Extracting high fidelity quantum computer hardware from random systems

An overview of current status and prospects of the development of quantum computer hardware based on inorganic crystals doped with rare-earth ions is presented. Major parts of the experimental work in this area has been done in two places, Canberra, Australia and Lund, Sweden, and the present description follows more closely the Lund work. Techniques will be described that include optimal filtering of the initially inhomogeneously broadened profile down to well separated and narrow ensembles, as well as the use of advanced pulse-shaping in order to achieve robust arbitrary single-qubit operations with fidelities above 90%, as characterized by quantum state tomography. It is expected that full scalability of these systems will require the ability to determine the state of single rare-earth ions. It has been proposed that this can be done using special readout ions doped into the crystal and an update is given on the work to find and characterize such ions. Finally, a few aspects on the possibilities for remote entanglement of ions in separate rare-earth-ion-doped crystals are considered.Comment: 19 pages, 9 figures. Written for The Proceedings of the Nobelsymposium on qubits for future quantum computers, Gothenburg, May-0

High fidelity readout scheme for rare-earth solid state quantum computing

We propose and analyze a high fidelity readout scheme for a single instance approach to quantum computing in rare-earth-ion-doped crystals. The scheme is based on using different species of qubit and readout ions, and it is shown that by allowing the closest qubit ion to act as a readout buffer, the readout error can be reduced by more than an order of magnitude. The scheme is shown to be robust against certain experimental variations, such as varying detection efficiencies, and we use the scheme to predict the expected quantum fidelity of a CNOT gate in these solid state systems. In addition, we discuss the potential scalability of the protocol to larger qubit systems. The results are based on parameters which we believed are experimentally feasible with current technology, and which can be simultaneously realized.Comment: 7 pages, 5 figure

Scalable designs for quantum computing with rare-earth-ion-doped crystals

Due to inhomogeneous broadening, the absorption lines of rare-earth-ion dopands in crystals are many order of magnitudes wider than the homogeneous linewidths. Several ways have been proposed to use ions with different inhomogeneous shifts as qubit registers, and to perform gate operations between such registers by means of the static dipole coupling between the ions. In this paper we show that in order to implement high-fidelity quantum gate operations by means of the static dipole interaction, we require the participating ions to be strongly coupled, and that the density of such strongly coupled registers in general scales poorly with register size. Although this is critical to previous proposals which rely on a high density of functional registers, we describe architectures and preparation strategies that will allow scalable quantum computers based on rare-earth-ion doped crystals.Comment: Submitted to Phys. Rev.

Storage and recall of weak coherent optical pulses with an efficiency of 25%

We demonstrate experimentally a quantum memory scheme for the storage of weak coherent light pulses in an inhomogeneously broadened optical transition in a Pr^{3+}: YSO crystal at 2.1 K. Precise optical pumping using a frequency stable (about 1kHz linewidth) laser is employed to create a highly controllable Atomic Frequency Comb (AFC) structure. We report single photon storage and retrieval efficiencies of 25%, based on coherent photon echo type re-emission in the forward direction. The coherence property of the quantum memory is proved through interference between a super Gaussian pulse and the emitted echo. Backward retrieval of the photon echo emission has potential for increasing storage and recall efficiency.Comment: 5,

Quantum memory for non-stationary light fields based on controlled reversible inhomogeneous broadening

We propose a new method for efficient storage and recall of non-stationary light fields, e.g. single photon time-bin qubits, in optically dense atomic ensembles. Our approach to quantum memory is based on controlled, reversible, inhomogeneous broadening. We briefly discuss experimental realizations of our proposal.Comment: 4 page

An Orthogonal Covalent Connector System for the Efficient Assembly of Enzyme Cascades on DNA Nanostructures

Combining structural DNA nanotechnology with the virtually unlimited variety of enzymes offers unique opportunities for generating novel biocatalytic devices. However, the immobilization of enzymes is still restricted by a lack of efficient covalent coupling techniques. The rational re-engineering of the genetically fusible SNAP-tag linker is reported here. By replacing five amino acids that alter the electrostatic properties of the SNAP_R5 variant, up to 11-fold increased coupling efficiency with benzylguanine-modified oligonucleotides and DNA origami nanostructures (DON) was achieved, resulting in typical occupancy densities of 75%. The novel SNAP_R5 linker can be combined with the equally efficient Halo-based oligonucleotide binding tag (HOB). Since both linkers exhibit neither cross-reactivity nor non-specific binding, they allowed orthogonal assembly of an enzyme cascade consisting of the stereoselective ketoreductase Gre2p and the cofactor-regenerating isocitrate dehydrogenase on DON. The cascade showed approximately 1.6-fold higher activity in a stereoselective cascade reaction than the corresponding free solubilized enzymes. The connector system presented here and the methods used to validate it represent important tools for further development of DON-based multienzyme systems to investigate mechanistic effects of substrate channeling and compartmentalization relevant for exploitation in biosensing and catalysis

Complete reconstruction of the quantum state of a single-photon wave packet absorbed by a doppler-broadened transition

The reconstruction of the quantum state of a nonstationary single-photon wave packet absorbed by a Doppler-broadened transition was analyzed. The proposed technique was based on the property that frequency shifts due to the Doppler effect were opposite for counterpropagating directions. The quantum state storage solution and the recall of nonstationary single-photon wave packets in the medium was investigated. In the two-level photon echo scheme, the reconstructed field was absorbed as it propagated in the sample, and although the state was correctly mapped onto the atomic ensemble if the sample had a high optical density, it was only partly extracted

Background reduction and sensitivity for germanium double beta decay experiments

Germanium detectors have very good capabilities for the investigation of rare phenomena like the neutrinoless double beta decay. Rejection of the background entangling the expected signal is one primary goal in this kind of experiments. Here, the attainable background reduction in the energy region where the neutrinoless double beta decay signal of 76Ge is expected to appear has been evaluated for experiments using germanium detectors, taking into consideration different strategies like the granularity of the detector system, the segmentation of each individual germanium detector and the application of Pulse Shape Analysis techniques to discriminate signal from background events. Detection efficiency to the signal is affected by background rejection techniques, and therefore it has been estimated for each of the background rejection scenarios considered. Finally, conditions regarding crystal mass, radiopurity, exposure to cosmic rays, shielding and rejection capabilities are discussed with the aim to achieve a background level of 10-3 c keV-1 kg-1 y-1 in the region of interest, which would allow to explore neutrino effective masses around 40 meV.Comment: 13 pages, 19 figures. Accepted by Astroparticle Physic