Realization of quantum photonic devices requires coupling single quantum emitters to the mode of optical resonators. In this work, a hybrid system consisting of defect centers in few‐layer hexagonal boron nitride (hBN) grown by chemical vapor deposition and a fiber‐based Fabry–Pérot cavity is presented. The sub 10‐nm thickness of hBN and its smooth surface enable efficient integration into the cavity mode. This hybrid platform is operated over a broad spectral range larger than 30 nm and its tuneability is used to explore different coupling regimes. Consequently, very large cavity‐assisted signal enhancement up to 50‐fold and strongly narrowed linewidths are achieved, which is owing to cavity funneling, a record for hBN‐cavity systems. Additionally, an excitation and readout scheme is implemented for resonant excitation that allows to establish cavity‐assisted photoluminescence excitation (PLE) spectroscopy. This work marks an important milestone for the deployment of 2D materials coupled to fiber‐based cavities in practical quantum technologies.

The CNOT gate is asymmetric with respect to parity. It requires interaction with the environment, and cannot be realized as an isolated quantum collision.An elastic collision between spinning particles might be viewed as a kind of logic gate, where the spin directions represent different bit values. Since each particle is a representation of the Poincaré group, characterized by mass, spin, and helicity, it is most natural to classify the various transitions in terms of helicity amplitudes. (1) This formalism provides a completely general description of quantum collisions involving spinning particles.For example, proton-proton scattering pp → pp is routinely analyzed in terms of five independent amplitudes. (1) Using a polarized beam, a polarized target, and polarized detectors, the cross section at a particular scattering angle is given by the square of the helicity amplitude for the polarizations selected. Measurements taken at many different angles could be converted into partial waves, but that is not necessary.Consider the general case AB → CD, where A = C and B = D. For particles with two possible spin states, there are a total of 16 helicity amplitudes. In Table 1, the amplitudes F 1 through F 6 are independent, while the other ten dependent amplitudes are related by mirror symmetry (parity), which flips all the helicities, and by time-reversal symmetry, which reverses the order of the initial and final states.

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If collapsed objects or black holes absorb energy from the vacuum, then their increasing mass will generate a repulsive gravitational force, which might account for the present cosmic acceleration. Direct evidence for vacuum instability should be sought in high-energy particle collisions, where energy conservation has not been seriously tested.