The authors thank Michael Lacerte for technical support. M.P.-L. acknowledges the Canadian Institute for Advanced Research (CIFAR), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovations (CFI) and Fonds de Recherche Qu&eacute;bec - Nature et Technologies (FRQNT) for financial support. The device presented here was fabricated at CRN2 and IMDQ facilities, funded in part by NanoQu&eacute;bec. The GaAs/AlGaAs substrate was fabricated by Z.R. Wasilewski from the Institute of Microstructural Sciences at the National Research Council Canada. J.C.L and C.B.-O. acknowledge CRSNG and FRQNT for financial support.

The fabrication process described below is done on a GaAs/AlGaAs substrate with dimensions of 1.04 x 1.04 cm. Twenty identical devices are fabricated on a substrate of this size. All steps of the process are done in a cleanroom and appropriate protective gear must be used at all times. Deionized water is used throughout the process, but is simply referred to as &#34;water&#34; in the protocol below.
1. Etching of the Mesa
The result of this fabrication step is shown in Figure 4a.
<ol>
	<li>Place the sample in a plasma asher with an O2 plasma at 75 W for 2 min to remove any traces of undesired resist or organic compounds.</li>
	<li>Clean the sample in a sonic bath with acetone (2x) and IPA for 5 min each. Blow-dry with a compressed N2 gun. Throughout the fabrication protocol, when using a sonic bath, avoid using high powers to prevent damaging the GaAs wafer that has a tendency to cleave.</li>
	<li>Bake the sample in an oven at 125 &#176;C for at least 15 min to dehydrate the surface. Dehydration can also be achieved by placing the sample on a hot plate at 180 &#176;C for at least 5 min.</li>
	<li>Spin coat the sample with Shipley S1818 photoresist at 3,500 rpm for 30 sec and bake on a hot plate at 115 &#176;C for 60 sec. The resulting layer of resist should be about 2.5 &#956;m thick. S1818 is used because it is thick and is affected very little by the solution used in step 1.11 to etch the substrate. Throughout the fabrication process, it is important to avoid overbaking photoresist and e-beam resist to ensure easy removal in subsequent steps.</li>
	<li>Use a photolithographic mask aligner and an edge bead mask to expose the resist on the outer edge of the sample. The edge bead mask consists of a 1 x 1 cm chrome square on a glass plate. Use light with a wavelength of 436 nm and expose the resist for 10 sec at 15m W/cm2. The wavelength and power are fixed and may vary with the machine used. Adjust the exposure and development times to obtain well-defined features of the proper size.</li>
	<li>Develop exposed resist by immersing the sample in MF-319 for 2 min 10 sec. Agitate slowly during this step. Rinse in water for 15 sec and blow dry with a compressed N2 gun. The edge bead should be removed at the end of this step, leaving a 1 x 1 cm surface with a uniform thickness of resist in the middle of the sample. The edge bead is removed before exposing the mesa mask to obtain a better contact between the mask and the sample, which leads to better results.</li>
	<li>Still using the photolithographic mask aligner, expose the resist using the mesa mask and 436 nm light for 7 sec at 15 mW/cm2.</li>
	<li>Develop the exposed resist by immersing the sample in MF-319 for 2 min 10 sec and agitating slowly. Rinse for 15 sec in water and blow dry with a compressed N2 gun. The resist remaining on the sample now has the shape of the mesa. The resist will protect this region of the sample from being etched away in the steps that follow.</li>
	<li>To remove all traces of resist in the previously exposed area, the sample is placed in a plasma asher and is exposed to an O2 plasma for 1 min at 75 W.</li>
	<li>A solution of H2SO4:H2O2:H2O (5:1:55) is used to etch the sample. Combine the proper proportions of H2O2 and water and then add the H2SO4. This solution is very reactive when it is first prepared. Wait 20 min before proceeding to the following step to avoid overetching.</li>
	<li>Etch the sample by immersing it in the acid solution for several seconds and immediately rinsing in water for 30 sec to stop the reaction. The surface of the sample should be etched past the layer of Si dopants and almost all the way down to the 2DEG to ensure depletion in these regions (see Figure 1). The etch depth can vary depending on the substrate. Generally, for the type of substrate used, a 34 sec immersion leads to the desired etch depth of 90-100 nm. However, it is important not to overetch to avoid damaging the 2DEG. Since the etch rate varies strongly with the ratio of H2SO4 and H2O2 in the solution as well as with the wait time before the etch, it is recommended to perform the etching by several immersions of 5-10 sec, instead of a single 34 sec etch, and measure the etched depth with a profilometer after each etch. The relatively slow etch rate of this solution allows for a good control of the etch depth. Cooling the etching solution below room temperature can lead to lower etch rates. However, since it is a shallow etch and the etch profile is unimportant, different etching solutions can also be used.</li>
	<li>Strip the resist by immersing the sample in 1165 Remover at 65 &#176;C for 3 hr. Rinse in acetone and IPA for 5 min each. Measure the etch profile with a profilometer.</li>
</ol>
2. Fabrication of the Ohmic Contacts
The result of this fabrication step is shown in Figure 4b.
<ol>
	<li>Clean the sample in a sonic bath by immersion in acetone (2x) and IPA for 5 min each. Blow dry with a compressed N2 gun.</li>
	<li>Bake the sample in an oven at 125 &#176;C for 15 min to dehydrate the surface.</li>
	<li>Spin coat with LOR5A at 2,500 rpm for 30 sec and bake on a hot plate at 150 &#176;C for 10 min. Then, spin coat with S1813 at 5,000 rpm for 30 sec and bake on a hot plate at 110 &#176;C for 60 sec. The resulting thicknesses of resist are ~600 nm and ~1.3 &#956;m, respectively. Two layers of resist are used to facilitate the lift-off of the metal that will be deposited in step 2.10.</li>
	<li>Using a photolithographic mask aligner and the edge bead mask, expose the resist on the outer edge of the sample with 436 nm light for 10 sec at 15 mW/cm2.</li>
	<li>Develop the exposed resist by immersing the sample in MF-319 for 2 min 10 sec while agitating smoothly. Rinse in water for 15 sec and blow dry with a compressed N2 gun.</li>
	<li>Still using the photolithographic mask aligner, but this time with the ohmic contact mask, align the patterns for the ohmic contacts on the etched mesa. Expose with 436 nm light for 6 sec at 15 mW/cm2.</li>
	<li>Develop the resist in MF-319 for 2 min 10 sec while agitating smoothly. Rinse in water for 15 sec and blow dry with a compressed N2 gun. There is no longer resist in the regions where the ohmic contacts will be deposited. The developer dissolves the LOR 5A faster than the S1813, leaving an undercut below the top layer of resist. This profile in the resist prevents the metal (that will be deposited in step 2.10) from forming walls at the edges of the resist and will also allow an easy removal of the remaining resist and undesirable metal.</li>
	<li>To remove all traces of resist in the previously exposed area, place the sample in a plasma asher and expose it to an O2 plasma for 1 min at 75 W. It is important not to let the sample sit for too long in the plasma because about 75 nm/min of resist is etched during this step.</li>
	<li>Remove the GaAs native oxide by immersing the sample in a solution of H2SO4:H2O (1:5) for 30 sec and rinsing in water for 30 sec. Blow dry with a compressed N2 gun. Step 2.10 should be performed as soon as possible to prevent the native oxide from reappearing.</li>
	<li>Use an e-beam evaporator to deposit 25 nm of Ni, 55 nm of Ge, and 80 nm of Au. The deposition rates are respectively 0.2 nm/sec, 0.5 nm/sec and 0.5 nm/sec. The deposition rates are chosen so that the evaporation time is short enough to avoid heating the chamber without being so short as to lose precision of the thickness of the deposited layer. The bilayer of Ge and Au can be replaced by a single layer of eutectic GeAu if it is available9.</li>
	<li>The previous step deposited metal on the entire surface of the sample. Removal of the metal that is deposited on the resist is done by dissolving the latter. To do this, immerse the sample in 1165 Remover at 65 &#176;C for 3 hr. To remove any unwanted metal that may not have lifted on its own, use a pipette to lightly spray the surface of the sample with hot remover. Rinse the sample in acetone and IPA for 5 min each.</li>
	<li>A rapid thermal anneal process in forming gas is used to diffuse the deposited metal down to the 2DEG of the sample. The temperature is increased at a rate of 50 &#176;C/sec until a temperature of 415 &#176;C is reached. The sample is left at this temperature for 20 sec and is then cooled down rapidly. The duration and the temperature of the anneal are chosen to obtain the smallest resistance through the ohmic contacts at low temperature. The optimal anneal time can vary depending on the depth of the 2DEG in the substrate.</li>
</ol>
3. Fabrication of the Ti/Au Schottky Leads
The result of this fabrication step is shown in Figure 4c.
<ol>
	<li>Clean the sample in a sonic bath by immersion in acetone (2x) and IPA for 5 min each. Blow dry with a compressed N2 gun.</li>
	<li>Bake the sample in an oven at 125 &#176;C for 15 min to dehydrate the surface.</li>
	<li>Spin coat PMMALMW4% at 5000rpm for 30 sec and bake on a hot plate at 180 &#176;C for 90 sec. Then, spin coat PMMA HMW 2% at 5,000 rpm for 30 sec and bake on a hot plate at 180 &#176;C for 90 sec. The resulting thicknesses of resist are ~75 nm and ~40 nm, respectively. Since the Ti/Au leads will be very thin, a single layer of PMMA can be used, but spinning, exposure and development parameters will need to be adjusted in consequence. This comment also applies to the Al leads and gates (step 4).</li>
	<li>E-beam process: This part of the process is highly dependent on the equipment used and the protocol needed can thus differ greatly from the one described below. The patterns for the leads are drawn in a CAD file. In this file, the leads must be drawn as closed polygons that are 2 &#956;m wide. The leads are needed to contact the bonding pads (fabricated in step 5) to the Al gates (fabricated in step 4). Four alignment marks must also be exposed to allow the vertical, horizontal, and rotational alignment of the Al leads and gates onto the Ti/Au Schottky leads.
	<ol>
		<li>Place the sample in a Scanning Electron Microscope (SEM) that is equipped to do e-beam lithography. Use an aperture of 10 &#956;m to reduce the beam spot size and set the acceleration voltage to 10 kV to ensure a good contrast during the alignment on the previously fabricated structures. Adjust the focus, stigmatism, and the aperture alignment. Measure the beam current with a Faraday cup.</li>
		<li>Align the beam with the previously etched mesa and set the magnification to 300X.</li>
		<li>Expose the leads. The SEM is programmed to expose the desired areas by filling them with an array of exposed dots. The distance between these dots (center-to-center distance) is 5.5 nm and the dose is 43 &#956;C/cm2 (the exposure time will depend on the beam current measured in step 3.5.1). Expose alignment marks as well to allow the alignment of the Al gates with the Ti/Au leads in step 4.</li>
		<li>Repeat steps 3.5.2 and 3.5.3 for each of the 20 devices on the sample.</li>
	</ol>
	</li>
	<li>Develop the resist by immersing the sample in IPA:H2O (9:1) for 30 sec. The solution must be at a temperature of 20 &#176;C. Rinse in water for 30 sec and blow dry with a compressed N2 gun. If preferred, the PMMA resist can be developed using an IPA:MIBK solution and rinsing in IPA to stop the reaction. A different development time will be needed if this option is chosen.</li>
	<li>Place the sample in a plasma asher and expose it to an O2 plasma for 4 sec at 50 W. This removes 5 nm of resist and ensures that there is no resist left at the bottom of the trenches formed during the e-beam process.</li>
	<li>Remove the GaAs native oxide by immersing the sample in a solution of H2SO4:H2O (1:5) for 30 sec and rinse in water for 30 sec. Blow dry with a compressed N2 gun. Step 3.8 should be performed as soon as possible to prevent the native oxide from reappearing.</li>
	<li>Place the sample in an e-beam evaporator and deposit 10 nm of Ti and 20 nm of Au, both at a rate of 0.1 nm/sec. It is important for the sample holder to be well grounded and placed at least 60 cm away from the source to prevent charges from accumulating on the sample during the deposition process.</li>
	<li>Lift-off excess metal by immersing the sample in 1165 Remover at 65 &#176;C for 15 min. To remove any unwanted metal that may not have lifted on its own, use a pipette to lightly spray the surface of the sample with hot remover. Rinse in acetone and IPA for 5 min each.</li>
</ol>
4. Fabrication of the Al Schottky Leads and Gates
The result of this fabrication step is shown in Figure 4d.
The fabrication of the Al Schottky leads and gates constitutes the most critical step of the fabrication process since these are the gates that define the dot. It is important that the electron beam be well focused and the beam current well adjusted in step 4.2. The exposure and development times must also be well adjusted to obtain small, continuous and well-defined gates. In many protocols, these gates and leads are fabricated in Ti/Au and are exposed simultaneously with the previous leads during step 3. However, an advantage of using Al is that it can be oxidized, therefore allowing for elements such as top gates to be deposited directly onto the surface of the sample without the need of a large insulating layer10.
<ol>
	<li>Repeat steps 3.1 to 3.4.2. However, do not use the sonic bath while cleaning the sample to avoid damaging the Ti/Au leads.</li>
	<li>E-beam process: The patterns for the Al leads and gates are drawn in a CAD file. The Al leads are used to contact the Ti/Au leads to the Al gates and are drawn as closed polygons with a width of 200 nm. The Al gates however are not drawn as polygons, but rather as two single lines separated by 20 nm.
	<ol>
		<li>Once the beam has been aligned with the mesa, set the magnification to 1,500X and align the beam with the Ti/Au Schottky leads.</li>
		<li>Expose the leads. Use a dose of 43 &#956;C/cm2 and a center-to-center distance of 3.3 nm.</li>
		<li>Expose the gates. Use a line dose of 0.149 nC/cm and a center-to-center distance of 1.1 nm. This will lead to a final gate width of 60 nm.</li>
		<li>Repeat steps 4.2.1 - 4.2.3 for each of the 20 devices on the sample.</li>
	</ol>
	</li>
	<li>Repeat steps 3.5 - 3.7.</li>
	<li>Place the sample in an e-beam evaporator and deposit 30 nm of Al at a rate of 0.3 nm/sec.</li>
	<li>Repeat step 3.10. It is important to be very delicate with the sample from this step onward to avoid damaging the small Al gates.</li>
</ol>
5. Fabrication of the Schottky Leads and Bonding Pads
The result of this fabrication step is shown in Figure 4e.
<ol>
	<li>Repeat steps 2.1 - 2.9, using a photolithographic mask for the Schottky leads instead of the mask for the ohmic pads.</li>
	<li>Place the sample in an e-beam evaporator and deposit 30 nm of Ti and 350 nm of Au at rates of 0.3 nm/sec and 1 nm/sec respectively. The thick layer of Au deposited facilitates bonding of the sample to the sample holder because it is less prone to tearing than thinner layers.</li>
	<li>Lift-off the excess metal and remaining resist by immersing the sample in 1165 Remover at 65 &#176;C for 3 hr. To remove any unwanted metal that may not have lifted on its own, use a pipette to lightly spray the surface of the sample with hot remover. Rinse the sample in acetone and IPA for 5 min each.</li>
</ol>
6. Dicing of the Samples
<ol>
	<li>Bake the sample in an oven at 125 &#176;C for 15 min to dehydrate the surface.</li>
	<li>Spin coat with S1818 at 3,500 rpm for 30 sec and bake on a hot plate at 115 &#176;C for 1 min. The thickness of this layer of resist is unimportant as long as it is thick enough to protect the surface of the sample during dicing.</li>
	<li>Place a layer of dicing tape on the top and bottom faces of the sample. The layer of tape on the top face offers an additional layer of protection to the device during dicing.</li>
	<li>Place the sample face-up in a dicer. Cut the sample into individual devices by dicing all the way through top layer of dicing tape and the GaAs/AlGaAs wafer without cutting through the bottom layer of dicing tape.</li>
	<li>Remove the dicing tape and the protective resist by placing the sample in acetone for a few minutes. Use tweezers to gently pull off pieces of dicing tape that do not lift on their own. Rinse the devices in IPA and blow dry using an N2 gun.</li>
</ol>

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Authors have nothing to disclose.

The process presented above describes the fabrication protocol of a double quantum dot able to reach the few-electron regime. However, the parameters given may vary depending on the model and calibration of the equipment used. Therefore, parameters such as the doses for exposures during the e-beam and photolithography steps will have to be calibrated before the fabrication of devices. The process can easily be adapted to the fabrication of gate-defined quantum dots on other types of substrates, such as Si/SiGe, that also...

Quantum information science has drawn a lot of attention ever since it was shown that quantum algorithms can be used to solve certain problems exponentially faster than with the best known classical algorithms1. An obvious candidate for a quantum bit (qubit) is the spin of single electron confined in a quantum dot since it is a two-level system. Numerous architectures have been suggested for the implementation of quantum dots, including semiconducting nanowires2, carbon nanotubes3, self-assembled quantum dots4, and semiconductor vertical5 and lateral quantum dots6. Gate-defined lateral quantum dots in GaAs/AlGaAs heterostructures have been very successful because of their versatility and their fabrication process is the focus of this paper.
In lateral quantum dots, the confinement of electrons in the direction perpendicular to the sample surface (z direction) is achieved by choosing the proper substrate. The GaAs/AlGaAs modulation-doped heterostructure presents a two-dimensional electron gas (2DEG) confined to the interface between the AlGaAs and the GaAs layers. These samples are grown by molecular beam epitaxy to obtain a low impurity density which, combined with the modulation-doping technique, leads to high electron mobility in the 2DEG. A schematic of the different layers of the heterostructure as well as its band structure are shown in Figure 1. A high electron mobility is needed in the 2DEG to ensure the coherence of electronic states over the entire surface of the quantum dot. The substrate used for the fabrication process described below was purchased from the National Research Council of Canada and presents an electron density of 2.2 x 1011cm-2 and an electron mobility of 1.69 x 106 cm2/Vsec.
The confinement of electrons in the directions parallel to the sample surface is achieved by placing metallic electrodes on the surface of the substrate. When these electrodes are deposited on the surface of the GaAs sample, Schottky barriers are formed7. Negative voltages applied to such electrodes lead to local barriers in the 2DEG below which only electrons with sufficient energy can cross. Depletion of the 2DEG occurs when the voltage applied is negative enough that no electrons have enough energy to cross the barrier. Therefore, by carefully choosing the geometry of the electrodes, it is possible to trap a small number of electrons between depleted regions of the sample. Control of the number of electrons on the dot as well as the tunneling energy between the dot and the 2DEG in the rest of the sample can be achieved by fine-tuning the voltages on the electrodes. A schematic of the gate electrodes and the depleted electron gas is shown in Figure 2. The design for the gate structures forming the dot is inspired by the design used by Barthel et al.8
To control and read out information regarding the number of electrons on the dot, it is useful to induce and measure current through the dot. Readout can also be done by using a Quantum Point Contact (QPC), which also requires a current through the 2DEG. The contact between the 2DEG and voltage sources is ensured by ohmic contacts. These are metallic pads that are diffused from the surface of the sample all the way down to the 2DEG using a standard rapid thermal anneal process7 (see Figures 3a and 4b). To avoid short circuits between the source and the drain, the surface of the sample is etched so that the 2DEG is depleted in certain regions and the current is forced to travel through certain specific channels (see Figures 3b and 4a). The region where the 2DEG still remains is referred to as the &#34;mesa&#34;.&#160;
The following protocol details the entire fabrication process of a gate-defined lateral quantum dot on a GaAs/AlGaAs substrate. The process is scalable since it remains the same regardless if the device being fabricated is a single, double, or triple quantum dot or even an array of quantum dots. Manipulation, measurement, and results for double quantum dots fabricated using this method are discussed in further sections.

<table border="1">
		<tbody>
 <tr>
 <td>Name of the reagent/material</td>
 <td>Company</td>
 <td>Product number</td>
 <td>CAS number</td>
 </tr>
 <tr>
 <td>Acetone - CH3COCH3</td>
 <td>Anachemia</td>
 <td>AC-0150</td>
 <td>67-64-1</td>
 </tr>
 <tr>
 <td>Isopropyl Alcohol (IPA) - (CH3)2CHOH</td>
 <td>Anachemia</td>
 <td>AC-7830</td>
 <td>67-63-0</td>
 </tr>
 <tr>
 <td>1165 Remover</td>
 <td>MicroChem Corp</td>
 <td>G050200</td>
 <td>872-50-4</td>
 </tr>
 <tr>
 <td>Microposit MF-319 Developer</td>
 <td>Shipley</td>
 <td>38460</td>
 <td>75-59-2</td>
 </tr>
 <tr>
 <td>Sulfuric Acid - H2SO4</td>
 <td>Anachemia</td>
 <td>AC-8750</td>
 <td>766-93-9</td>
 </tr>
 <tr>
 <td>Hydrogen Peroxide (30%) - H2O2</td>
 <td>Fisher Scientific</td>
 <td></td>
 <td>7722-84-1</td>
 </tr>
 <tr>
 <td>LOR 5A Lift-off resist</td>
 <td>MicroChem Corp</td>
 <td>G516608</td>
 <td>120-92-3</td>
 </tr>
 <tr>
 <td>Microposit S1813 Photo Resist</td>
 <td>Shipley</td>
 <td>41280</td>
 <td>108-65-6</td>
 </tr>
 <tr>
 <td>Microposit S1818 Photo Resist</td>
 <td>Shipley</td>
 <td>41340</td>
 <td>108-65-6</td>
 </tr>
 <tr>
 <td>PMMA LMW 4% in anisole</td>
 <td>MicroChem Corp</td>
 <td></td>
 <td>100-66-3, 9011-14-7</td>
 </tr>
 <tr>
 <td>PMMA HMW 2% in anisole</td>
 <td>MicroChem Corp</td>
 <td></td>
 <td>100-66-3, 9011-14-7</td>
 </tr>
 <tr>
 <td >GaAs/AlGaAs wafer</td>
 <td>National Research Council Canada</td>
 <td>See detailed layer structure in Figure 1.</td>
 <td></td>
 </tr>
 <tr>
 <td>Ni (99.0%)</td>
 <td>Anachemia</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Ge (99.999%)</td>
 <td>CERAC inc. </td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Au (99.999%)</td>
 <td>Kamis inc. </td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Ti (99.995%)</td>
 <td>Kurt J Lesker</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Al</td>
 <td>Kamis inc.</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Silver Epoxy</td>
 <td>Epoxy Technology</td>
 <td>H20E</td>
 <td></td>
 </tr>
	 </tbody>
 </table>

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.

One of the critical steps in the process described above is the etching of the mesa (step 1). It is important to etch enough to remove the 2DEG below while avoiding overetching. Therefore, it is recommended to use a bulk GaAs dummy sample to test the etching solution before performing the etch on the GaAs/AlGaAs sample. The etch rate of the GaAs/AlGaAs heterostructure is larger than that of GaAs, but the etching of the dummy can give an indication to whether the solution is more or less reactive than usual and the etch t...

Watch this Scientific Journal Video about Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots at JoVE.com

Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots

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 ...

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16.0K Views.  Université de Sherbrooke. 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.

Video: Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots

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).

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 ...

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 ...

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 ...

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 ...

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

Experimental Methods for Trapping Ions Using Microfabricated Surface Ion Traps

Ions trapped in a quadrupole Paul trap have been considered one of the strong physical candidates to implement quantum information processing. This is due to their long coherence time and their capability to manipulate and detect individual quantum bits (qubits). In more recent years, microfabricated surface ion traps have received more attention for large-scale integrated qubit platforms. This paper presents a microfabrication methodology for ion traps using micro-electro-mechanical system (MEMS) technology, including the fabrication method for a 14 &#181;m-thick dielectric layer and metal overhang structures atop the dielectric layer. In addition, an experimental procedure for trapping ytterbium (Yb) ions of isotope 174 (174Yb+) using 369.5 nm, 399 nm, and 935 nm diode lasers is described. These methodologies and procedures involve many scientific and engineering disciplines, and this paper first presents the detailed experimental procedures. The methods discussed in this paper can easily be extended to the trapping of Yb ions of isotope 171 (171Yb+) and to the manipulation of qubits.

This paper presents a microfabrication methodology for surface ion traps, as well as a detailed experimental procedure for trapping ytterbium ions in a room-temperature environment.

Ions trapped in a quadrupole Paul trap have been considered one of the strong physical candidates to implement quantum information processing. This is due ...

Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). Resonant excitation without fluorescence detection &#8211; for example, a differential transmission measurement &#8211; can determine some properties of the emitting system, but does not allow applications or measurements based on the emitted photons. For example, the measurement of photon correlations, observation of the Mollow triplet, and realization of single photon sources all require collection of the fluorescence. Incoherent excitation with fluorescence detection &#8211; for example, above band-gap excitation &#8211; can be used to create single photon sources, but the disturbance of the environment due to the excitation reduces the indistinguishability of the photons. Single photon sources based on QDs will have to be resonantly excited to have high photon indistinguishability, and simultaneous collection of the photons will be necessary to make use of them. We demonstrate a method to resonantly excite a single QD embedded in a planar cavity by coupling the excitation beam into this cavity from the cleaved face of the sample while collecting the fluorescence along the sample&#39;s surface normal direction. By carefully matching the excitation beam to the waveguide mode of the cavity, the excitation light can couple into the cavity and interact with the QD. The scattered photons can couple to the Fabry-Perot mode of the cavity and escape in the surface normal direction. This method allows complete freedom in the detection polarization, but the excitation polarization is restricted by the propagation direction of the excitation beam. The fluorescence from the wetting layer provides a guide to align the collection path with respect to the excitation beam. The orthogonality of the excitation and detection modes enables resonant excitation of a single QD with negligible laser scattering background.

Resonant excitation of a single self-assembled quantum dot can be achieved using an excitation mode orthogonal to the fluorescence collection mode. We demonstrate a method using the waveguide and Fabry-Perot modes of a planar microcavity surrounding the quantum dots. The method allows complete freedom in the detection polarization.

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). ...