Speaker: Jelena Vuckovic (Stanford University)

Title: Quantum dots in photonic crystals: from quantum information processing to optical switching at a single photon level

By: Jelena Vuckovic, Andrei Faraon, Dirk Englund, Arka Majumdar, Pierre Petroff

Ginzton Laboratory, Stanford University

Quantum dots in photonic crystals are interesting both as a testbed for fundamental cavity quantum electrodynamics (QED) experiments, as well as a platform for quantum and classical information processing.

Quantum dot-photonic crystal cavity QED has been probed both in photoluminescence and coherently, by resonant light scattering from such a system [1]. In the latter case, both intensity and photon statistics of the reflected beam have been analyzed as a function of wavelength, leading to observation of effects such as photon blockade and photon induced tunneling - for the first time in solid state [2]. The system has also been employed to achieve a controlled phase and amplitude modulation between two modes of light at the single photon level [3] - nonlinearity observed so far only in atomic physics systems.

These demonstrations lie at the core of a number of proposals for quantum information processing, and could also be employed to build novel devices, such as optical switches controlled at a single photon level.

[1]. Dirk Englund, Andrei Faraon, Ilya Fushman, Nick Stoltz, Pierre Petroff, and Jelena Vuckovic, "Controlling cavity reflectivity with a single quantum dot," Nature, vol. 450, No. 7171, pp. 857-861, December 2007

[2]. Andrei Faraon, Ilya Fushman, Dirk Englund, Nick Stoltz, Pierre Petroff, and Jelena Vuckovic, "Coherent generation of nonclassical light on a chip via photon-induced tunneling and blockade," Nature Physics, Vol 4, pp. 859 - 863 (2008)

[3]. Ilya Fushman, Dirk Englund, Andrei Faraon, Nick Stoltz, Pierre Petroff, and Jelena Vuckovic, "Controlled phase shift with a single quantum dot," Science, vol. 320, number 5877, pp. 769-772 ( 2008)

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Speaker: Steve Flammia (Perimeter Institute)

Title: Ultrafast quantum state tomography

Everybody hates tomography. And with good reason! Experimentalists hate it because it is inefficient and difficult. Theorists hate it because it isn't very "quantum." But because of our current lack of meso-scale quantum computers capable of convincingly performing non-classical calculations, tomography seems like a necessary evil. In this talk, I will attempt to banish quantum state tomography to the Hell of Lost Paradigms where it belongs. I hope to achieve this by introducing several heuristics for learning quantum states more efficiently, in some cases exponentially so. One such heuristic runs in polynomial time and outputs a polynomial-sized classical approximation of the state (in matrix product state form.) Another takes advantage of the fact that most interesting states are close to pure states to get a quadratic speedup using ideas from compressed sensing. Both algorithms come with rigorous error bounds.

This is joint work with S. Bartlett, D. Gross, R. Somma (first result), and S. Becker, J. Eisert, D. Gross, and Y.-K. Liu (second result).

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Speaker: Yuuki Tokunaga (NTT, Osaka University)

Title: Complete process tomography of experimental one-way quantum computation

Coauthors: Satoru Okamoto, Rikizo Ikuta, Takashi Yamamoto, Masato Koashi, and Nobuyuki Imoto

We present full quantum process tomography of photonic cluster-state quantum computation. We demonstrated basic gates of one-way quantum computation and reconstructed these process matrices by using maximum likelihood estimation. From the completely reconstructed process matrices, we can evaluate several gate performances such as process fidelity, purity, and entanglement capability. We also discuss the relation between experimentally obtained process matrices and fault-tolerant threshold theory with several error models such as independent stochastic Pauli errors, independent stochastic completely-positive trace-preserving errors, and general local unitary errors (e.g. slight over-rotation).