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Control over quantum dynamics of open systems is one of the central challenges in quantum science and engineering. Coherent optical techniques, such as coherent population trapping involving dark resonances (1,2), are widely used to control quantum states of isolated atoms and ions. In conjunction with spontaneous emission, they allow for laser cooling of atomic motion (3), preparation and manipulation of atomic states (4), and rapid quantum optical measurements that are essential for applications in metrology (5-7). Here we show that these techniques can be applied to monitor and control individual atom-like impurities, and their local environment (8-11), in the solid state. Using all-optical manipulation of the electronic spin of an individual nitrogen-vacancy colour centre in diamond, we demonstrate optical cooling, real-time measurement and conditional preparation of its nuclear spin environment by post-selection. These methods offer potential applications ranging from all-optical nanomagnetometry to quantum feedback control of solid-state qubits, and may lead to new approaches for quantum information storage and processing, Over the past two decades, coherent population trapping (CPT) has been used in the laser cooling of neutral atoms and ions (3), the creation of ultracold molecules (4), optical magnetometry [...]

Quantum entanglement is among the most fascinating aspects of quantum theory (1). Entangled optical photons are now widely used for fundamental tests of quantum mechanics (2) and applications such as quantum cryptography (1). Several recent experiments demonstrated entanglement of optical photons with trapped ions (3), atoms (4,5) and atomic ensembles (6-8), which are then used to connect remote long-term memory nodes in distributed quantum networks (9-11). Here we realize quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond. Our experimental entanglement verification uses the quantum eraser technique (5,12), and demonstrates that a high degree of control over interactions between a solid-state qubit and the quantum light field can be achieved. The reported entanglement source can be used in studies of fundamental quantum phenomena and provides a key building block for the solid-state realization of quantum optical networks (13,14)., A quantum network (13) consists of several nodes, each containing a long-lived quantum memory and a small quantum processor, that are connected via entanglement. Its potential applications include long-distance quantum [...]

Detection of weak magnetic fields with nanoscale spatial resolution is an outstanding problem in the biological and physical sciences (1-5). For example, at a distance of 10 run, the spin [...]

We experimentally demonstrate a dipolar polariton based electric-field sensor. We tune and optimize the sensitivity of the sensor by varying the dipole moment of polaritons. We show polariton interactions play an important role in determining the conditions for optimal electric-field sensing, and achieve a sensitivity of 0.12   V m^{-1} Hz^{-0.5}. Finally, we apply the sensor to illustrate that excitation of polaritons modifies the electric field in a spatial region much larger than the optical excitation spot.

Nonperturbative coupling between cavity photons and excitons leads to the formation of hybrid light-matter excitations, termed polaritons. In structures where photon absorption leads to the creation of excitons with aligned permanent dipoles, the elementary excitations, termed dipolar polaritons, are expected to exhibit enhanced interactions. Here, we report a substantial increase in interaction strength between dipolar polaritons as the size of the dipole is increased by tuning the applied gate voltage. To this end, we use coupled quantum well structures embedded inside a microcavity where coherent electron tunneling between the wells creates the excitonic dipole. Modifications of the interaction strength are characterized by measuring the changes in the reflected light intensity when polaritons are driven with a resonant laser. The factor of 6.5 increase in the interaction-strength-to-linewidth ratio that we obtain indicates that dipolar polaritons could constitute an important step towards a demonstration of the polariton blockade effect, and thereby to form the building blocks of many-body states of light.

Neutral particles subject to artificial gauge potentials can behave as charged particles in magnetic fields. This fascinating premise has led to demonstrations of one-way waveguides, topologically protected edge states and Landau levels for photons. In ultracold neutral atoms, effective gauge fields have allowed the emulation of matter under strong magnetic fields leading to realization of Harper-Hofstadter and Haldane models. Here we show that application of perpendicular electric and magnetic fields effects a tunable artificial gauge potential for two-dimensional microcavity exciton polaritons. For verification, we perform interferometric measurements of the associated phase accumulated during coherent polariton transport. Since the gauge potential originates from the magnetoelectric Stark effect, it can be realized for photons strongly coupled to excitations in any polarizable medium. Together with strong polariton-polariton interactions and engineered polariton lattices, artificial gauge fields could play a key role in investigation of non-equilibrium dynamics of strongly correlated photons.