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Quantum trajectories are crucial to understanding the evolution of open systems. We consider an open cavity mode undergoing up and down multistate quantum jumps due to the emission and absorption of photons. We prove that among all subtrajectories, starting simultaneously from different photon number states, only one survives a long single-run evolution. A random Fock state terminating the subtrajectory becomes known for the ergodic case via the key-the processed record of the input and output photocounts, and the trajectory duration. Based on this result, we propose a robust protocol to infer the Fock state, a valuable resource for quantum applications.

Highly sensitive room-temperature vectorial magnetic-field gradiometry is demonstrated using optically detected magnetic resonance (ODMR) in fiber-coupled nitrogen-vacancy (NV) centers in diamond. With a bulk NV-diamond magnetometer coupled to a pair of optical fibers integrated with a microwave transmission line, the differential ODMR measurements are implemented in both space and time, with magnetic-field gradient measurements supplemented with differential ODMR signal detection in the time domain, allowing efficient noise cancellation and providing a sensitivity of magnetogradiometry at the level of 10 -7   nT/(nmHz 1/2 ).

Differential optical detection of a magnetic resonance induced in nitrogen-vacancy (NV) centers in diamond is shown to enable a high-spatial-resolution room-temperature magnetic-field gradiometry on a fiber platform. An ultracompact design of this fiber-based solid-state magnetic gradiometer is achieved by integrating an NV-diamond magnetic sensor with a two-fiber opto-microwave interface, which couples NV centers to microwave and optical fields, used to resonantly drive and interrogate the spin of NV centers.

We demonstrate a scanning fiber-optic probe for magnetic-field imaging where nitrogen-vacancy (NV) centers are coupled to an optical fiber integrated with a two-wire microwave transmission line. The electron spin of NV centers in a diamond microcrystal attached to the tip of the fiber probe is manipulated by a frequency-modulated microwave field and is initialized by laser radiation transmitted through the optical tract of the fiber probe. The two-dimensional profile of the magnetic field is imaged with a high speed and high sensitivity using the photoluminescence spin-readout return from NV centers, captured and delivered by the same optical fiber.

We explore a three-level Lambda scheme to demonstrate that the phenomenon of lasing without inversion (LWI) can be observed in the transient regime. We demonstrate that the pure LWI contribution to the gain of a probe field is distinct from both the resonant absorption (or gain) and the coherent Raman gain (or absorption) by choosing specific initial populations in the dressed-state basis. The suppression of the non-LWI (resonant and Raman) processes is followed by the "rich get richer" (capitalistic) effect for the ground-state population dynamics: Initially, the more populated ground state becomes even more populated. The conditions for the observation of the effect are specified.