The authors would like to thank Dr. Hong Yi at the Emory University Integrated Microscopy Core for electron microscopy imaging. This work was sponsored by NIH grants (PN2EY018244, R01 CA108468, U54CA119338, and 1K99CA154006-01).

The following synthesis procedures involve standard air-free techniques and the use of a vacuum/inert gas manifold; detailed methodology can be found in references 10 and 11. MSDS for all potentially toxic and flammable substances should be consulted before use and all flammable and/or air-labile compounds should be aliquoted into septum-sealed vials in a glove box or glove bag.
1. Synthesis of Mercury Cadmium Selenide (HgxCd1-xSe) Quantum Dot Cores
<ol>
 <li>Prepare a 0.4 M solution of selenium in trioctylphosphine (TOP). Add selenium (0.316 g, 4 mmol) to a 50 ml 3-necked flask, then evacuate and fill with argon using a Schlenk line. Under air-free conditions (dry nitrogen or argon atmosphere), add 10 ml TOP and heat to 100 &deg;C while stirring for 1 hr to yield a clear, colorless solution. Cool the solution to room temperature and set the flask aside.</li>
 <li>To a 250 ml 3-necked flask, add cadmium oxide (CdO 0.0770 g, 0.6 mmol), tetradecylphosphonic acid (TDPA, 0.3674 g, 1.32 mmol), and octadecene (ODE, 27.6 ml), and evacuate the solution using a Schlenk line while stirring. Increase the temperature to 100 &deg;C and evacuate for an additional 15 min to remove low-boiling point impurities.</li>
 <li>Under argon or nitrogen gas, heat the mixture to 300 &deg;C for 1 hr to fully dissolve the CdO. The solution will change from a reddish color to clear and colorless. Cool the solution to room temperature.</li>
 <li>Add hexadecylamine (HDA, 7.0 g) to the cadmium solution, heat to 70 &deg;C, and evacuate. Once a constant pressure is attained, increase the temperature to 100-110 &deg;C and reflux the solution for 30 min. Switch the Schlenk line valve to inert gas and insert the thermocouple directly into the solution.</li>
 <li>Under air-free conditions, add diphenylphosphine (DPP, 100 &mu;l) to the solution and increase the temperature to 310 &deg;C. Remove 7.5 ml of the 0.4 M TOP-Se solution (3 mmol selenium) in a disposable plastic syringe attached to a 16 gauge needle.</li>
 <li>Once the temperature equilibrates at 310 &deg;C, set the temperature controller to 0 &deg;C and swiftly inject the TOP-Se solution directly into the cadmium solution. The solution will change from colorless to yellow-orange and the temperature will quickly drop and increase again to ~280 &deg;C. After 1 min of reaction, remove the flask from the heating mantle and quickly cool with a stream of air until the temperature is less than 200 &deg;C.</li>
 <li>When the temperature reaches ~40 &deg;C, dilute with 30 ml hexane; most of the remaining cadmium precursor will settle out of solution. Remove this precipitate by centrifugation (5,000 x g, 10 min).</li>
 <li>In each of six 50 ml polypropylene conical centrifuge tube, dilute 12 ml of the crude nanocrystal solution with 40 ml acetone, centrifuge (5,000 x g for 10 min), and carefully decant and discard the supernatant.</li>
 <li>Dissolve the nanocrystal pellets in hexane (25 ml total volume). Extract this solution 3 times with an equal volume of methanol, retaining the top phase. For the third extraction, the volume of methanol may be adjusted to ~15 ml to obtain a concentrated hexane solution of pure CdSe QDs at roughly 200 &mu;M. The typical yield of this reaction is 3 &mu;mol of CdSe nanocrystals with a diameter of 2.3 nm (50-60% reaction yield).</li>
 <li>Determine the nanocrystal diameter and concentration by measuring the UV-Vis absorption spectrum and consulting the size-fitting chart of Mulvaney and coworkers 12 and the extinction correlations of Bawendi and coworkers 13. See Appendix for details.</li>
 <li>Mercury cation exchange: the nanocrystals may be partially exchanged with mercury to red-shift the absorption and fluorescence emission. Mix the following together in order in a 20 ml glass vial with stirbar (this reaction may be scaled as desired): 3 ml hexane, 2 ml chloroform, 1 ml 200 &mu;M CdSe QD solution (200 nmol), 15 &mu;l oleylamine (OLA), and 500 &mu;l of a 0.1 M solution of Hg(OT)2 in chloroform. Mercury octanethioate (HgOT2) may be prepared by reacting mercury acetate and octanethiol in methanol (see appendix). As the cation exchange reaction proceeds, the extent of red-shift may be monitored with UV-Vis absorption spectrophotometry. After the desired absorption band has been attained, measure the absorption of the nanocrystal solution at 350 nm and determine the new extinction coefficient, assuming that the nanocrystal concentration has not changed (30.7 &mu;M in this example). Quench the reaction by removing the unreacted mercury: add 5 ml decane, 10 ml hexane, and 7 ml methanol and extract the solution, keeping the top phase containing the nanocrystals. Extract twice more with hexane and methanol, and adjust the volume of methanol so that the top phase is ~7 ml. If the phases are slow to separate, the solution may be centrifuged (5,000 x g, 10 min). Add 100 &mu;l TOP, 100 &mu;l OLA, and 100 &mu;l oleic acid to the nanocrystals followed by 40 ml acetone to induce precipitation. Collect the nanocrystals via centrifugation and disperse in 3 ml hexane. Centrifuge again to remove insoluble components and determine the nanocrystal concentration again, using the new extinction coefficient at 350 nm. Allow the nanocrystal solution to age for at least 24 hr at room temperature before proceeding to the next step.</li>
</ol>
2. Growth of Cadmium Zinc Sulfide (CdyZn1-yS) Shell
<ol>
 <li>Prepare 0.1 M shell precursor solutions in 50 ml 3-necked flasks. Cadmium precursor: cadmium acetate hydrate (230.5 mg, 1 mmol) and 10 ml oleylamine (OLA). Zinc precursor: zinc acetate (183.5 mg, 1 mmol) and 10 ml OLA. Sulfur precursor: sulfur (32.1 mg, 1 mmol) and 10 ml ODE. Under vacuum, heat each solution to reflux for 1 hr to yield clear solutions, and then charge with argon. The sulfur solution may be cooled to room temperature, but the cadmium and zinc precursors are maintained at approximately 50 &deg;C. Calculations of shell precursor quantities can be found in reference 14.</li>
 <li>Add to a 3-necked flask: HgxCd1-xSe QDs (120 nmol, 2.3 nm diameter), ODE (2 ml), and trioctylphosphine oxide (TOPO, 250 mg). Evacuate off the hexane at room temperature using the Schlenk line. Increase the temperature to 100 &deg;C and reflux for 15 min. Change the Schlenk line valve to argon or nitrogen gas and insert the thermocouple in the nanocrystal solution.</li>
 <li>Increase the temperature to 120 &deg;C, add 0.5 monolayers of sulfur precursor solution (140 &mu;l), and allow the reaction to proceed for 15 min. Small aliquots (&lt;50 &mu;l) may be removed using a glass syringe to monitor the progress of the reaction using fluorescence and/or UV-Vis absorption spectrophotometry. Increase the temperature to 140 &deg;C, add 0.5 monolayers of cadmium precursor solution (140 &mu;l), and allow the reaction to proceed for 15 min. Add 500 &mu;l anhydrous OLA to the reaction solution.</li>
 <li>At 160 &deg;C add 0.5 monolayers of sulfur precursor solution (220 &mu;l) followed by an equal quantity of zinc precursor solution at 170 &deg;C with 15 min between each addition. Then at 180 &deg;C add 0.25 monolayers of sulfur precursor solution (150 &mu;l) and zinc precursor solution in 15 min intervals.</li>
 <li>Cool the solution to room temperature and again calculate a new extinction coefficient for these particles using a UV-Vis spectrum, assuming that the number of nanocrystals has not changed (120 nmol in 3.8 ml reaction solution). Store the reaction solution as a crude mixture in a freezer; the nanocrystals may be thawed and purified as needed using the same method described in sections 1.8 and 1.9.</li>
 <li>The nanocrystals may be characterized using electron microscopy, UV-Vis absorption spectroscopy, and fluorescence spectroscopy. Quantum yield may be calculated absolutely using an integrating sphere or relatively in comparison to a known standard using the methods of reference 15.</li>
</ol>
3. Phase Transfer
<ol>
 <li>Add purified core/shell HgxCd1-xSe/CdyZn1-yS QDs (5 ml, 20 &mu;M) to a 50 ml 3-necked flask and remove the hexane under a high vacuum to yield a dry film. Fill the flask with argon, add anhydrous pyridine (3 ml) to the nanoparticle film and heat the slurry to 80 &deg;C. Over the course of 1-2 hr the nanoparticles will fully dissolve.</li>
 <li>Add 1-thioglycerol (1 ml) to the solution and stir at 80 &deg;C for 2 hr. Then cool the solution to room temperature and add triethylamine (0.5 ml) to deprotonate thioglycerol. Stir for 30 min. The solution may become cloudy after the addition of triethylamine due to the poor solubility of polar nanocrystals in this solvent mixture.</li>
 <li>Transfer the QD solution into a 50 ml conical centrifuge tube containing a mixture of 20 ml hexane and 20 ml acetone, and mix well. Isolate the precipitated nanocrystals via centrifugation (5,000 x g, 10 min), and wash the pellet with acetone.</li>
 <li>Dissolve the QD pellet in DMSO (5 ml) with bath sonication, and then centrifuge (7,000 x g, 10 min) to remove possible aggregates. Determine the nanoparticle concentration from a UV-Vis absorption spectrum. This solution of pure QDs should be used within 3 hr, as the surface thiols can slowly oxidize under ambient conditions in air.</li>
 <li>Dilute the QD solution to 10 &mu;M or less with DMSO and transfer to a 50 ml flask. Prepare a 5 mg/ml solution of thiolated polyacrylic acid (synthesis described in Appendix) in DMSO. Add the polymer solution (0.15 mg polymer per nmol QDs) dropwise to the QD solution while stirring and degas the solution at room temperature for 5 min.</li>
 <li>Purge the QD/polymer solution with argon and heat to 80 &deg;C for 90 min. Then cool the solution to room temperature and dropwise add an equal volume of 50 mM sodium borate, pH 8. Stir for 10 min.</li>
 <li>Purify the QDs via dialysis (20 kDa cutoff) in 50 mM sodium borate, pH 8, and then concentrate the particles using a centrifugal filter (10 kDa cutoff). Determine the concentration from a UV-Vis absorption spectrum.</li>
</ol>
4. PEG Coating
<ol>
 <li>In a 4 ml glass vial with stirbar, mix 1 nmol QDs in borate buffer with a 40,000 x molar excess of 750 Da monoamino-polyethylene glycol (30 mg, 40 &mu;mol). If a specific chemical functionality is to be added to the nanocrystals (e.g. hydrazide or maleimide), it may be introduced by replacing a fraction of the amino-PEG with a heterobifunctional amino-PEG (30% mole fraction typically works well). Dilute the nanocrystal solution to 1 &mu;M with borate buffer. This reaction may be scaled as desired.</li>
 <li>Prepare a fresh solution of DMTMM (20 mg, 72 &mu;mol) in DMSO (144 &mu;l). This solution can be briefly heated under a stream of warm tap-water or submerged in a bath sonicator to fully dissolve the DMTMM. Quickly add a 25,000 x molar excess of this 0.5 M DMTMM solution (50 &mu;l) to the QD solution and stir at room temperature for 30 min.</li>
 <li>Repeat step 4.2 four more times to saturate the nanocrystal surface with PEG. Finally, add 200 &mu;l 1 M Tris buffer to quench the reaction and purify the nanocrystals using dialysis, centrifugal filters, or ultracentrifugation.</li>
 <li>The nanocrystals may be analyzed for monodispersity, hydrodynamic size, and surface charge using liquid chromatography, agarose gel electrophoresis, and fluorescence microscopy. To determine hydrodynamic size and size distribution using an automated liquid chromatography system (GE AKTAprime Plus), use a Superose 6 column, a flow rate of 0.5 ml/min with PBS buffer eluent, and absorption detection at 260 or 280 nm. Compare nanoparticle elution times with those of molecular weight standards. For agarose gel electrophoresis, prepare a 0.5% agarose gel in 50 mM sodium borate buffer (pH 8.5) or 50 mM sodium phosphate buffer (pH 7.4), mix 1 &mu;M samples with 10% glycerol and load into wells, and run at 100 V for 30 min. Image the nanocrystals in the gel using a UV hand wand or UV transilluminator and for fluorescence excitation. To image the nanocrystals at the single molecule level using fluorescence microscopy, dilute the particles to 0.2 nM in 10 mM phosphate buffer, drop 2.5 &mu;l of the solution on a glass coverslip, and carefully place a second coverslip on top of the liquid bead to spread a film between the coverslips. Image the surface-bound particles using a high numerical aperture objective (ideally at least 1.40) in either epifluorescence or TIRF mode with excitation at wavelengths between 400-580 nm and an electron-multiplying CCD camera. Exact imaging parameters will vary between microscopy setup.</li>
</ol>

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No conflicts of interest declared.

Compared to conventional CdSe quantum dots, ternary alloy HgxCd1-xSe nanocrystals can be tuned in size and fluorescence wavelength independently. The size is first selected during the synthesis of CdSe nanocrystal cores, and the fluorescence wavelength is chosen in a secondary mercury cation exchange step, which does not substantially alter the nanocrystal size 9. It is important to allow the purified HgxCd1-xSe nanocrystals to incubate at room temperature fo...

<table border="1">
 <tbody>
 <tr>
 <td>Name of the reagent</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments (optional)</td>
 </tr>
 <tr>
 <td>Selenium</td>
 <td>Sigma-Aldrich</td>
 <td>229865</td>
 <td></td>
 </tr>
 <tr>
 <td>Tri-n-octylphosphine</td>
 <td>Strem</td>
 <td>15-6655</td>
 <td>97% pure, unstable in air</td>
 </tr>
 <tr>
 <td>Cadmium oxide</td>
 <td>Sigma-Aldrich</td>
 <td>202894</td>
 <td>Highly toxic: use caution </td>
 </tr>
 <tr>
 <td>Tetradecylphosphonic acid</td>
 <td>PCI Synthesis</td>
 <td>4671-75-4</td>
 <td></td>
 </tr>
 <tr>
 <td>Octadecene</td>
 <td>Alfa Aesar</td>
 <td>L11004</td>
 <td>Technical grade</td>
 </tr>
 <tr>
 <td>Hexadecylamine</td>
 <td>Sigma-Aldrich</td>
 <td>H7408</td>
 <td></td>
 </tr>
 <tr>
 <td>Diphenylphosphine</td>
 <td>Sigma-Aldrich</td>
 <td>252964</td>
 <td>Pyrophoric</td>
 </tr>
 <tr>
 <td>Mercury acetate</td>
 <td>Sigma-Aldrich</td>
 <td>456012</td>
 <td>Highly toxic: use caution</td>
 </tr>
 <tr>
 <td>1-Octanethiol</td>
 <td>Sigma-Aldrich</td>
 <td>471836</td>
 <td>Strong odor</td>
 </tr>
 <tr>
 <td>Oleic acid</td>
 <td>Sigma-Aldrich</td>
 <td>W281506</td>
 <td></td>
 </tr>
 <tr>
 <td>Zinc acetate </td>
 <td>Alfa Aesar</td>
 <td>35792</td>
 <td></td>
 </tr>
 <tr>
 <td>Cadmium acetate hydrate</td>
 <td>Sigma-Aldrich</td>
 <td>229490</td>
 <td>Highly toxic: use caution</td>
 </tr>
 <tr>
 <td>Oleylamine</td>
 <td>Fisher Scientific</td>
 <td>AC12954</td>
 <td>Unstable in air</td>
 </tr>
 <tr>
 <td>Sulfur</td>
 <td>Sigma-Aldrich</td>
 <td>344621</td>
 <td></td>
 </tr>
 <tr>
 <td>Trioctylphosphine oxide</td>
 <td>Strem</td>
 <td>15-6661</td>
 <td>99%</td>
 </tr>
 <tr>
 <td>Pyridine</td>
 <td>VWR</td>
 <td>EM-PX2012-6</td>
 <td>Anhydrous</td>
 </tr>
 <tr>
 <td>Thioglycerol</td>
 <td>Sigma-Aldrich</td>
 <td>M1753</td>
 <td>Strong odor</td>
 </tr>
 <tr>
 <td>Triethylamine</td>
 <td>Sigma-Aldrich</td>
 <td>471283</td>
 <td>Anhydrous</td>
 </tr>
 <tr>
 <td>Dialysis tubing</td>
 <td>Spectrum Labs</td>
 <td>131342</td>
 <td>20 kDa cutoff</td>
 </tr>
 <tr>
 <td>Centrifugal filter</td>
 <td>Millipore</td>
 <td>UFC801024</td>
 <td>10 kDa cutoff</td>
 </tr>
 <tr>
 <td>Monoamino-PEG</td>
 <td>Rapp Polymere</td>
 <td>12 750-2</td>
 <td>750 Da</td>
 </tr>
 <tr>
 <td>DMTMM, 4-(4,6-Dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride hydrate</td>
 <td>Alfa Aesar</td>
 <td>H26333</td>
 <td></td>
 </tr>
 <tr>
 <td>AKTAprime Plus Chromatography System</td>
 <td>GE HealthCare</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Superose 6 10/300 GL chromatography column</td>
 <td>GE HealthCare</td>
 <td>17-5172-01</td>
 <td></td>
 </tr>
 <tr>
 <td>Agarose, OmniPur</td>
 <td>VWR</td>
 <td>EM-2120</td>
 <td></td>
 </tr>
 <tr>
 <td></td>
 <td></td>
 <td></td>
 <td>Appendix
Synthesis of mercury octanethiolate: Slowly add a methanol solution of mercury acetate (1 eq.) to a stirring solution of 1-octanethiol (3 eq.) and potassium hydroxide (3 eq.) in methanol at room temperature. Isolate the mercury(II) octanethiolate precipitate via filtration, wash two times with methanol and once with ether, and then dry under vacuum.
Synthesis of multidentate polymer: Dissolve polyacrylic acid (1 g, 1,773 Da) in 25 ml dimethylformamide (DMF) in a 150 ml three-necked flask and bubble with argon for 30 min. Add an anhydrous solution of cysteamine (374 mg, 4.87 mmol) in 10 ml DMF. At room temperature with vigorous stirring, slowly add anhydrous diisopropylcarbodiimide (DIC, 736 mg, 5.83 mmol) over 30 min, followed by triethylamine (170 &mu;l, 1.22 mmol), and allow the reaction to proceed for 72 hr at 60 &deg;C. Add mercaptoethanol (501 mg, 6.41 mmol) to quench the reaction, and stir for 2 hr at room temperature. Remove DMF via rotary evaporation and isolate the polymer with the addition of a 2:1 mixture of ice-cold acetone:chloroform, followed by centrifugation. Dissolve the polymer in ~5 ml anhydrous DMF, filter, precipitate again with diethyl ether, and repeat. Dry the product under vacuum and store under argon.
Determination of CdSe core diameter: From the UV-Vis absorption spectrum determine the wavelength of the first exciton peak (&lambda;, in nm), which is the longest-wavelength peak (e.g. roughly 498 nm for CdSe in Figure 2a), and use the sizing curve of Mulvaney and coworkers 12:
<img src="/files/ftp_upload/4236/4236eq1.jpg" alt="Equation 1" />
Determination of CdSe nanocrystal concentration: From a background-subtracted UV-Vis spectrum of an optically clear solution of CdSe nanocrystals, determine the absorption at 350 nm wavelength. Serial dilutions can be used to determine if the optical absorption is within the linear range of Beer's Law. The nanocrystal concentration (QD, in M) can be determined by plugging in the nanocrystal diameter (D, in nm), the optical absorption value (A3sa), and the cuvette path length (l, in cm) into the following equation from the empirical correlation of Bawendi and coworkers 13:
<img src="/files/ftp_upload/4236/4236eq2.jpg" alt="Equation 2" /></td>
 </tr>
 </tbody>
</table>

compact quantum dots for single-molecule imaging

Single-molecule imaging is an important tool for understanding the mechanisms of biomolecular function and for visualizing the spatial and temporal heterogeneity of molecular behaviors that underlie cellular biology 1-4. To image an individual molecule of interest, it is typically conjugated to a fluorescent tag (dye, protein, bead, or quantum dot) and observed with epifluorescence or total internal reflection fluorescence (TIRF) microscopy. While dyes and fluorescent proteins have been the mainstay of fluorescence imaging for decades, their fluorescence is unstable under high photon fluxes necessary to observe individual molecules, yielding only a few seconds of observation before complete loss of signal. Latex beads and dye-labeled beads provide improved signal stability but at the expense of drastically larger hydrodynamic size, which can deleteriously alter the diffusion and behavior of the molecule under study.
Quantum dots (QDs) offer a balance between these two problematic regimes. These nanoparticles are composed of semiconductor materials and can be engineered with a hydrodynamically compact size with exceptional resistance to photodegradation 5. Thus in recent years QDs have been instrumental in enabling long-term observation of complex macromolecular behavior on the single molecule level. However these particles have still been found to exhibit impaired diffusion in crowded molecular environments such as the cellular cytoplasm and the neuronal synaptic cleft, where their sizes are still too large 4,6,7.
Recently we have engineered the cores and surface coatings of QDs for minimized hydrodynamic size, while balancing offsets to colloidal stability, photostability, brightness, and nonspecific binding that have hindered the utility of compact QDs in the past 8,9. The goal of this article is to demonstrate the synthesis, modification, and characterization of these optimized nanocrystals, composed of an alloyed HgxCd1-xSe core coated with an insulating CdyZn1-yS shell, further coated with a multidentate polymer ligand modified with short polyethylene glycol (PEG) chains (Figure 1). Compared with conventional CdSe nanocrystals, HgxCd1-xSe alloys offer greater quantum yields of fluorescence, fluorescence at red and near-infrared wavelengths for enhanced signal-to-noise in cells, and excitation at non-cytotoxic visible wavelengths. Multidentate polymer coatings bind to the nanocrystal surface in a closed and flat conformation to minimize hydrodynamic size, and PEG neutralizes the surface charge to minimize nonspecific binding to cells and biomolecules. The end result is a brightly fluorescent nanocrystal with emission between 550-800 nm and a total hydrodynamic size near 12 nm. This is in the same size range as many soluble globular proteins in cells, and substantially smaller than conventional PEGylated QDs (25-35 nm).

Figure 2 depicts representative absorption and fluorescence spectra for CdSe nanocrystals, HgxCd1-xSe nanocrystals after cation exchange, and HgxCd1-xSe/CdyZn1-yS nanocrystals after shell growth. The core CdSe nanocrystals have a quantum yield of fluorescence near 15% (including long-wavelength deep-trap emission) but this efficiency drops to less than 1% after mercury exchange, likely due to charge carrier traps introduced through surface...

Watch this Scientific Journal Video about Compact Quantum Dots for Single-molecule Imaging at JoVE.com

Compact Quantum Dots for Single-molecule Imaging

We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

Single-molecule imaging is an important tool for  understanding the mechanisms of biomolecular function and for visualizing the  spatial and temporal ...

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18.0K Views.  Emory University. We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

Video: Compact Quantum Dots for Single-molecule Imaging

Gradient Echo Quantum Memory in Warm Atomic Vapor

Gradient echo memory (GEM) is a protocol for storing optical quantum states of light in atomic ensembles. The primary motivation for such a technology is that quantum key distribution (QKD), which uses Heisenberg uncertainty to guarantee security of cryptographic keys, is limited in transmission distance. The development of a quantum repeater is a possible path to extend QKD range, but a repeater will need a quantum memory. In our experiments we use a gas of rubidium 87 vapor that is contained in a warm gas cell. This makes the scheme particularly simple. It is also a highly versatile scheme that enables in-memory refinement of the stored state, such as frequency shifting and bandwidth manipulation. The basis of the GEM protocol is to absorb the light into an ensemble of atoms that has been prepared in a magnetic field gradient. The reversal of this gradient leads to rephasing of the atomic polarization and thus recall of the stored optical state. We will outline how we prepare the atoms and this gradient and also describe some of the pitfalls that need to be avoided, in particular four-wave mixing, which can give rise to optical gain.

The gradient echo memory is a protocol for storing optical quantum states of light in atomic ensembles. Quantum memory is a key element of a quantum repeater, which can extend the range of quantum key distribution. We outline the operation of the scheme when implemented in a 3-level atomic ensemble.

Gradient  echo memory (GEM) is a protocol for storing optical quantum states of light in  atomic ensembles. The primary motivation for such a technology ...

Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and entanglement, to solve certain problems exponentially faster than with the best known algorithms on a classical computer. Gate-defined lateral quantum dots on GaAs/AlGaAs are one of many avenues explored for the implementation of a qubit. When properly fabricated, such a device is able to trap a small number of electrons in a certain region of space. The spin states of these electrons can then be used to implement the logical 0 and 1 of the quantum bit. Given the nanometer scale of these quantum dots, cleanroom facilities offering specialized equipment- such as scanning electron microscopes and e-beam evaporators- are required for their fabrication. Great care must be taken throughout the fabrication process to maintain cleanliness of the sample surface and to avoid damaging the fragile gates of the structure. This paper presents the detailed fabrication protocol of gate-defined lateral quantum dots from the wafer to a working device. Characterization methods and representative results are also briefly discussed. Although this paper concentrates on double quantum dots, the fabrication process remains the same for single or triple dots or even arrays of quantum dots. Moreover, the protocol can be adapted to fabricate lateral quantum dots on other substrates, such as Si/SiGe.

This paper presents a detailed fabrication protocol for gate-defined semiconductor lateral quantum dots on gallium arsenide heterostructures. These nanoscale devices are used to trap few electrons for use as quantum bits in quantum information processing or for other mesoscopic experiments such as coherent conductance measurements.

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and ...

Analysis of Contact Interfaces for Single GaN Nanowire Devices

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO2 surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO2 substrate.

A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit  a strong degradation after annealing due to the  occurrence of void formation at the ...

Construction and Characterization of External Cavity Diode Lasers for Atomic Physics

Since their development in the late 1980s, cheap, reliable external cavity diode lasers (ECDLs) have replaced complex and expensive traditional dye and Titanium Sapphire lasers as the workhorse laser of atomic physics labs1,2. Their versatility and prolific use throughout atomic physics in applications such as absorption spectroscopy and laser cooling1,2 makes it imperative for incoming students to gain a firm practical understanding of these lasers. This publication builds upon the seminal work by Wieman3, updating components, and providing a video tutorial. The setup, frequency locking and performance characterization of an ECDL will be described. Discussion of component selection and proper mounting of both diodes and gratings, the factors affecting mode selection within the cavity, proper alignment for optimal external feedback, optics setup for coarse and fine frequency sensitive measurements, a brief overview of laser locking techniques, and laser linewidth measurements are included.

This is an instructional paper to guide the construction and diagnostics of external cavity diode lasers (ECDLs), including component selection and optical alignment, as well as the basics of frequency reference spectroscopy and laser linewidth measurements for applications in the field of atomic physics.

Since their development in the late 1980s, cheap, reliable external cavity diode lasers (ECDLs) have replaced complex and expensive traditional dye and ...

Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states are indeed the resources for implementing various protocols, ranging from enhanced metrology to quantum communication and computing. A variety of devices can be used to generate non-classical states, such as single emitters, light-matter interfaces or non-linear systems3. We focus here on the use of a continuous-wave optical parametric oscillator3,4. This system is based on a non-linear &#967;2 crystal inserted inside an optical cavity and it is now well-known as a very efficient source of non-classical light, such as single-mode or two-mode squeezed vacuum depending on the crystal phase matching. 
Squeezed vacuum is a Gaussian state as its quadrature distributions follow a Gaussian statistics. However, it has been shown that number of protocols require non-Gaussian states5. Generating directly such states is a difficult task and would require strong &#967;3 non-linearities. Another procedure, probabilistic but heralded, consists in using a measurement-induced non-linearity via a conditional preparation technique operated on Gaussian states. Here, we detail this generation protocol for two non-Gaussian states, the single-photon state and a superposition of coherent states, using two differently phase-matched parametric oscillators as primary resources. This technique enables achievement of a high fidelity with the targeted state and generation of the state in a well-controlled spatiotemporal mode.

We describe the reliable generation of non-Gaussian states of traveling optical fields, including single-photon states and coherent state superpositions, using a conditional preparation method operated on the non-classical light emitted by optical parametric oscillators. Type-I and type-II phase-matched oscillators are considered and common procedures, such as the required frequency filtering or the high-efficiency quantum state characterization by homodyning, are detailed.

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states ...

Investigating Single Molecule Adhesion by Atomic Force Spectroscopy

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. At the same time the AFM tip is passivated to prevent unspecific interactions between the tip and the substrate, which is a prerequisite to study single molecules attached to the AFM tip. Analyses to determine the adhesion force, the adhesion length, and the free energy of these molecules on solid surfaces and bio-interfaces are shortly presented and external references for further reading are provided. Example molecules are the poly(amino acid) polytyrosine, the graft polymer PI-g-PS and the phospholipid POPE (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine). These molecules are desorbed from different surfaces like CH3-SAMs, hydrogen terminated diamond and supported lipid bilayers under various solvent conditions. Finally, the advantages of force spectroscopic single molecule experiments are discussed including means to decide if truly a single molecule has been studied in the experiment.

A protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. Procedures and examples to determine the adhesion force and free energy of these molecules on solid supports and bio-interfaces are provided.

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Silicon Metal-oxide-semiconductor Quantum Dots for Single-electron Pumping

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by quantum mechanical effects such as tunneling through single dopants, scattering via interface defects, and discrete trap charge states. However, progress in silicon technology has shown that these phenomena can be harnessed and exploited for a new class of quantum-based electronics. Among others, multi-layer-gated silicon metal-oxide-semiconductor (MOS) technology can be used to control single charge or spin confined in electrostatically-defined quantum dots (QD). These QD-based devices are an excellent platform for quantum computing applications and, recently, it has been demonstrated that they can also be used as single-electron pumps, which are accurate sources of quantized current for metrological purposes. Here, we discuss in detail the fabrication protocol for silicon MOS QDs which is relevant to both quantum computing and quantum metrology applications. Moreover, we describe characterization methods to test the integrity of the devices after fabrication. Finally, we give a brief description of the measurement set-up used for charge pumping experiments and show representative results of electric current quantization.

The fabrication process and experimental characterization techniques relevant to single-electron pumps based on silicon metal-oxide-semiconductor quantum dots are discussed.

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by ...

Use of a Multi-compartment Dynamic Single Enzyme Phantom for Studies of Hyperpolarized Magnetic Resonance Agents

Imaging of hyperpolarized substrates by magnetic resonance shows great clinical promise for assessment of critical biochemical processes in real time. Due to fundamental constraints imposed by the hyperpolarized state, exotic imaging and reconstruction techniques are commonly used. A practical system for characterization of dynamic, multi-spectral imaging methods is critically needed. Such a system must reproducibly recapitulate the relevant chemical dynamics of normal and pathological tissues. The most widely utilized substrate to date is hyperpolarized [1-13C]-pyruvate for assessment of cancer metabolism. We describe an enzyme-based phantom system that mediates the conversion of pyruvate to lactate. The reaction is initiated by injection of the hyperpolarized agent into multiple chambers within the phantom, each of which contains varying concentrations of reagents that control the reaction rate. Multiple compartments are necessary to ensure that imaging sequences faithfully capture the spatial and metabolic heterogeneity of tissue. This system will aid the development and validation of advanced imaging strategies by providing chemical dynamics that are not available from conventional phantoms, as well as control and reproducibility that is not possible in vivo.

A multi-compartment dynamic phantom is used to simulate some biology of interest for metabolic studies using hyperpolarized magnet resonance agents.

Imaging of hyperpolarized substrates by magnetic resonance shows great clinical promise for assessment of critical biochemical processes in real time. Due ...

Compact Lens-less Digital Holographic Microscope for MEMS Inspection and Characterization

A micro-electro-mechanical-system (MEMS) is a widely used component in many industries, including energy, biotechnology, medical, communications, and automotive. However, effective inspection and characterization metrology systems are needed to ensure the functional reliability of MEMS. This study presents a system based on digital holography as a tool for MEMS metrology. Digital holography has gained increasing attention in the past 20 years. With the fast development and decreasing cost of sensor arrays, resolution of such systems has increased broadening potential applications. Thus, it has attracted attention from both research and industry sides as a potential reliable tool for industrial metrology. Indeed, by recording the interference pattern between an object beam (which contains sample height information) and a reference beam on a CCD camera, one can retrieve the quantitative phase information of an object. However, most of digital holographic systems are bulky and thus not easy to implement on industry production lines. The novelty of the system presented is that it is lens-less and thus very compact. In this study, it is shown that the Compact Digital Holographic Microscope (CDHM) can be used to evaluate several characteristics typically consider as criteria in MEMS inspections. The surface profiles of MEMS in both static and dynamic conditions are presented. Comparison with AFM is investigated to validate the accuracy of the CDHM.

We present a compact reflection digital holographic system (CDHM) for inspection and characterization of MEMS devices. A lens-less design using a diverging input wave providing natural geometrical magnification is demonstrated. Both static and dynamic studies are presented.

A micro-electro-mechanical-system (MEMS) is a widely used component in many industries, including energy, biotechnology, medical, communications, and ...