Quantum Limits of Thermometry

The precision of typical thermometers consisting of $N$ particles is shot noise limited, improving as $\sim1/\sqrt{N}$. For high precision thermometry and thermometric standards this presents an important theoretical noise floor. Here it is demonstrated that thermometry may be mapped onto the problem of phase estimation, and using techniques from optimal phase estimation, it follows that the scaling of the precision of a thermometer may in principle be improved to $\sim1/N$, representing a Heisenberg limit to thermometry.Comment: 4 page

Dephasing-assisted Gain and Loss in Mesoscopic Quantum Systems

Motivated by recent experiments, we analyse the phonon-assisted steady-state gain of a microwave field driving a double quantum-dot in a resonator. We apply the results of our companion paper, which derives the complete set of fourth-order Lindblad dissipators using Keldysh methods, to show that resonator gain and loss are substantially affected by dephasing-assisted dissipative processes in the quantum-dot system. These additional processes, which go beyond recently proposed polaronic theories, are in good quantitative agreement with experimental observationsComment: 5 pages, 3 Figures, published together with arXiv:1608.0416

"Be war, ye wemen, of youre subtyle fo" : a study of Chaucer's Legend of good women : a thesis presented in partial fulfilment of the requirements for the degree of Master of Arts in English at Massey University

The principal aim of this thesis is to survey the two main critical opinions which have been expressed upon the Legend of Good Women, and to ascertain the merits of each case in conjunction with an attempt at bringing about some reconciliation of the two. The thesis begins by pointing out the relatively cursory critical attention that the Legend has in fact received. The Prologue alone has appealed to critics generally because of its two versions, Chaucer's use of courtly love conventions, and because Queen Anne may have commanded him to write it. The weight of critical opinion maintains that the legends them-selves are generally monotonous and tedious, and that Chaucer himself was bored. A survey is made of the development and entrenchment of this, the bored thesis. A rare and recent case against this thesis is then examined in detail. As this essentially rests on the recognition and assessment of the rhetorical technique abbreviatio, a brief survey of medieval rhetorical theory is then made. This is followed by an attempt to set the work in its wider medieval context as a counter to prejudice against the Legend due to a modern perspective. It is possible then to endeavour to look at the poem from Chaucer's viewpoint.It is concluded that the poem was essentially an experiment for him. Also, the smallness of scale and repetitious theme of the legends must have bothered Chaucer as much as they do modern critics. This comes out in the ambivalent position of the narrator and it is here that a possible reconciliation of the opposing theses is suggested. Finally, Chaucer neglected the rather flat Legend for the infinitely more varied and human Canterbury Tales

Decoding Schemes for Foliated Sparse Quantum Error Correcting Codes

Foliated quantum codes are a resource for fault-tolerant measurement-based quantum error correction for quantum repeaters and for quantum computation. They represent a general approach to integrating a range of possible quantum error correcting codes into larger fault-tolerant networks. Here we present an efficient heuristic decoding scheme for foliated quantum codes, based on message passing between primal and dual code 'sheets'. We test this decoder on two different families of sparse quantum error correcting code: turbo codes and bicycle codes, and show reasonably high numerical performance thresholds. We also present a construction schedule for building such code states.Comment: 23 pages, 15 figures, accepted for publication in Phys. Rev.

Photon Frequency Mode Matching using Acousto-Optic Frequency Beam Splitters

It is a difficult engineering task to create distinct solid state single photon sources which nonetheless emit photons at the same frequency. It is also hard to create entangled photon pairs from quantum dots. In the spirit of quantum engineering we propose a simple optical circuit which can, in the right circumstances, make frequency distinguishable photons frequency indistinguishable. Our circuit can supply a downstream solution to both problems, opening up a large window of allowed frequency mismatches between physical mechanisms. The only components used are spectrum analysers/prisms and an Acousto-Optic Modulator. We also note that an Acousto-Optic Modulator can be used to obtain Hong-Ou-Mandel two photon interference effects from the frequency distinguishable photons generated by distinct sources.Comment: 4 pages, 4 figure

Fault tolerant quantum computation with very high threshold for loss errors

Many proposals for fault tolerant quantum computation (FTQC) suffer detectable loss processes. Here we show that topological FTQC schemes, which are known to have high error thresholds, are also extremely robust against losses. We demonstrate that these schemes tolerate loss rates up to 24.9%, determined by bond percolation on a cubic lattice. Our numerical results show that these schemes retain good performance when loss and computational errors are simultaneously present.Comment: 4 pages, comments still very welcome. v2 is a reasonable approximation to the published versio

The effect of noise correlations on randomized benchmarking

Among the most popular and well studied quantum characterization, verification and validation techniques is randomized benchmarking (RB), an important statistical tool used to characterize the performance of physical logic operations useful in quantum information processing. In this work we provide a detailed mathematical treatment of the effect of temporal noise correlations on the outcomes of RB protocols. We provide a fully analytic framework capturing the accumulation of error in RB expressed in terms of a three-dimensional random walk in "Pauli space." Using this framework we derive the probability density function describing RB outcomes (averaged over noise) for both Markovian and correlated errors, which we show is generally described by a gamma distribution with shape and scale parameters depending on the correlation structure. Long temporal correlations impart large nonvanishing variance and skew in the distribution towards high-fidelity outcomes -- consistent with existing experimental data -- highlighting potential finite-sampling pitfalls and the divergence of the mean RB outcome from worst-case errors in the presence of noise correlations. We use the Filter-transfer function formalism to reveal the underlying reason for these differences in terms of effective coherent averaging of correlated errors in certain random sequences. We conclude by commenting on the impact of these calculations on the utility of single-metric approaches to quantum characterization, verification, and validation.Comment: Updated and expanded to include full derivation. Related papers available from http://www.physics.usyd.edu.au/~mbiercuk/Publications.htm

Bragg-induced orbital angular-momentum mixing in paraxial high-finesse cavities

Numerical calculation of vector electromagnetic modes of plano-concave microcavities reveals that the polarization-dependent reflectivity of a flat Bragg mirror can lead to unexpected cavity field distributions for nominally paraxial modes. Even in a rotationally symmetric resonator, certain pairs of orbital angular momenta are necessarily mixed in an excitation-independent way to form doublets. A characteristic mixing angle is identified, which even in the paraxial limit can be designed to have large values. This correction to Gaussian theory is zeroth-order in deviations from paraxiality. We discuss the resulting nonuniform polarization fields. Observation will require small cavities with sufficiently high Q. Possible applications are proposed.Comment: Corrected typos in Fig. 2 and text. Added Journal Ref. For higher-quality figures, see http://darkwing.uoregon.edu/~noeckel/papers.php#xref3