Linear optics underpins tests of fundamental quantum mechanics and computer science, as well as quantum technologies. Here we experimentally demonstrate the longstanding goal of a single reprogrammable optical circuit that is sufficient to implement all possible linear optical protocols up to the size of that circuit. Our six-mode universal system consists of a cascade of 15 MachZehnder interferometers with 30 thermo-optic phase shifters integrated into a single photonic chip that is electrically and optically interfaced for arbitrary setting of all phase shifters, input of up to six photons and their measurement with a 12 single-photon detector system. We programmed this system to implement heralded quantum logic and entangling gates, boson sampling with verification tests, and six-dimensional complex Hadamards. We implemented 100 Haar random unitaries with average fidelity 0.999 ± 0.001. Our system is capable of switching between these and any other linear optical protocol in seconds. These results point the way to applications across fundamental science and quantum technologies.Photonics has been crucial in establishing the foundations of quantum mechanics [1], and more recently has pushed the vanguard of efforts in understanding new non-classical computational possibilities. Typical protocols involve nonlinear operations, such as the generation of quantum states of light through optical frequency conversion [2,3], or measurement-induced nonlinearities for quantum logic gates [4], together with linear operations between optical modes to implement core processing functions [5]. Encoding qubits in the polarisation of photons has been particularly appealing for the ability to implement arbitrary linear operations on the two polarisation modes using a series of wave plates [6]. For path encoding the same operations can be mapped to a sequence of beamsplitters and phase shifters. In fact, since any linear optical (LO) circuit is described by a unitary operator, and a specific array of basic two-mode operations is mathematically sufficient to implement any unitary operator on optical modes [7], it is theoretically possible to construct a single device with sufficient versatility to implement any possible LO operation up to the specified number of modes.Here we report the realisation of this longstanding goal with a six-mode device that is completely reprogrammable and universal for LO. We demonstrate the versatility of this universal LO processor (LPU) by applying it to several quantum information protocols, including tasks that were previously not possible. We im- * anthony.laing@bristol.ac.uk plement heralded quantum logic gates at the heart of the circuit model of LO quantum computing [4] and new heralded entangling gates that underpin the measurementbased model of LO quantum computing [8][9][10], both of which are the first of their kind in integrated photonics. We perform 100 different boson sampling [11][12][13][14][15] experiments and simultaneously realise new verification protocols. Finally, we use multi-p...

Unlike most previously reported kelp forests, which exhibit long-term persistence over large spatial scales, the kelp forest community on the west end of San Nicolas Island, California, occurs as a dynamic patchwork of barren areas characterized by grazing sea urchins and an algal assemblage consisting of upright and encrusting coralline algae, and kelp-dominated areas characterized by high densities of perennial brown algae, including the giant kelp Macrocystis pyrifera. In this study we examine the factors that regulate the grazing activity of sea urchins (Strongylocentrotusfranciscanus), which in tum determines the structure of this community. In the barren area, drift algae were sparse, sea urchins were poorly nourished, occupied open, unprotected microhabitats, and actively grazed the substratum. In the kelp-dominated area, drift algae were abundant, sea urchins were well nourished, moved little, occupied cracks and crevices, and probably fed on drift algae. Early in the study substantial recruitment of brown macroalgae occurred in both sites, and the barren area gradually transformed into a kelp-dominated area. Concomitant with this change, the abundance of drift algae in the barren area increased. Urchins in this area abandoned open microhabitats for protected crevices and pockets, and sea urchin grazing intensity was reduced to levels characteristic of the kelp-dominated area. From these observations we present a qualitative model in which the transformation from one configuration to another is triggered by a behavioral switch in the mode of feeding of red sea urchins. This switch is controlled by the availability of drift algae, which in tum is dependent upon the abundance of attached macroalgae. The adult algal standing stocks appear to be regulated by prevailing hydrographic conditions, which are either favorable or unfavorable for kelp recruitment, survivorship, and growth. In this model grazing intensity is independent of urchin density and therefore provides a new view of how kelp forest communities function.

Advances in control techniques for vibrational quantum states in molecules present new challenges for modelling such systems, which could be amenable to quantum simulation methods. Here, by exploiting a natural mapping between vibrations in molecules and photons in waveguides, we demonstrate a reprogrammable photonic chip as a versatile simulation platform for a range of quantum dynamic behaviour in different molecules. We begin by simulating the time evolution of vibrational excitations in the harmonic approximation for several four-atom molecules, including HCS, SO, HNCO, HFHF, N and P. We then simulate coherent and dephased energy transport in the simplest model of the peptide bond in proteins-N-methylacetamide-and simulate thermal relaxation and the effect of anharmonicities in HO. Finally, we use multi-photon statistics with a feedback control algorithm to iteratively identify quantum states that increase a particular dissociation pathway of NH. These methods point to powerful new simulation tools for molecular quantum dynamics and the field of femtochemistry.

Sporophyte recrultment, holclfast growth and mortality of giant kelp Macrocyst~spynfera were measured seasonally on permanent transects at 3 sltes [protected Intermediate and exposed) along a wave exposure gradient on the Monterey Peninsula, c e n t~a l Cal~fornia (USA) between 1988 and 1991 The constant presence of cold, nutnent-nch water and the relative absence of o t h e~ kelps and large grazers allowed the dynanucs of M pynfera populations to be examined under conditions in which wave exposure was highly vanable and ~nfluences of other abiotlc and biotic factors were minimized Recovery of M pynfera populations fiom decreased adult density (presumably due to stormlnduced mortality, adult density was negatively correlated with storm activity) was a 2-stage process requiring the establishment of luvenile populations and condit~ons suitable for ]uvenile growth to adult size Sporophyte recrultment was negatively correlated with M pynfera canopy cover, and thus appeared to be related to irradiance Recruitment was low and continuous under a temporally stable M pyrifera canopy at the protected slte At the intermediate and exposed sites, canopy cover was more variable, canopy loss was greater, and durdt~ons of low canopy cover were longer than at the protected site resulting in episodic sporophyte recruitment These distinct patterns in sporophyte lecruitment resulted In continuous luvenile populations at the protected site and intermittent luvenlle populations at the intermediate and exposed sites Growth of luveniles to adult size required additional irrad~ance probably d u e to gleater light requirements for luvenlle growth than for sporophyte recrultment We observed that luveniles grew to adult slze when canopy cover was low and adults were below a threshold denslty estimated at -10 plants 100 m 2 , but ~uveniles also occasionally grew to adult size following periods of low canopy cover only Ep~sodic sporophyte recruitment at the intermediate and exposed sites resulted in decreased coincidence of the 2 recovery stages, adult dens~ties were often decreased below threshold piior to the establishment of luvenile populations Recovery time, that required by populations to return to densities above threshold was equal to the time lag between occurrence of the 2 recovery stages and was therefore greatest at the more exposed sites Compansons between central and southern Cal~tornia M pyl~fera populat~ons suggest that by altering recovery time variable frequency and magnitude of storm disturbance may result in different periodlclties of adult population cycles K E Y WORDS. Glant kelp. Macrocyst~spyr~fera . Wave exposure. Demography. Recruitment. Recovery

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Remotely operated vehicle (ROV) surveys were conducted over a 3-year period to evaluate the importance of macroalgal drift exported from nearshore forests of giant kelp (Macrocystis pyrifera) to adjacent submarine-canyon and continental-shelf benthic habitats. Abundant macroalgal drift was found in the Carmel Submarine Canyon (153-454-m depth) but was rare at Pt. Joe, a continental shelf habitat 9 km away (87-357-m depth). Most of the drift parcels consisted of brown algae, but 10 other categories were found, including green and red algae and the surfgrass Phyllospadix spp. M. pyrifera drift accounted for as much as 20% of the density of drift parcels in the Carmel Submarine Canyon, and up to 50% of the drift parcels at Pt. Joe. From previously determined turnover rates of drift M. pyrifera and the standing stock estimates from the present study, we estimated that the rate of carbon flux associated with drift M. pyrifera to the benthos of the Cannel Submarine Canyon was 45.2 mg C m z d I. Based on published estimates of carbon flux from vertically sinking particulate organic carbon at 400-m depth in the nearby Monterey Submarine Canyon, and our estimates of kelp-derived carbon flux, drift M. pyrifera can account for 20-83% of the total particulate organic carbon reaching the sea floor in the Carmel Submarine Canyon. We conclude that drift macrophytes provide significant enrichment of organic carbon to the benthos above that provided by vertically sinking particulate organic material. The extent to which this alters the structure of benthic communities compared to areas without organic enrichment by drift macrophytes remains unknown and awaits further study.

In January 1986, a sea-urchm mediated deforestation began in C a m e l Bay, California. USA, near the center of the California sea otter's range. The deforestation occurred on a n offshore rocky reef where depths range from l ? to > 30 m. Both purple (Strongylocentrotus purpuratus) and red (S. franciscanus) sea urchins were abundant and probably belonged to a cohort that settled in 1984. By October 1986, nearly all non-crustose algae had been removed, as were most sessile invertebrates (excluding the cnidarians Balanophyllia elegans and Corynactis californica). Concurrent samples taken from 2 nearby forested sites at comparable depths showed no evidence of overgrazing by urchins. Populations of red sea urchins at the deforested site remained fairly stable, but purple sea urchins declined to zero by July 1989. Algal spores were available during the deforestation, but recruitment of seaweeds d d not occur until after sea urchln abundance had decllned Reasons for the decline of the purple sea urchin population are not clear, although predation by sea otters 1s unlikely. Circumstances surrounding these events indicate kelp forest deforestations may b e controlled not only by mortality of adult sea urchins (from predation or disease) and fluctuation of their food supply, but also by factors affecting recruitment such a s larval abundance and early post-settlement sunrival.

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