Coherent Quantum Emitters in Hexagonal Boron Nitride

Abstract

Hexagonal boron nitride is an emerging 2D material with far-reaching applications in fields like nanophotonics or nanomechanics. Its layered architecture plays a key role for new materials such as Van der Waals heterostructures. The layered structure has also unique implications for hosted, optically active defect centers. A very special type of defect center arises from the possibility to host mechanically isolated orbitals localized between the layers. The resulting absence of coupling to low-frequency acoustic phonons turns out to be the essential element to protect the coherence of optical transitions from mechanical interactions with the environment. Consequently, the spectral transition linewidth remains unusually narrow even at room temperature, thus paving a new way toward coherent quantum optics under ambient conditions. In this review, the state-of-the-art of defect centers in hexagonal boron nitride is summarized with a focus on optically coherent defect centers. The current understanding of the defect centers, remaining questions, and potential research directions to overcome pervasive challenges are discussed. The field is put into a broad perspective with impact on quantum technology such as quantum optics, quantum photonics, as well as spin optomechanics.

Unveiling the Zero-Phonon Line of the Boron Vacancy Center by Cavity-Enhanced Emission

“…Negatively charged boron vacancies ( V B – ) in hexagonal boron nitride (hBN) are novel quantum emitters that currently attract broad interest due to their spin-dependent optical properties, even at room temperature. − The electronic states of V B – stagger in hBN coupled strongly to local vibrational modes. − Combined with singlet and triplet electronic subsystems that are coupled via spin–orbit interaction and mix via Jahn–Teller effects, this results in a broad and largely featureless emission spectrum that complicates comparison of experimental results with theoretical calculations. − Typically, the photoluminescence (PL) spectrum of ensembles of V B – stagger exhibit a broad and featureless spectrum dominated by phonon-assisted emissions peaked around 800 nm − even at liquid helium temperature . Therefore, it is difficult to unambiguously identify the zero-phonon line (ZPL) of V B – from transitions involving vibrational modes of the surrounding hBN matrix, in contrast to the nitrogen vacancies where the ZPL peak can be directly distinguished in the PL spectra .…”

“…Typical results are presented in Figure c that shows the typical broad photoluminescence recorded at room temperature with a maximum around 800 nm. The inset of Figure c shows a typical continuous wave ODMR spectrum exhibiting two characteristic transitions in a zero magnetic field, arising from redistribution of spin among the triplet m s = 0 and ±1 triplet ground state manifold of V B – . − …”

Plasmon-Triggered Ultrafast Operation of Color Centers in Hexagonal Boron Nitride Layers

“…hBN is an ultrawide-band-gap (∼6 eV) material , that can host a myriad of emission centers with different resonance wavelengths. − The exact nature and properties of the various defects are under theoretical investigation. , Emission centers in hBN have presented a plethora of phenomena in quantum optics, among them single-photon emission , with high quantum efficiency, and Rabi oscillations probed by the phonon sideband emission . Moreover, controlled placement of CCs in hBN layers at specific positions has been demonstrated …”

Silicon Nitride Waveguides with Intrinsic Single-Photon Emitters for Integrated Quantum Photonics

“…The possibility to integrate quantum emitters with QPICs depends critically on the properties of the host materials. Well-established solid-state quantum emitters include III–V semiconductor quantum dots (QDs) ,, and defect-based color centers in diamond, − silicon carbide (SiC), − and hexagonal boron nitride (hBN), − to name a few. The high brightness, single-photon purity, indistinguishability, and optically addressable spins of these quantum emitters make them promising for applications in quantum communication, computing, and sensing.…”