All-Optical Noise Spectroscopy of a Solid-State Spin.

Noise spectroscopy elucidates the fundamental noise sources in spin systems, thereby serving as an essential tool toward developing spin qubits with long coherence times for quantum information processing, communication, and sensing. But existing techniques for noise spectroscopy that rely on microwave fields become infeasible when the microwave power is too weak to generate Rabi rotations of the spin. Here, we demonstrate an alternative all-optical approach to performing noise spectroscopy. Our approach utilizes coherent Raman rotations of the spin state with controlled timing and phase to implement Carr-Purcell-Meiboom-Gill pulse sequences. Analyzing the spin dynamics under these sequences enables us to extract the noise spectrum of a dense ensemble of nuclear spins interacting with a single spin in a quantum dot, which has thus far been modeled only theoretically. By providing spectral bandwidths of over 100 MHz, our approach enables studies of spin dynamics and decoherence for a broad range of solid-state spin qubits.

Optical Transparency Induced by a Largely Purcell Enhanced Quantum Dot in a Polarization-Degenerate Cavity.

Optically active spin systems coupled to photonic cavities with high cooperativity can generate strong light-matter interactions, a key ingredient in quantum networks. However, obtaining high cooperativities for quantum information processing often involves the use of photonic crystal cavities that feature a poor optical access from the free space, especially to circularly polarized light required for the coherent control of the spin. Here, we demonstrate coupling with a cooperativity as high as 8 of an InAs/GaAs quantum dot to a fabricated bullseye cavity that provides nearly degenerate and Gaussian polarization modes for efficient optical accessing. We observe spontaneous emission lifetimes of the quantum dot as short as 80 ps (an ∼15 Purcell enhancement) and a ∼80% transparency of light reflected from the cavity. Leveraging the induced transparency for photon switching while coherently controlling the quantum dot spin could contribute to ongoing efforts of establishing quantum networks.

Universality of local weak interactions and its application for interferometric alignment.

The modification of the effect of interactions of a particle as a function of its preselected and postselected states is analyzed theoretically and experimentally. The universality property of this modification in the case of local interactions of a spatially preselected and postselected particle has been found. It allowed us to define an operational approach for the characterization of the presence of a quantum particle in a particular place: the way it modifies the effect of local interactions. The experiment demonstrating this universality property provides an efficient interferometric alignment method, in which the position of the beam on a single detector throughout one phase scan yields all misalignment parameters.