Jacques Carolan

Universal linear optics

Complex quantum optical circuitry Encoding and manipulating information in the states of single photons provides a potential platform for quantum computing and communication. Carolan et al. developed a reconfigurable integrated waveguide device fabricated in a glass chip (see the Perspective by Rohde and Dowling). The device allowed for universal linear optics transformations on six wave-guides using 15 integrated Mach-Zehnder interferometers, each of which was individually programmable. Functional performance in a number of applications in optics and quantum optics demonstrates the versatility of the devices reprogrammable architecture. Science, this issue p. 711; see also p. 696 A reconfigurable optical circuit provides a platform for a photonically-based quantum computer. [Also see Perspective by Rohde and Dowling] Linear optics underpins fundamental tests of quantum mechanics and quantum technologies. We demonstrate 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 Mach-Zehnder 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 an average fidelity of 0.999 ± 0.001. Our system can be rapidly reprogrammed to implement these and any other linear optical protocol, pointing the way to applications across fundamental science and quantum technologies.

On the experimental verification of quantum complexity in linear optics

Scalable methods employing a random unitary chip and a quantum walk chip are developed to experimentally verify correct operation for large-scale boson sampling. Experimental analysis reveals that the resulting statistics of the output of a linear interferometer fed by indistinguishable single-photon states exhibits true non-classical characteristics.

Relative multiplexing for minimising switching in linear-optical quantum computing

Many existing schemes for linear-optical quantum computing (LOQC) depend on multiplexing (MUX), which uses dynamic routing to enable near-deterministic gates and sources to be constructed using heralded, probabilistic primitives. MUXing accounts for the overwhelming majority of active switching demands in current LOQC architectures. In this manuscript we introduce relative multiplexing (RMUX), a general-purpose optimisation which can dramatically reduce the active switching requirements for MUX in LOQC, and thereby reduce hardware complexity and energy consumption, as well as relaxing demands on performance for various photonic components. We discuss the application of RMUX to the generation of entangled states from probabilistic single-photon sources, and argue that an order of magnitude improvement in the rate of generation of Bell states can be achieved. In addition, we apply RMUX to the proposal for percolation of a 3D cluster state by Gimeno-Segovia et al (2015 Phys. Rev. Lett. 115 020502), and we find that RMUX allows an 2.4× increase in loss tolerance for this architecture.

Simulating the vibrational quantum dynamics of molecules using photonics

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 H2CS, SO3, HNCO, HFHF, N4 and P4. 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 H2O. Finally, we use multi-photon statistics with a feedback control algorithm to iteratively identify quantum states that increase a particular dissociation pathway of NH3. These methods point to powerful new simulation tools for molecular quantum dynamics and the field of femtochemistry.By mapping vibrations in molecules to photons in waveguides, the vibrational quantum dynamics of various molecules are simulated using a photonic chip.

Verifying Quantum Complexity in Linear Optical Experiments

We develop techniques to verify the computational complexity of a class of analogue quantum computers known as boson samplers. We demonstrate these techniques with up to 5 photons in two different types of integrated linear optical circuit, observing Hilbert spaces of up to 50,000 dimensions.

Integrated quantum photonics

This paper reviews recent advances in integrated waveguide circuits, lithographically fabricated for quantum optics. With the increase in complexity of realizable quantum architectures, the need for stability and high quality nonclassical interference within large optical circuits has become a matter of concern in modern quantum optics. Using integrated waveguide structures, we demonstrate a high performance platform from which to further develop quantum technologies and experimental quantum physics using single photons. We review the performance of directional couplers in Hong-Ou-Mandel experiments, together with inherently stable interferometers with controlled phase shifts for quantum state preparation, manipulation, and measurement as well as demonstrating the first on-chip quantum metrology experiments. These fundamental components of optical quantum circuits are used together to construct integrated linear optical realizations of two-photon quantum controlled logic gates. The high quality quantum mechanical performance observed at the single photon level signifies their central role in future optical quantum technologies.