot microcavities

Wereport on measurements and modeling of the mode structure of tunable Fabry–Pérot optical microcavities with imperfect mirrors.Wefind that non-spherical mirror shape and finite mirror size leave the fundamental mode mostly unaffected, but lead to loss, mode deformation, and shifted resonance frequencies at particular mirror separations. For small mirror diameters, the useful cavity length is limited to values significantly below the expected stability range.Weexplain the observations by resonant coupling between different transverse modes of the cavity and mode-dependent diffraction loss. A model based on resonant state expansion that takes into account the measured mirror profile can reproduce the measurements and identify the parameter regime where detrimental effects of mode mixing are avoided

Beyond the Jaynes-Cummings model: circuit QED in the ultrastrong coupling regime

In cavity quantum electrodynamics (QED), light-matter interaction is probed at its most fundamental level, where individual atoms are coupled to single photons stored in three-dimensional cavities. This unique possibility to experimentally explore the foundations of quantum physics has greatly evolved with the advent of circuit QED, where on-chip superconducting qubits and oscillators play the roles of two-level atoms and cavities, respectively. In the strong coupling limit, atom and cavity can exchange a photon frequently before coherence is lost. This important regime has been reached both in cavity and circuit QED, but the design flexibility and engineering potential of the latter allowed for increasing the ratio between the atom-cavity coupling rate and the cavity transition frequency above the percent level. While these experiments are well described by the renowned Jaynes-Cummings model, novel physics is expected in the ultrastrong coupling limit. Here, we report on the first experimental realization of a superconducting circuit QED system in the ultrastrong coupling limit and present direct evidence for the breakdown of the Jaynes-Cummings model.Comment: 5 pages, 3 figure