[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Quantum cryptography could well be the first application of quantum mechanics at the individual quanta level. The very fast progress in both theory and experiments over the recent years are reviewed, with emphasis on open questions and technological issues.

Quantum Cryptography

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Quantum computers promise to increase greatly the efficiency of solving problems such as factoring large integers, combinatorial optimization and quantum physics simulation. One of the greatest challenges now is to implement the basic quantum-computational elements in a physical system and to demonstrate that they can be reliably and scalably controlled. One of the earliest proposals for quantum computation is based on implementing a quantum bit with two optical modes containing one photon. The proposal is appealing because of the ease with which photon interference can be observed. Until now, it suffered from the requirement for non-linear couplings between optical modes containing few photons. Here we show that efficient quantum computation is possible using only beam splitters, phase shifters, single photon sources and photo-detectors. Our methods exploit feedback from photo-detectors and are robust against errors from photon loss and detector inefficiency. The basic elements are accessible to experimental investigation with current technology.

A scheme for efficient quantum computation with linear optics.

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

We propose a new method of generating nonclassical optical field states. The method uses a semiconductor device, which consists of a single quantum dot as active medium embedded in a $p$- $i$- $n$ junction and surrounded by a microcavity. Resonant tunneling of electrons and holes into the quantum dot ground states, together with the Pauli exclusion principle, produce regulated single photons or regulated pairs of photons. We propose that this device also has the unique potential to generate pairs of entangled photons at a well-defined repetition rate.

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Regulated and Entangled Photons from a Single Quantum Dot

We demonstrate that efficient shape control may be achieved in the shell of colloidally grown semiconductor nanocrystals (independent of the core), allowing the combination of a 0-D spherical CdSe core with a 1-D rodlike CdS shell. Besides exhibiting linearly polarized emission with a room-temperature quantum efficiency above 70%, these mixed-dimensionality colloidal heterostructures display large, length-dependent Stokes shifts as well as giant extinction coefficients approaching 107cm-1 M-1.

Highly Emissive Colloidal CdSe/CdS Heterostructures of Mixed Dimensionality

We have reconstructed the quantum state of optical pulses containing single photons using the method of phase-randomized pulsed optical homodyne tomography. The single-photon Fock state 1> was prepared using conditional measurements on photon pairs born in the process of parametric down-conversion. A probability distribution of the phase-averaged electric field amplitudes with a strongly non-Gaussian shape is obtained with the total detection efficiency of (55+/-1)%. The angle-averaged Wigner function reconstructed from this distribution shows a strong dip reaching classically impossible negative values around the origin of the phase space.

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Quantum State Reconstruction of the Single-Photon Fock State

In this Letter we report the plasmon-enhanced upconversion in single NaYF4 nanocrystals codoped with Yb3+/Er3+. Single nanocrystals and gold nanospheres are investigated and assembled in a combined confocal and atomic force microscope setup. The nanocrystals show strong upconversion emission in the green and red under excitation with a continuous wave laser in the near-infrared at 973 nm. By the use of the atomic force microscope, we couple single nanocrystals with gold spheres (30 and 60 nm in diameter) to obtain enhanced upconversion emission. An overall enhancement factor of 3.8 is reached. A comparison of time-resolved measurements on the bare nanocrystal and the coupled nanocrystal−gold sphere systems unveil that faster excitation as well as faster emission occurs in the nanocrystals.

Plasmon-enhanced upconversion in single NaYF4:Yb3+/Er3+ codoped nanocrystals.

In this Letter we present the controlled coupling of a single nitrogen vacancy center to a plasmonic structure. With the help of an atomic force microscope, a single nanodiamond containing a single nitrogen vacancy center and two gold nanospheres are assembled step-by-step. We show that both the excitation rate and the radiative decay rate of the color center are enhanced by about 1 order of magnitude, while the single photon character of the emission is maintained. Hot spots between diamond and gold nanoparticles provide an efficient near-field coupling, despite the mismatch in size and shape. Our approach provides hybrid systems as important building blocks for novel nanophotonic light sources in advanced plasmonic devices stable even at room temperature.

Oliver Benson

Papers

Regulated and Entangled Photons from a Single Quantum Dot

Highly Emissive Colloidal CdSe/CdS Heterostructures of Mixed Dimensionality

Quantum State Reconstruction of the Single-Photon Fock State

Plasmon-enhanced upconversion in single NaYF4:Yb3+/Er3+ codoped nanocrystals.

Plasmon-enhanced single photon emission from a nanoassembled metal-diamond hybrid structure at room temperature.