Deterministic coupling of delta-doped NV centers to a nanobeam photonic crystal cavity

The negatively-charged nitrogen vacancy center (NV) in diamond has generated significant interest as a platform for quantum information processing and sensing in the solid state. For most applications, high quality optical cavities are required to enhance the NV zero-phonon line (ZPL) emission. An outstanding challenge in maximizing the degree of NV-cavity coupling is the deterministic placement of NVs within the cavity. Here, we report photonic crystal nanobeam cavities coupled to NVs incorporated by a delta-doping technique that allows nanometer-scale vertical positioning of the emitters. We demonstrate cavities with Q up to ~24,000 and mode volume V ~ $0.47({\lambda}/n)^{3}$ as well as resonant enhancement of the ZPL of an NV ensemble with Purcell factor of ~20. Our fabrication technique provides a first step towards deterministic NV-cavity coupling using spatial control of the emitters.Comment: 13 pages, 3 figure

Quantum Metrology with Spin Defects in Nanodiamonds

The negatively charged nitrogen-vacancy center in diamond has become a prominent room temperature spin qubit platform for quantum information and sensing applications owing to its inherent optical addressability and long spin coherence times. In particular, a major research effort has focused on exploiting the metrology capabilities of the nitrogen-vacancy center to investigate the details of biological and magnetic systems at the nanoscale using diamond micro- and nanoparticles (we will mostly refer to these as nanoparticles in what follows). These particles behave like highly localized, low-thermal-mass spin systems and can therefore be used for high spatial resolution, high sensitivity mapping of their environment. Nonetheless, some outstanding challenges limit the sensing capabilities of these nanosensors. On the one hand, the nitrogen-vacancy centers contained in nanoparticles present strongly degraded coherence properties compared to what is common in bulk diamond due to a higher density of structural defects and impurities. On the other hand, in order to investigate systems that produce a signal that decay fast with distance (e.g. single or small ensembles of spins), the qubit needs to be placed within a few nanometer of the diamond surface. Because of the presence of magnetic and electric noise resulting from electronic spin and charges at the interface, the coherence of the qubits is further diminished. As the nitrogen-vacancy center’s coherence properties ultimately limit the achievable sensitivity, improving thse properties has been a central research focus in the scientific community. In this thesis two main approaches to extend the capabilities of diamond nanoparticles as nanoscale sensors are investigated. Firstly, we discuss a fabrication process developed to produce nanodiamonds with controlled crystal properties, shape, size, and nitrogen-vacancy center placement and density. We show that these accomplishments are conducive to enhanced photoluminescence properties and bulk-like spin coherence times. We additionally demonstrate that, thanks to their precisely controlled geometry, the nanoparticles can be optically trapped within a microfluidic system and behave as highly stable, contactless nanoprobes in solution. In a separate effort, we study hybrid quantum architectures that promise to improve the sensitivity of the nitrogen-vacancy centers by enabling the amplification of the signal to be measured. In this way, the requirements on the position of the qubits with respect to the diamond’s interface can be relaxed. In particular, we first develop a technique to create portable arrays of nanoparticles embedded within a flexible and transparent matrix that can be placed in contact with a sample and provide two-dimensional mapping capabilities. We then use this sensing platform to create a ferromagnet-nanodiamond hybrid system to study the interactions between collective spin excitations (spin waves) in the ferromagnet and ensembles of nitrogen-vacancy centers in the nanodiamonds. We show that surface confined spin waves interact strongly with the qubits’ spins and can mediate and enhance their coherent magnetic interactions with a microwave source. These results pave the way to the use of spin wave mediated coupling as a way to improve the sensitivity of the nitrogen-vacancy centers both in nanoparticles and bulk material

Research Data Supporting: Ultrafast melting and recovery of collective order in the excitonic insulator Ta2NiSe5

This data includes that which is shown in the main text figures 1-4 of the associated publication. We have used pump-probe measurements to investigate the ordered phase in excitonic insulator candidate, Ta2NiSe5. The measurements are performed with a single pump, and two pump configuration (also sometimes called pump-push-probe). While all the data shown is taken in reflection, we did similar measurements also in a transmission configuration

Ultrafast melting and recovery of collective order in the excitonic insulator Ta2NiSe5

AbstractThe layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 and demonstrate that the transient reflectivity in the near-infrared range is connected to the system’s low-energy physics. We track the status of the ordered phase using this optical signature, establishing that high-fluence photoexcitations can suppress this order. From the sub-50 fs quenching timescale and the behaviour of the photoinduced coherent phonon modes, we conclude that electronic correlations provide a decisive contribution to the excitonic order formation. Our results pave the way towards the ultrafast control of an exciton condensate at room temperature.We are also grateful to the Engineering and Physical Science Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability for funding. We acknowledge the financial support from the Department of Science and Technology (DST), India [Grant No. SR/WOS-A/PM-33/2018 (G)] and IISER Pune for providing the facilities for crystal growth and characterization. We acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement 758826). We thank the Department of Science and Technology, India for support under Nanoemission and Year of Science Professorship

Research data supporting ''Imaging the coherent propagation of collective modes in the excitonic insulator Ta2NiSe5 at room temperature''

Using a widefield pump-probe microscope (with ~10 nm spatial precision and ~10 fs time resolution) we probe the temperature and fluence dependent dynamics of the collective modes in excitonic insulator candidate Ta2NiSe5. We pump with a broadband pulse centered around 500 nm and probe ~800 nm using a bandpass filter.The calculations were run on the Beo05 cluster at the University of Fribourg. We acknowledge the CECAM workshop “Excitonic insulator: New perspectives in long-range interacting systems” at EPFL Lausanne for insights and discussions and for providing the opportunity to start this collaboration. We also thank A. Boris (Max Planck Institute for Solid State Research) and A. Musser (Cornell University) for helpful discussions. Funding: H.M.B., P.A., and A.R. acknowledge support from the Winton Programme for the Physics of Sustainability, the Engineering and Physical Sciences Research Council, and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement 758826). N.R.C. acknowledges funding from the Engineering and Physical Sciences Research Council (EPSRC grant no. EP/P034616/1). Y.M. thanks the Japan Society for the Promotion of Science and the Japan Science and Technology Agency for funding through KAKENHI grant no. JP19K23425 and JST CREST grant no. JPMJCR1901. P.W. acknowledges funding from the ERC Consolidator grant no. 724103 and from the Swiss National Science Foundation via NCCR Marvel. A.K.S. thanks the Department of Science and Technology, India for support under Nanomission and Year of Science Professorship. L.H. acknowledges the financial support from the Department of Science and Technology (DST), India [grant no. SR/WOS-A/PM-33/2018 (G)] and IISER Pune for providing the facilities for crystal growth and characterization. B.R. acknowledges support from the Cambridge International Trust and Wolfson College, Cambridge. D.G. is supported by the Slovenian Research Agency (ARRS) under program nos. P1-0044 and J1-2455. N.R.C., A.J.M., and D.G. acknowledge support from the Simons Foundation

Imaging the coherent propagation of collective modes in the excitonic insulator Ta₂NiSe₅ at room temperature.

Excitonic insulators host a condensate of electron-hole pairs at equilibrium, giving rise to collective many-body effects. Although several materials have emerged as excitonic insulator candidates, evidence of long-range coherence is lacking and the origin of the ordered phase in these systems remains controversial. Here, using ultrafast pump-probe microscopy, we investigate the possible excitonic insulator Ta2NiSe5 Below 328 K, we observe the anomalous micrometer-scale propagation of coherent modes at velocities of ~105 m/s, which we attribute to the hybridization between phonon modes and the phase mode of the condensate. We develop a theoretical framework to support this explanation and propose that electronic interactions provide a substantial contribution to the ordered phase in Ta2NiSe5 These results allow us to understand how the condensate's collective modes transport energy and interact with other degrees of freedom. Our study provides a unique paradigm for the investigation and manipulation of these properties in strongly correlated materials