Studying Light-Harvesting Models with Superconducting Circuits

The process of photosynthesis, the main source of energy in the animate world, converts sunlight into chemical energy. The surprisingly high efficiency of this process is believed to be enabled by an intricate interplay between the quantum nature of molecular structures in photosynthetic complexes and their interaction with the environment. Investigating these effects in biological samples is challenging due to their complex and disordered structure. Here we experimentally demonstrate a new approach for studying photosynthetic models based on superconducting quantum circuits. In particular, we demonstrate the unprecedented versatility and control of our method in an engineered three-site model of a pigment protein complex with realistic parameters scaled down in energy by a factor of $10^5$. With this system we show that the excitation transport between quantum coherent sites disordered in energy can be enabled through the interaction with environmental noise. We also show that the efficiency of the process is maximized for structured noise resembling intramolecular phononic environments found in photosynthetic complexes.Comment: 8+12 pages, 4+12 figure

Toolbox for Quantum Computing and Digital Quantum Simulation with Superconducting Qubits

Quantum computers make use of the coherent time evolution of a quantum system to map an input to an output. The coherent quantum dynamics allows the system to take on superpositions of states which is not possible in the laws of classical physics. Once quantum computers are built, they can solve certain problems exponentially faster than a classical computer. However, a quantum computer will most likely not be a stand-alone component but rather needs a host of classical electronics for control and readout of the quantum state. In the present thesis we develop tools for the realization of quantum computing and simulation experiments with superconducting circuits. We develop a real-time digital signal processing unit based on a field programmable gate array (FPGA). A practical quantum computer might require several rounds of measurements where, in each step, a set of quantum bits (qubits) has to be reset into a known state. We demonstrate active reset of a qubit using the FPGA unit on timescales of a few hundred nanoseconds. As a further step, we experimentally demonstrate the usage of the FPGA instrument to realize an active feedforward operation for deterministic quantum teleportation. Quantum teleportation allows to transfer the state of a qubit from one location to the other using a classical communication channel and an entangled pair of qubits as a resource. Quantum teleportation thus might be a useful means for data transfer in future quantum computing and communication systems. One of the most promising applications of a quantum computer is the simulation of quantum mechanical models which are hard to simulate with a classical computer. We perform a proof-of-principle experiment, where we demonstrate the digital quantum simulation of the time evolution under three different kinds of interactions between two spins. In particular, we simulate the XY model, the Heisenberg XYZ model, and the quantum mechanical Ising model with transverse magnetic field. The digital quantum simulation is based on a stroboscopic decomposition of the coherent time evolution into a sequence of up to ten two-qubit gates with variable duration and intertwined with single qubit gates. In future experiments, the ability to digitally simulate spin–spin interactions with superconducting qubits could form a building block for digital quantum simulations of complex systems