A ratiometric luminescence probe for selective detection of Ag+ based on thiolactic acid-capped gold nanoclusters with near-infrared emission and employing bovine serum albumin as a signal amplifier.

When thiolactic acid-capped gold nanoclusters (AuNCs@TLA) with strong near-infrared (NIR, 800 nm) emission were applied to detect metal ions, only Ag+ induced the generation of two new emission peaks at 610 and 670 nm in sequence and quenching the original NIR emission. The new peak at 670 nm generated after the 800-nm emission disappeared utterly. The ratiometric and turn-on responses showed different linear concentration ranges (0.10-4.0 µmol·L-1 and 10-50 µmol·L-1) toward Ag+, and the limit of detection (LOD) was 40 nmol·L-1. Especially, the probe exhibited extremely high selectivity and strong anti-interference from other metal ions. Mechanism studies showed that the novel responses were attributed to the anti-galvanic reaction of AuNCs to Ag+ and formation of bimetallic nanoclusters. The two new emission peaks were due to the composition change and size growth of the metal core. Besides, bovine serum albumin (BSA) has been employed as a signal amplifier based on the assembly-induced emission enhancement properties of AuNCs, which improved the LOD to 10 nmol·L-1. Moreover, the ratiometric method is feasible for Ag+ detection in diluted serum with high recovery rates, showing large application potential in the biological system. The present study supplies a novel ratiometric probe for Ag+ with a two-stage response and provides a novel signal amplifier of BSA, which will facilitate and promote the application of NIR-emitted metal nanoclusters in biological system.

Observation of three-photon bound states in a quantum nonlinear medium.

Bound states of massive particles, such as nuclei, atoms, or molecules, constitute the bulk of the visible world around us. By contrast, photons typically only interact weakly. We report the observation of traveling three-photon bound states in a quantum nonlinear medium where the interactions between photons are mediated by atomic Rydberg states. Photon correlation and conditional phase measurements reveal the distinct bunching and phase features associated with three-photon and two-photon bound states. Such photonic trimers and dimers possess shape-preserving wave functions that depend on the constituent photon number. The observed bunching and strongly nonlinear optical phase are described by an effective field theory of Rydberg-induced photon-photon interactions. These observations demonstrate the ability to realize and control strongly interacting quantum many-body states of light.

Symmetry-protected collisions between strongly interacting photons.

Realizing robust quantum phenomena in strongly interacting systems is one of the central challenges in modern physical science. Approaches ranging from topological protection to quantum error correction are currently being explored across many different experimental platforms, including electrons in condensed-matter systems, trapped atoms and photons. Although photon-photon interactions are typically negligible in conventional optical media, strong interactions between individual photons have recently been engineered in several systems. Here, using coherent coupling between light and Rydberg excitations in an ultracold atomic gas, we demonstrate a controlled and coherent exchange collision between two photons that is accompanied by a π/2 phase shift. The effect is robust in that the value of the phase shift is determined by the interaction symmetry rather than the precise experimental parameters, and in that it occurs under conditions where photon absorption is minimal. The measured phase shift of 0.48(3)π is in excellent agreement with a theoretical model. These observations open a route to realizing robust single-photon switches and all-optical quantum logic gates, and to exploring novel quantum many-body phenomena with strongly interacting photons.

Attractive photons in a quantum nonlinear medium.

The fundamental properties of light derive from its constituent particles--massless quanta (photons) that do not interact with one another. However, it has long been known that the realization of coherent interactions between individual photons, akin to those associated with conventional massive particles, could enable a wide variety of novel scientific and engineering applications. Here we demonstrate a quantum nonlinear medium inside which individual photons travel as massive particles with strong mutual attraction, such that the propagation of photon pairs is dominated by a two-photon bound state. We achieve this through dispersive coupling of light to strongly interacting atoms in highly excited Rydberg states. We measure the dynamical evolution of the two-photon wavefunction using time-resolved quantum state tomography, and demonstrate a conditional phase shift exceeding one radian, resulting in polarization-entangled photon pairs. Particular applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light.

Quantum nonlinear optics with single photons enabled by strongly interacting atoms.

The realization of strong nonlinear interactions between individual light quanta (photons) is a long-standing goal in optical science and engineering, being of both fundamental and technological significance. In conventional optical materials, the nonlinearity at light powers corresponding to single photons is negligibly weak. Here we demonstrate a medium that is nonlinear at the level of individual quanta, exhibiting strong absorption of photon pairs while remaining transparent to single photons. The quantum nonlinearity is obtained by coherently coupling slowly propagating photons to strongly interacting atomic Rydberg states in a cold, dense atomic gas. Our approach paves the way for quantum-by-quantum control of light fields, including single-photon switching, all-optical deterministic quantum logic and the realization of strongly correlated many-body states of light.