Creating nuclear spin entanglement using an optical degree of freedom

Funding: Marie Curie Early Stage Training network QIPEST (Grant No. MESTCT-2005-020505), the EPSRC through QIP IRC (Grant Nos. GR/S82176/01 and GR/S15808/01), the National Research Foundation and Ministry of Education, Singapore, the DAAD (German Academic Exchange Service), Linacre College, Oxford, and the Royal Society.Molecular nanostructures are promising building blocks for future quantum technologies, provided methods of harnessing their multiple degrees of freedom can be identified and implemented. Due to low decoherence rates, nuclear spins are considered ideal candidates for storing quantum information, while optical excitations can give rise to fast and controllable interactions for information processing. A recent paper [M. Schaffry et al., Phys. Rev. Lett. 104, 200501 (2010)] proposed a method for entangling two nuclear spins through their mutual coupling to a transient optically excited electron spin. Building on the same idea, we present here an extended and much more detailed theoretical framework, showing that this method is in fact applicable to a much wider class of molecular structures than previously discussed in the original proposal.Publisher PDFPeer reviewe

Entanglement distribution for a practical quantum-dot-based quantum processor architecture

We propose a quantum dot (QD) architecture for enabling universal quantum information processing. Quantum registers, consisting of arrays of vertically stacked self-assembled semiconductor QDs, are connected by chains of in-plane self-assembled dots. We propose an entanglement distributor, a device for producing and distributing maximally entangled qubits on demand, communicated through in-plane dot chains. This enables the transmission of entanglement to spatially separated register stacks, providing a resource for the realization of a sizeable quantum processor built from coupled register stacks of practical size. Our entanglement distributor could be integrated into many of the present proposals for self-assembled QD-based quantum computation (QC). Our device exploits the properties of simple, relatively short, spin-chains and does not require microcavities. Utilizing the properties of self-assembled QDs, after distribution the entanglement can be mapped into relatively long-lived spin qubits and purified, providing a flexible, distributed, off-line resource. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

High fidelity all-optical control of quantum dot spins: detailed study of the adiabatic approach

Confined electron spins are preferred candidates for embodying quantum information in the solid state. A popular idea is the use of optical excitation to achieve the ``best of both worlds'', i.e. marrying the long spin decoherence times with rapid gating. Here we study an all-optical adiabatic approach to generating single qubit phase gates. We find that such a gate can be extremely robust against the combined effect of all principal sources of decoherence, with an achievable fidelity of 0.999 even at finite temperature. Crucially this performance can be obtained with only a small time cost: the adiabatic gate duration is within about an order of magnitude of a simple dynamic implementation. An experimental verification of these predictions is immediately feasible with only modest resources

Quantum dynamics in a tiered non-Markovian environment

We introduce a new analytical method for studying the open quantum systems problem of a discrete system weakly coupled to an environment of harmonic oscillators. Our approach is based on a phase space representation of the density matrix for a system coupled to a two-tiered environment. The dynamics of the system and its immediate environment are resolved in a non-Markovian way, and the environmental modes of the inner environment can themselves be damped by a wider `universe'. Applying our approach to the canonical cases of the Rabi and spin-boson models we obtain new analytical expressions for an effective thermalisation temperature and corrections to the environmental response functions as direct consequences of considering such a tiered environment. A comparison with exact numerical simulations confirms that our approximate expressions are remarkably accurate, while their analytic nature offers the prospect of deeper understanding of the physics which they describe. A unique advantage of our method is that it permits the simultaneous inclusion of a continuous bath as well as discrete environmental modes, leading to wide and versatile applicability.Comment: Video abstract available at http://iopscience.iop.org/1367-2630/17/2/023063. 15 pages, 6 figure

Practicality of spin chain 'wiring' in diamond quantum technologies

Coupled spin chains are promising candidates for 'wiring up' qubits in solid-state quantum computing (QC). In particular, two nitrogen-vacancy centers in diamond can be connected by a chain of implanted nitrogen impurities; when driven by a suitable global fields the chain can potentially enable quantum state transfer at room temperature. However, our detailed analysis of error effects suggests that foreseeable systems may fall far short of the fidelities required for QC. Fortunately the chain can function in the more modest role as a mediator of noisy entanglement, enabling QC provided that we use subsequent purification. For instance, a chain of 5 spins with inter-spin distances of 10 nm has finite entangling power as long as the T2 time of the spins exceeds 0.55 ms. Moreover we show that re-purposing the chain this way can remove the restriction to nearest-neighbor interactions, so eliminating the need for complicated dynamical decoupling sequences.Comment: 5 pages (plus 5-page supplement

Measurement-based approach to entanglement generation in coupled quantum dots

Measurements provide a novel mechanism for generating the entanglement resource necessary for performing scalable quantum computation. Recently, we proposed a method for performing parity measurements in a coupled quantum dot system. In this paper we generalise this scheme and perform a comprehensive analytic and numerical study of environmental factors. We calculate the effects of possible error sources including non-ideal photon detectors, ineffective spin-selective excitation and dot distinguishability (both spatial and spectral). Furthermore, we present an experimental approach for verifying the success of the parity measurement

Freezing distributed entanglement in spin chains

We show how to freeze distributed entanglement that has been created from the natural dynamics of spin chain systems. The technique that we propose simply requires single-qubit operations and isolates the entanglement in specific qubits at the ends of branches. Such frozen entanglement provides a useful resource, for example for teleportation or distributed quantum processing. The scheme can be applied to a wide range of systems -- including actual spin systems and alternative qubit embodiments in strings of quantum dots, molecules or atoms.Comment: 5 pages, to appear in Phys. Rev. A (Rapid Communication

A general approach to quantum dynamics using a variational master equation: Application to phonon-damped Rabi rotations in quantum dots

We develop a versatile master equation approach to describe the non-equilibrium dynamics of a two-level system in contact with a bosonic environment, which allows for the exploration of a wide range of parameter regimes within a single formalism. As an experimentally relevant example, we apply this technique to the study of excitonic Rabi rotations in a driven quantum dot, and compare its predictions to the numerical Feynman integral approach. We find excellent agreement between the two methods across a generally difficult range of parameters. In particular, the variational master equation technique captures effects usually considered to be non-perturbative, such as multi-phonon processes and bath-induced driving renormalisation, and can give reliable results even in regimes in which previous master equation approaches fail.Comment: 5 pages, 2 figures. Published version, revised title, minor changes to the tex

Quantum-enhanced capture of photons using optical ratchet states

Natural and artificial light harvesting systems often operate in a regime where the flux of photons is relatively low. Besides absorbing as many photons as possible it is therefore paramount to prevent excitons from annihilation via photon re-emission until they have undergone an irreversible energy conversion process. Taking inspiration from photosynthetic antenna structures, we here consider ring-like systems and introduce a class of states we call ratchets: excited states capable of absorbing but not emitting light. This allows our antennae to absorb further photons whilst retaining the excitations from those that have already been captured. Simulations for a ring of four sites reveal a peak power enhancement by up to a factor of 35 under ambient conditions owing to a combination of ratcheting and the prevention of emission through dark-state population. In the slow extraction limit the achievable power enhancement due to ratcheting alone exceeds 20%.Comment: major revision with improved model (all data and figures updated