Dimitris I. Tsomokos

Quantum simulator of an open quantum system using superconducting qubits: exciton transport in photosynthetic complexes

Open quantum system approaches are widely used in the description of physical, chemical and biological systems. A famous example is electronic excitation transfer in the initial stage of photosynthesis, where harvested energy is transferred with remarkably high efficiency to a reaction center. This transport is affected by the motion of a structured vibrational environment, which makes simulations on a classical computer very demanding. Here we propose an analog quantum simulator of complex open system dynamics with a precisely engineered quantum environment. Our setup is based on superconducting circuits, a well established technology. As an example, we demonstrate that it is feasible to simulate exciton transport in the Fenna–Matthews–Olson photosynthetic complex. Our approach allows for a controllable single-molecule simulation and the investigation of energy transfer pathways as well as non-Markovian noise-correlation effects.

Fully connected network of superconducting qubits in a cavity

A fully connected qubit network is considered, where every qubit interacts with every other one. When the interactions between the qubits are homogeneous, the system is a special case of the finite Lipkin–Meshkov–Glick (LMG) model. We propose a natural implementation of this model using superconducting qubits in state-of-the-art circuit QED. The ground state, the low-lying energy spectrum and the dynamical evolution are investigated. We find that, under realistic conditions, highly entangled states of Greenberger–Horne–Zeilinger (GHZ) and W types can be generated. We also comment on the influence of disorder on the system and discuss the possibility of simulating complex quantum systems, such as Sherrington–Kirkpatrick (SK) spin glasses, with superconducting qubit networks.

Entanglement dynamics in chains of qubits with noise and disorder

The entanglement dynamics of arrays of qubits is analysed in the presence of some general sources of noise and disorder. In particular, we consider linear chains of Josephson qubits in experimentally realistic conditions. Electromagnetic and other (spin or boson) fluctuations due to the background circuitry and surrounding substrate, finite temperature in the external environment, and disorder in the initial preparation and the control parameters are embedded into our model. We show that the amount of disorder that is typically present in current experiments does not affect the entanglement dynamics significantly, while the presence of noise can have a drastic influence on the generation and propagation of entanglement. We examine under which circumstances the system exhibits steady-state entanglement for both short (N 30) chains and show that, remarkably, there are parameter regimes where the steady-state entanglement is strictly non-monotonic as a function of the noise strength. We also present optimized schemes for entanglement verification and quantification based on simple correlation measurements that are experimentally more economic than state tomography.

Using superconducting qubit circuits to engineer exotic lattice systems

We propose an architecture based on superconducting qubits and resonators for the implementation of a variety of exotic lattice systems, such as spin and Hubbard models in higher or fractal dimensions and higher-genus topologies. Spin systems are realized naturally using qubits, while superconducting resonators can be used for the realization of Bose-Hubbard models. Fundamental requirements for these designs, such as controllable interactions between arbitrary qubit pairs, have recently been implemented in the laboratory, rendering our proposals feasible with current technology.

State transfer in highly connected networks and a quantum Babinet principle

Original article can be found at: http://pra.aps.org/ Copyright American Physical Society. DOI: 10.1103/PhysRevA.78.062310

Control-limited perfect state transfer, quantum stochastic resonance and many-body entangling gate in imperfect qubit registers

Original article can be found at: http://pra.aps.org/ Copyright American Physical Society. DOI: 10.1103/PhysRevA.77.062337

Quantum walks on complex networks with connection instabilities and community structure

A continuous-time quantum walk is investigated on complex networks with the characteristic property of community structure, which is shared by most real-world networks. Motivated by the prospect of viable quantum networks, I focus on the effects of network instabilities in the form of broken links, and examine the response of the quantum walk to such failures. It is shown that the reconfiguration of the quantum walk is determined by the community structure of the network. In this context, quantum walks based on the adjacency and Laplacian matrices of the network are compared, and their responses to link failures is analyzed.

Topological order following a quantum quench

We determine the conditions under which topological order survives a rapid quantum quench. Specifically, we consider the case where a quantum spin system is prepared in the ground state of the toric code model and, after the quench, it evolves with a Hamiltonian that does not support topological order. We provide analytical results supported by numerical evidence for a variety of quench Hamiltonians. The robustness of topological order under nonequilibrium situations is tested by studying the topological entropy and a dynamical measure, which makes use of the similarity between partial density matrices obtained from different topological sectors.

Chiral entanglement in triangular lattice models

Original article can be found at: http://pra.aps.org/ Copyright American Physical Society. DOI: 10.1103/PhysRevA.77.012106

Entanglement of topological phase factors

The topological phase factor induced on interfering electrons by external quantum electromagnetic fields has been studied. Two and three electron interference experiments inside distant cavities are considered and the influence of correlated photons on the phase factors is investigated. It is shown that the classical or quantum correlations of the irradiating photons are transferred to the topological phases. The effect is quantified in terms of Weyl functions for the density operators of the photons and illustrated with particular examples. The scheme employs the generalized phase factor as a mechanism for information transfer from the photons to the electric charges. In this sense, the scheme may be useful in the context of flying qubits (corresponding to photons) and stationary qubits (electrons), as well as the conversion from one type to the other.