Roland Albrecht

Coupling of a single nitrogen-vacancy center in diamond to a fiber-based microcavity

We report on the coupling of a single nitrogen-vacancy (NV) center in a nanodiamond to a fiber-based microcavity at room temperature. Investigating the very same NV center inside the cavity and in free space allows us to systematically explore a regime of phonon-assisted cavity feeding. Making use of the NV centers strongly broadened emission, we realize a widely tunable, narrow band single photon source. A master equation model well reproduces our experimental results and predicts a transition into a Purcell-enhanced emission regime at low temperatures.

Narrow-band single photon emission at room temperature based on a single nitrogen-vacancy center coupled to an all-fiber-cavity

We report the realization of a device based on a single Nitrogen-vacancy (NV) center in diamond coupled to a fiber-cavity for use as single photon source (SPS). The device consists of two concave mirrors each directly fabricated on the facets of two optical fibers and a preselected nanodiamond containing a single NV center deposited onto one of these mirrors. Both, cavity in- and output are directly fiber-coupled and the emission wavelength is easily tunable by variation of the separation of the two mirrors with a piezo-electric crystal. By coupling to the cavity we achieve an increase of the spectral photon rate density by two orders of magnitude compared to free-space emission of the NV center. With this work we establish a simple all-fiber based SPS with promising prospects for the integration into photonic quantum networks.

Electronic transitions of single silicon vacancy centers in the near-infrared spectral region

Photoluminescence (PL) spectra of single silicon vacancy (SiV) centers in diamond frequently feature very narrow room temperature PL lines in the near-infrared (NIR) spectral region, mostly between 820 nm and 840 nm, in addition to the well known zero-phonon line (ZPL) at approximately 738 nm [E. Neu et al., Phys. Rev. B 84, 205211 (2011)]. We here exemplarily prove for a single SiV center that this NIR PL is due to an additional purely electronic transition (ZPL). For the NIR line at 822.7 nm, we find a room temperature linewidth of 1.4 nm (2.6 meV). The line saturates at similar excitation power as the ZPL. The ZPL and NIR line exhibit identical polarization properties. Cross-correlation measurements between the ZPL and the NIR line reveal anticorrelated emission and prove that the lines originate from a single SiV center, furthermore indicating a fast switching between the transitions (0.7 ns).

Coupling of a single N-V center in diamond to a fiber-based microcavity

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Quantum frequency conversion of visible single photons from a quantum dot to a telecom band

autocorrelation measurements exclude that the NIR line is a vibronic sideband or that it arises due to a transition from/to a metastable (shelving) state.

Towards optimized single photon sources based on color centers in diamond

We here demonstrate coupling of a single N-V center located in a nanodiamond to a fiber-based Fabry-Perot cavity.

Design of photonic crystal microcavities in diamond films for quantum information

The work demonstrates frequency downconversion of single photons from a quantum dot and prove the preservation of the single photon character. The authors were able to translate the frequency of single photons of a semiconductor quantum dot from the red spectral range to the telecom O-band. It was proven that this conversion process leaves the quantum properties of the photons untouched. The high over-all efficiency of 32% for this conversion interface makes it well suited for an application in long-range quantum transmission links.

Visible-to-telecom quantum frequency conversion of light from a single quantum emitter.

In recent years color centers in diamond have attracted significant interest as solid-state single photon sources [1]. Preferable properties for this application are narrow emission bandwidth, high collection efficiency, photostability and operation at room temperature. Single photon emission has been demonstrated with nitrogen-vacancy (NV), nickel-nitrogen (NE8) [1] and silicon-vacancy (SiV) color centers [2]. Further steps towards efficient single photon sources and realization of various quantum information protocols have to include the coupling of single defect centers to cavity structures with small modal volumes and high Q-factors [3].

Quantum frequency down-conversion of single photons from a quantum dot to the telecom band

We here consider photonic crystal (PhC) microcavities in diamond films for QI applications. The PhC structures consist of a thin free-standing diamond membrane (slab) with a periodic array of circular air holes. Cavities are formed by point or line defects in the triangular lattice of air holes. Analogous PhC slab structures have been successfully employed in semiconductor quantum optics yielding strong coupling of single semiconductor quantum dots. For diamond, however, the crucial parameter is the relatively low refractive index of n = 2.4, leading to smaller PhC bandgaps and weaker mode localization. It is thus important to investigate whether one can still realize small mode volume high-Q cavities. Even taking into account experimental imperfections as fabrication tolerances, material absorption and imperfect spatial placement of the emitters, such high Q factors should be sufficient for demonstrations of emitter-cavity coupling and QI applications.