Enhancing coherent transport in a photonic network using controllable decoherence
Devon N. Biggerstaff, René Heilmann, Aidan A. Zecevik, Markus Gräfe, Matthew A. Broome, Alessandro Fedrizzi, Stefan Nolte, Alexander Szameit, Andrew G. White, Ivan Kassal
Transport phenomena on a quantum scale appear in a variety of systems, ranging from photosynthetic complexes to engineered quantum devices. It has been predicted that the efficiency of coherent transport can be enhanced through dynamic interaction between the system and a noisy environment. We report an experimental simulation of environment-assisted coherent transport, using an engineered network of laser-written waveguides, with relative energies and inter-waveguide couplings tailored to yield the desired Hamiltonian. Controllable-strength decoherence is simulated by broadening the bandwidth of the input illumination, yielding a significant increase in transport efficiency relative to the narrowband case. We show integrated optics to be suitable for simulating specific target Hamiltonians as well as open quantum systems with controllable loss and decoherence.
Transport phenomena on a quantum scale appear in a variety of systems, ranging from photosynthetic complexes to engineered quantum devices. It has been predicted that the efficiency of coherent transport can be enhanced through dynamic interaction between the system and a noisy environment. We report an experimental simulation of environment-assisted coherent transport, using an engineered network of laser-written waveguides, with relative energies and inter-waveguide couplings tailored to yield the desired Hamiltonian. Controllable-strength decoherence is simulated by broadening the bandwidth of the input illumination, yielding a significant increase in transport efficiency relative to the narrowband case. We show integrated optics to be suitable for simulating specific target Hamiltonians as well as open quantum systems with controllable loss and decoherence.
Abstract Quantum effects in photosynthetic energy transport in nature, especially for the typical Fenna-Matthews-Olson (FMO) complexes, are extensively studied in quantum biology. Such energy transport processes can be investigated as open quantum systems that blend the quantum coherence and environmental noise, and have been experimentally simulated on a few quantum devices. However, the existing experiments always lack a solid quantum simulation for the FMO energy transport due to their constraints to map a variety of issues in actual FMO complexes that have rich biological meanings. Here we successfully map the full coupling profile of the seven-site FMO structure by comprehensive characterisation and precise control of the evanescent coupling of the three-dimensional waveguide array. By applying a stochastic dynamical modulation on each waveguide, we introduce the base site energy and the dephasing term in coloured noise to faithfully simulate the power spectral density of the FMO complexes. We show our photonic model well interprets the phenomena including reorganisation energy, vibrational assistance, exciton transfer and energy localisation. We further experimentally demonstrate the existence of an optimal transport efficiency at certain dephasing strength, providing a window to closely investigate environment-assisted quantum transport.
Quantum information science has the potential to greatly enhance our capabilities for securencommunication through quantum encryption and communication technologies; for measurementnsensitivity through quantum metrology; and for certain computational tasks throughnquantum computers. In particular, simulations of complex quantum systemsmusing eithernquantum computers or other, better-controllable quantum systemsmhave the potential tongreatly improve our understanding of the states and dynamics of processes such as the foldingnof proteins, dynamics in chemistry, and energy transport on molecular and cellular scales.nThis thesis concerns experimental quantum information science, in particular quantum simulations,nconducted using classical and quantum states of light in arrays of coupled laser-writtennwaveguides. Integrated quantum photonics, wherein optical quantum states are manipulatednin waveguide arrays, will be crucial for the miniaturization and ultimate practical scalabilitynof optical quantum simulation and computing. Such waveguide arrays are also particularlynwell-suited for some quantum simulations due to the similarity between the scalar, paraxialnoptical wave equation and Schrodingerrs equation for a bound quantum particle.nFour experiments are described in this thesis where laser-written waveguide arrays arenemployed for optical quantum information science. Two of these concern waveguide opticalnsimulations inspired by exciton transport dynamics in photosynthetic structures. For the firstnof these, we detail the design, modeling, and testing of a waveguide array for analog simulationnof the Hamiltonian governing exciton dynamics in a particular bacterial photosynthetic subunit.nWe ultimately find that such a demanding simulation may exceed current capabilitiesnin the rapidly-maturing field of laser-written waveguides, but the modeling and measurementntechniques developed inform our further experiments. In the other such experiment we studynan important quantum transport phenomenon, environmentally-assisted quantum transportn(ENAQT), which is hypothesized to partially explain high energy transport in photosyntheticnlight-harvesting. We show that the transport efficiency of light in a specific disorderednwaveguide array can be enhanced through the addition of quantum decoherence which wenimplement by broadening the illumination bandwidth.Another experiment concerns the behavior of two-photon states in a continuous-time quantum walk with periodic boundary conditions implemented in an elliptic waveguide array. Wenuse results from classical-light characterization to predict the outcomes of measurements ofntwo-photon correlations in the array. The final experiment in this thesis details the design,nmodeling, fabrication, and testing of laser-written unitary circuits for heralded two-photonnentangling quantum gates. We study the action of such devices in the presence of fabricationnimperfections, characterize our circuits using classical and quantum interference techniques,nand show such circuits to be good candidates for heralded photonic gate fabrication.n
The transport of excitations governs fundamental properties of matter. Particularly rich physics emerges in the interplay between disorder and environmental noise, even in small systems such as photosynthetic biomolecules. Counterintuitively, noise can enhance coherent quantum transport, which has been proposed as a mechanism behind the high transport efficiencies observed in photosynthetic complexes. This effect has been called ``environment-assisted quantum transport''. Here, we propose a quantum simulation of the excitation transport in an open quantum network, taking advantage of the high controllability of current trapped-ion experiments. Our scheme allows for the controlled study of various different aspects of the excitation transfer, ranging from the influence of static disorder and interaction range, over the effect of Markovian and non-Markovian dephasing, to the impact of a continuous insertion of excitations. Our paper discusses experimental error sources and realistic parameters, showing that it can be implemented in state-of-the-art ion-chain experiments.
We study, under very general conditions and in a variety of geometries, quantum enhancement of transport in open systems. Both static disorder and dephasing associated with dynamical disorder (or finite temperature) are fully included in the analysis. We show that quantum coherence effects may significantly enhance transport in open quantum systems even in the semiclassical regime (where the decoherence rate is greater than the inter-site hopping amplitude), as long as the static disorder is sufficiently strong. When the strengths of static and dynamical disorder are fixed, there is an optimal opening strength at which the coherent transport enhancement is optimized. Analytic results are obtained in two simple paradigmatic tight-binding models of large systems: the linear chain and the fully connected network. The physical behavior is also reflected, for example, in the FMO photosynthetic complex, which may be viewed as being intermediate between these paradigmatic models. We furthermore show that a nonzero dephasing rate assists transport in an open linear chain when the disorder strength exceeds a critical value, and obtain this critical disorder strength as a function of the degree of opening.
In work towards the development and creation of practical quantum devices for use in quantum computing and simulation, the importance of long lived coherence is key to the advantages offered over classical systems. Isolation from environmental sources of decoherence is of integral importance to such platforms. Recent experiments of biological systems seem to indicate the existence of certain structures that utilise the decohering effects of their surrounding environments to enhance performance. In this thesis, we present a novel experimental set-up used to simulate the effect of decoherence-enhanced systems by mapping the energy level structure of laser cooled Rubidium 87 atoms interacting with electromagnetic fields to a quantum network of connected sites. Classical noise is controllably added to the states to create a tunable system to explore these effects. We present the design and implementation of the system, with results of several technical aspects, along with initial work on the effect of the applied noise on transport through the network. We present a key result that transport through the network is non-trivially enhanced by the presence of an optimum amplitude of noise, pointing to the interplay between the coherent nature of the quantum system and the decoherence provided by the noise.
We consider the problem of energy transport in a chain of coupled dissipative quantum systems in the presence of non-Markovian dephasing. We use a model of non-Markovianity which is experimentally realizable in the context of controlled quantum systems. We show that non-Markovian dephasing can significantly enhance quantum transport, and we characterize this phenomenon in terms of internal coupling strengths of the chain for some chain lengths. Finally, we show that the phenomenon of dephasing-assisted quantum transport is also enhanced in the non-Markovian scenario when compared to the Markovian case. Our work brings together engineered environments, which are a reality in quantum technologies, and energy transport, which is typically discussed in terms of complex molecular systems. We then expect that it may motivate experimental work and further theoretical investigations on resources which can enhance transport efficiency in a controllable way. This can help in the design of quantum devices with lower dissipation rates, an important concern in any practical application.