We thank Ji Liu and Wen Xu for involving in the experimental setup. Le Luo is a member of the Indiana University Center for Spacetime Symmetries (IUCSS). This work was supported by IUPUI and IUCRG.

NOTE: This protocol requires a home-built ultracold atom apparatus including the following equipment: two external cavity diode lasers (ECDL), a locking setup for the ECDL offset frequency locking10, a fiber laser for the ODT, an AOM for laser intensity modulation, an radio frequency (rf) antenna system with a source generator and a power amplifier, an absorption imaging system with a CCD camera, a computer program for timing sequence and data acquisition (DAQ), a computer program for imaging processing and data analysis, a pair of electromagnets for the MOT and bias magnetic fields, and an ultrahigh vacuum chamber including a 6Li vapor oven and a Zeeman slower (shown in Figure 1).
Caution: Three lasers of different powers and wavelengths are used. Please consult the relevant laser safety data sheets and choose the proper laser safety goggles.
1. Timing Control
NOTE: All timing sequences are controlled by a 128 channel PCI DAQ card through a timing control program. The resolution of the timing sequence is 100 &#181;s. Several instrumentation control programs are used to control the settings of the instruments, such as fiber laser arbitrary function generator (AFG), ODT AFG, arbitrary pulse generator (APG), parametric modulation AFG, MOT multiplexer, rf generator, etc.
<ol>
	<li>Open the timing control program and the control programs for the instruments. 
	NOTE: The timing control program sends TTL (Transistor-transistor logic) signals to the control terminals for running the timing control files. Some instruments are connected to the computer by GPIB (IEEE 488) for real-time control.</li>
	<li>Write the experiment timing file and set the timing parameters as listed in Table 1. 
	NOTE: The after MOT timing sequence is also illustrated by Figure 2.</li>
</ol>
2. CCD Camera Preparation
NOTE: CCD camera is used to record the absorption imaging of the cold atoms, which is the main diagnostic tool of cold atoms.
<ol>
	<li>Turn on the CCD camera driver and its control program. Set the CCD camera to Particle image velocimetry (PIV) mode11. Set the CCD exposure time to 5 ms. 
	NOTE: PIV mode reduces the time gap between the signal and reference frame, which increases the signal-to-noise ratio of the absorption imaging.</li>
	<li>Use an external trigger to control the CCD exposure 
	NOTE: The CCD trigger time is listed in Table 1.</li>
</ol>
3. 671 nm Laser Preparation
NOTE: A 671 nm single frequency ECDL with 500 mW output power is used to generate the MOT cooling and trapping beams. Another 671 nm ECDL of 35 mW is used for absorption imaging. A digital laser current modulation method (DLCM) is applied for laser frequency stabilization10. The related 6Li energy levels are shown in Figure 3a. Room temperature stability of 20 &#177; 1 &#176;C is required for the optimal stability of laser frequency locking.
<ol>
	<li>MOT Laser Preparation 
	NOTE: The optical setup and relevant results of the DLCM method is presented in Reference 10.
	<ol>
		<li>Turn on the 6Li atomic vapor cell heater and warm it up to 340 &#176;C.</li>
		<li>Warm up the laser locking AOM for 1 h.</li>
		<li>Turn on the laser frequency lock controller and open its software. Turn on the laser grating and current modulation of the ECDL in the software. 
		NOTE: The modulation frequency and amplitude of the grating modulation are set to 5 Hz and 1.0 V respectively. The modulation frequency and amplitude of the current modulation are set to 100 kHz and 0.0015 Vpp respectively to reduce the laser linewidth10.</li>
		<li>Turn on the ECDL emission. 
		NOTE: The laser light passes through the MOT optical setup and reaches the experiment vacuum chamber.</li>
		<li>Slightly adjust the current of the ECDL laser manually to tune the laser frequency until the lock-in error signal of the 6Li D2 line is observed, as shown in Figure 3b.</li>
		<li>Set the lock point in the control software to the 22S1/2 (F = 3/2) &#8594; 22P3/2 transition (see Figures 3a, 3b). Then lock the laser frequency to this transition, and adjust the lock point to the center of the transition10. 
		NOTE: Once the laser frequency is locked, the lock-in error signal shows a small fluctuation at the lock point corresponding to the frequency fluctuation around the lock point.</li>
	</ol>
	</li>
	<li>Imaging Laser Preparation 
	NOTE: The optical setup and relevant results of the offset locking method are presented in Reference 10.
	<ol>
		<li>Turn on the offset locking rf signal generator.</li>
		<li>Turn on the modulation of the grating, and increase the modulation amplitude to 2 V.</li>
		<li>Repeat the frequency tuning process in 3.1.4.-3.1.5. to get the laser frequency beating error signal in the oscilloscope and the rf spectrum analyzer.</li>
		<li>Lock the laser frequency to the beating signal of the offset locking through two PID feedback modules. 
		NOTE: Once the laser frequency is locked, the spectrum of the beating signal in the rf spectrum will stop at the locking point.</li>
	</ol>
	</li>
</ol>
4. Absorption Imaging Preparation
NOTE: The atoms are probed with absorption imaging, which needs two image frames. The first one with the atoms is the signal frame, and the second one without atoms is the reference frame.
<ol>
	<li>Turn on an APG and the imaging beam AOM.</li>
	<li>Set the imaging pulse duration to 10 &#181;s, and set the separation time between the two imaging frames to 5.5 ms.</li>
	<li>Set the imaging beam intensity to about 0.3Isat, where Isat = 2.54 mW/cm2 is the saturated absorption intensity of the 6Li D2 line.</li>
</ol>
5. Cooling Atoms with MOT
NOTE: MOT is a widely-used cooling method in ultracold atoms experiments. This section generates a MOT of around one billion 6Li atoms at about 300 &#181;K.
<ol>
	<li>Slow Atom Source
	<ol>
		<li>Turn on the oven heaters.</li>
		<li>After the oven temperatures reach the operational region (refer to Table 2), turn on the cooling fans for the Zeeman slower. Then slowly increase the current of the slower to 9.2 A. Turn on the current of the two crossover coils to 7 A and 1 A respectively. 
		NOTE: The temperature distribution of the oven listed in Table 2 is optimized for collimation and lifetime of the atomic source12. The location of the heaters on the oven is shown in Figure 4.</li>
		<li>Unblock the Zeeman slower laser beam manually by opening the atomic shutter. Set the frequency of the laser beam to 192 MHz red-detuned with the 22S1/2 (F = 3/2) &#8594; 22P3/2 transition. 
		NOTE: With this setup, the speed of the atoms is slowed down from 1,400 m/s to 100 m/s. The Zeeman slower is shown in Figure 5.</li>
	</ol>
	</li>
	<li>Magnetic Field Gradient 
	NOTE: This apparatus uses a pair of coils controlled by an H-bridge switch circuit to produce either an anti-Helmholtz or Helmholtz magnetic field. The coils are water cooled to prevent overheating.
	<ol>
		<li>Slowly turn on the water flow to 6 gal/min.</li>
		<li>Set the H-bridge for anti-Helmholtz magnetic field configuration by running the timing control program with the MOT loading timing file.</li>
		<li>Turn on the magnets&#39; power supplies, and set the current of each coil to about 18 A via its control program, which creates a magnetic field gradient of about 22 G/cm for the MOT. 
		NOTE: A static MOT is observed in the experiment chamber after the magnetic field gradient is turned on.</li>
	</ol>
	</li>
	<li>Dynamic MOT 
	NOTE: The optical setup of the 6Li MOT contains three pairs of counter propagating MOT beams with all pairs orthogonal to each other. Each MOT beam includes a cooling beam and a repumping beam. The intensities and frequency detunings of the beams, which are controlled by AOMs, are varied for the three phases. The control voltages of the AOMs are set via multiplexer circuits commanded by a timing control system. The parameters for three phases are listed in Table 3. The optical layout of the MOT beams is shown in Figure 6.
	<ol>
		<li>Load, compile and run the experiment timing file in the timing control program on a loop with the software control. The experiment timing starts with the MOT loading phase. Monitor the MOT fluorescence signal in the photodetector to reach 2 V, which indicates around 109 atoms in the MOT. 
		NOTE: The fluorescence of the MOT is collected by a lens with spatial angle of about 10-4 rad. The loading phase atom number can be calculated by the method in Reference 13.</li>
		<li>Use the optical shutter to block the slowing beam before the loading phase ends. 
		NOTE: The timing of the slowing beam shutter is also under control of the experiment timing, which is listed in Table 1.</li>
		<li>Set intensities and frequency detunings of the MOT laser beams according to Table 3 for the cooling phase. 
		NOTE: After the cooling phase, the temperature of the MOT is reduced to about 300 &#181;K.</li>
		<li>For the pumping phase, program the experiment timing file to turn off the repumping beams with the AOM. 
		NOTE: The pumping phase pumps all the atoms into the lowest hyperfine states 22S1/2 (F = 1/2).</li>
		<li>Turn off the MOT beams and shift the laser frequency 30 MHz below the atomic transition resonance by AOM, and block the leaking light from the AOMs with optical shutters. 
		NOTE: After the MOT stage, any leakage of the resonant light to the atomic cloud will result in atom loss. The timing of the AOM control and MOT beam shutter are all listed in Table 1.</li>
		<li>After the dynamic MOT, acquire the imaging frames from the camera. Get the absorption imaging of the MOT. 
		NOTE: The atomic number of the MOT is about 107 after the pumping phase. A typical absorption image of the MOT is shown in Figure 7a.</li>
	</ol>
	</li>
</ol>
6. Preparing an Ultracold Fermi Gas with ODT
<ol>
	<li>Optical Dipole Trap 
	NOTE: ODT is the main tool to generate ultracold Fermi gases. In order to generate a deep ODT, a fiber laser with 100 W emission power at 1064 nm wavelength is used. The setup of ODT is shown in Figure 8.
	<ol>
		<li>Turn on the water flow for cooling the laser beam dumps.</li>
		<li>Set the ODT AOM control voltage to 1 V manually. Turn on the fiber laser with 13 W emission power.</li>
		<li>Check the ODT optics with an infrared light viewer, and remove any dust with argon gas flow. 
		NOTE: Dust on the optics can change the spatial profile of the ODT, and cause instability of the ODT.</li>
		<li>Command the fiber laser AFG to generate a laser pulse via the AFG control program. 
		NOTE: The output of the laser pulse is triggered by the experiment timing, and the starting time of this pulse is set to 14 ms before the end of the MOT loading phase. The pulse sequence control is shown in Figure 1, and the timing is listed in Table 1.</li>
		<li>Manually set the ODT AOM control voltage to 8 V (80% of the saturated rf power). 
		NOTE: The maximum rf power of the AOM driver should be limited to 80% of the saturated power to reduce the thermal lensing effect.</li>
		<li>Acquire the absorption images of the MOT and ODT from the camera. 
		NOTE: Check the overlap of the MOT and ODT through their absorption imaging. Figure 7b shows typical absorption images of the MOT and ODT, respectively.</li>
	</ol>
	</li>
	<li>Bias Magnetic Field and Spin Mixing rf Field 
	NOTE: In order to generate an interacting Fermi gas, a bias magnetic field in the vertical direction is applied to tune the s-wave scattering length.
	<ol>
		<li>Set the H-bridge in the experiment timing program so that the magnetic field configuration changes from anti-Helmholtz to Helmholtz. 
		NOTE: The Helmholtz coils generate the bias magnetic field for tuning interatomic interaction.</li>
		<li>Set the bias magnetic field to 330 G in channel 2 and 527.3 G in channel 3 of the magnets control program.</li>
		<li>Program the experiment timing sequence to sweep the magnetic field from 0 G to 330 G after the MOT is turned off. 
		NOTE: This magnetic field sweep prepares a weakly interacting 6Li Fermi gas for standard evaporative cooling.</li>
		<li>Program a magnetic field sweep from 330 G to 527 G for a noninteracting Fermi gas14. 
		NOTE: The magnetic field sequence from 6.2.1-6.2.4. is shown in Figure 1, and the timing is listed in Table 1.</li>
		<li>Apply a noisy rf pulse to create a 50:50 mixture of the two lowest hyperfine states 22S1/2(F = 1/2, mF= &#177;1/2) of 6Li.</li>
		<li>Tune the locked laser frequency resonant with the atoms at 527.3 G (corresponding to the transition 22S1/2 (F = 1/2, mF = -1/2)&#160;&#8594; 22P3/2 at the low magnetic field) by changing the output frequency of the rf signal generator. 
		NOTE: The resonant frequency maximizes the atom number of the absorption imaging, which is used to guide the frequency adjustment. Only the spin-down atoms are imaged to present the atomic cloud because the 50:50 spin mixtures are used for the experiment.</li>
	</ol>
	</li>
	<li>Evaporative Cooling by Trap Lowering 
	NOTE: A standard evaporative cooling is used to cool the fermionic atoms of 6Li near the degenerate regime. The first stage of evaporative cooling is controlled by the pulse of the fiber laser and the second is controlled by the ODT AOM. The near-degenerate Fermi gas will be used as the sample for parametric cooling.
	<ol>
		<li>Start the first stage of evaporative cooling with the control software by pulsing the fiber laser power, which increases the trap depth of the ODT to U0, then back to 0.1U0 (U0 is the full trap depth with the laser power of 100 W). The total time of this stage is 0.5 s. 
		NOTE: The pulse duration corresponding to U0 should be limited to 0.5 s to avoid the thermal lensing effect.</li>
		<li>Program the ODT AOM with an exponential curve as shown in Figure 1. After the first stage of evaporative cooling is finished, wait 30 ms, and then start the second stage of evaporative cooling by lowering the trap depth from 0.1U0 to 0.01U0 through the ODT AOM. The total time of this stage is 1.5 s.</li>
		<li>Acquire the absorption imaging of the cold atoms after the evaporative cooling. 
		NOTE: About 105 atoms are left in the ODT after evaporative cooling, which can be calculated from the absorption image.</li>
	</ol>
	</li>
</ol>
7. Parametric Cooling
<ol>
	<li>Trap Depth Modulation
	<ol>
		<li>Wait 100 ms after the magnetic sweep to 527.3 G. Modulate the trap depth with the ODT AOM by U(tm) = 0.01U0(1+&#948;cos(&#969;m tm)), where &#948; is the modulation depth and &#969;m is the modulation frequency. Set the modulation time tm in the parametric modulation AFG control program. The time sequence of the modulation is shown in Figure 1. 
		NOTE: This is the key step of implementing parametric cooling.</li>
		<li>Program the APG to release the atoms from the ODT by abruptly turning off the trapping beams. Let the gas ballistically expand for 300 &#181;s before applying absorption imaging. 
		NOTE: The ballistic expansion is used with TOF absorption imaging to get the temperature of the cold atoms.</li>
		<li>Acquire the absorption image of the cold atoms after parametric cooling.</li>
	</ol>
	</li>
	<li>Time Dependence Measurement 
	NOTE: In our previous work7, we found the optimized frequency of the parametric cooling to be 1.45&#969;x, where &#969;x is the radial trapping frequency of ODT at 0.01U0. Using this frequency, we can selectively remove high-energy atoms along the axial direction.
	<ol>
		<li>Set the modulation depth to &#948; = 0.5 via the parametric modulation AFG control program.</li>
		<li>Use the external trigger control function of the parametric modulation AFG to change the modulation time from 0 to 600 ms by varying the modulation cycle numbers. 
		NOTE: With the increasing of modulation time, the size of the atomic cloud will be reduced, especially the axial direction. The relevant results are shown in Figure 9.</li>
		<li>Acquire the imaging frames from the camera. Save and analyze the images through the CCD control program.</li>
	</ol>
	</li>
</ol>

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We present an experimental protocol for parametric cooling of a noninteracting Fermi gas in a crossed-beam optical trap. The critical steps of this protocol include: First, the optically-trapped Fermi gas needs to be cooled close to the degenerate temperature by lowering the trap depth. Second, a modulation frequency is chosen that is resonant with the anharmonic component of the trapping potential. Third, the intensity of the trapping beam is modulated to cool the atomic cloud and measure the dependence of the cloud ene...

In the past two decades, various cooling techniques have been developed for generating Bose-Einstein condensates (BEC) and degenerate Fermi gases (DFG) from hot atomic vapors1,2,3,4,5. BEC and DFG are novel phases of matter that exist in extremely low temperatures, usually one millionth of a degree above absolute zero temperature, far below those normally found on Earth or in space. To obtain such low temperatures, most cooling methods rely on lowering the trapping potential to evaporatively cool the atoms. However, the lowering scheme also decreases the collision rate of the atoms, which limits the cooling efficiency when the gas reaches the quantum regime6. In this article, we present an &#34;expelling&#34; method to evaporatively cool an ultracold Fermi gas in an ODT without lowering the trap depth. This method is based on our recent study of parametric cooling7, showing several advantages compared to the lowering schemes7,8,9.
The key idea of the parametric scheme is to employ the anharmonicity of the crossed-beam ODT, which makes the hotter atoms near the edge of the trapping potential feel the lower trapping frequencies than the colder atoms in the center. This anharmonicity allows the hotter atoms to be selectively expelled from the trap when modulating the trapping potential at frequencies resonant with the high-energy atoms.
The experimental protocol of parametric cooling requires a pre-cooled noninteracting Fermi gas near the degenerate temperature. To implement this protocol, an acousto-optic modulator (AOM) is used to modulate the intensity of the trapping beams by controlling the modulation frequency, depth and time. To verify the cooling effect, the atomic cloud is probed by absorption imaging of time-of-flight (TOF), where a resonant laser beam illuminates the atomic cloud and the absorption shadow is captured by a charge coupled device (CCD) camera. The cloud properties, such as the atom number, energy, and temperature, are determined by the column density. To characterize the cooling effect, we measure the dependence of the cloud energies on the various modulation times.

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cooling an optically trapped ultracold fermi gas by periodical driving

We present a cooling method for a cold Fermi gas by parametrically driving atomic motions in a crossed-beam optical dipole trap (ODT). Our method employs the anharmonicity of the ODT, in which the hotter atoms at the edge of the trap feel the anharmonic components of the trapping potential, while the colder atoms in the center of the trap feel the harmonic one. By modulating the trap depth with frequencies that are resonant with the anharmonic components, we selectively excite the hotter atoms out of the trap while keeping the colder atoms in the trap, generating parametric cooling. This experimental protocol starts with a magneto-optical trap (MOT) that is loaded by a Zeeman slower. The precooled atoms in the MOT are then transferred to an ODT, and a bias magnetic field is applied to create an interacting Fermi gas. We then lower the trapping potential to prepare a cold Fermi gas near the degenerate temperature. After that, we sweep the magnetic field to the noninteracting regime of the Fermi gas, in which the parametric cooling can be manifested by modulating the intensity of the optical trapping beams. We find that the parametric cooling effect strongly depends on the modulation frequencies and amplitudes. With the optimized frequency and amplitude, we measure the dependence of the cloud energy on the modulation time. We observe that the cloud energy is changed in an anisotropic way, where the energy of the axial direction is significantly reduced by parametric driving. The cooling effect is limited to the axial direction because the dominant anharmonicity of the crossed-beam ODT is along the axial direction. Finally, we propose to extend this protocol for the trapping potentials of large anharmonicity in all directions, which provides a promising scheme for cooling quantum gases using external driving.

Using this protocol, we study the dependence of the parametric cooling on the modulation time with the optimized modulation frequency and amplitude, both of which have been determined in our previous publication7. We first prepare a noninteracting Fermi gas of 6Li atoms in the two lowest hyperfine states with a temperature of T/TF &#8776; 1.2. Here, TF = (6N)1/3&#295;&#969;/<em...

Watch this Scientific Journal Video about Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving at JoVE.com

Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

We present a parametric driving method to cool an ultracold Fermi gas in a crossed-beam optical dipole trap. This method selectively removes high-energy atoms from the trap by periodically modulating the trap depth with frequencies that are resonant with the anharmonic components of the trapping potential.

We present a cooling method for a cold Fermi gas by parametrically driving atomic motions in a crossed-beam optical dipole trap (ODT). Our method employs ...

cooling-an-optically-trapped-ultracold-fermi-gas-by-periodical-driving

Research

JoVE Journal

Engineering

7.4K Views.  Indiana University–Purdue University Indianapolis (IUPUI). We present a parametric driving method to cool an ultracold Fermi gas in a crossed-beam optical dipole trap. This method selectively removes high-energy atoms from the trap by periodically modulating the trap depth with frequencies that are resonant with the anharmonic components of the trapping potential. 

Video: Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

Hyperpolarized Xenon for NMR and MRI Applications

Nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) suffer from intrinsic low sensitivity because even strong external magnetic fields of ~10 T generate only a small detectable net-magnetization of the sample at room temperature 1. Hence, most NMR and MRI applications rely on the detection of molecules at relative high concentration (e.g., water for imaging of biological tissue) or require excessive acquisition times. This limits our ability to exploit the very useful molecular specificity of NMR signals for many biochemical and medical applications. However, novel approaches have emerged in the past few years: Manipulation of the detected spin species prior to detection inside the NMR/MRI magnet can dramatically increase the magnetization and therefore allows detection of molecules at much lower concentration 2.
Here, we present a method for polarization of a xenon gas mixture (2-5% Xe, 10% N2, He balance) in a compact setup with a ca. 16000-fold signal enhancement. Modern line-narrowed diode lasers allow efficient polarization 7 and immediate use of gas mixture even if the noble gas is not separated from the other components. The SEOP apparatus is explained and determination of the achieved spin polarization is demonstrated for performance control of the method.
The hyperpolarized gas can be used for void space imaging, including gas flow imaging or diffusion studies at the interfaces with other materials 8,9. Moreover, the Xe NMR signal is extremely sensitive to its molecular environment 6. This enables the option to use it as an NMR/MRI contrast agent when dissolved in aqueous solution with functionalized molecular hosts that temporarily trap the gas 10,11. Direct detection and high-sensitivity indirect detection of such constructs is demonstrated in both spectroscopic and imaging mode.

The production of hyperpolarized xenon by means of spin exchange optical pumping (SEOP) is described. This method yields a ~10000-fold enhancement of the nuclear spin polarization of Xe-129 and has applications in nuclear magnetic resonance spectroscopy and imaging. Examples of gas phase and solution state experiments are given.

Nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) suffer from intrinsic low sensitivity because even strong external magnetic fields of ~10 ...

Optical Trapping of Nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam1. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles1. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec1, which has serious implications for biological matter2,3.
A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime4. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure.
The double-nanohole structure has been shown to give a strong local field enhancement5,6. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres7 and 3.4 nm hydrodynamic radius bovine serum albumin proteins8. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.

The following setup approach details low power optical trapping of dielectric nanoparticles using a double-nanohole in metal film.

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping ...

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 ...

Fabrication and Operation of a Nano-Optical Conveyor Belt

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and&#160;limits the&#160;resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.

The scalability and resolution of conventional optical manipulation techniques are limited by diffraction. We circumvent the diffraction limit and describe a method of optically transporting nanoparticles across a chip using a gold surface patterned with a path of closely spaced C-shaped plasmonic resonators.

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological ...

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 ...

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Chemistry

Optical Trap Loading of Dielectric Microparticles In Air

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. 
In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly ...

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 ...

Construction and Operation of a Light-driven Gold Nanorod Rotary Motor System

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and artificial nanomotors and may provide new routes towards single cell analysis, studies of non-equilibrium thermodynamics, and mechanical actuation of nanoscale systems. A facile way to drive rotation is to use focused circularly polarized laser light in optical tweezers. Using this approach, metallic nanoparticles can be operated as highly efficient scattering-driven rotary motors spinning at unprecedented rotation frequencies in water.
In this protocol, we outline the construction and operation of circularly-polarized optical tweezers for nanoparticle rotation and describe the instrumentation needed for recording the Brownian dynamics and Rayleigh scattering of the trapped particle. The rotational motion and the scattering spectra provides independent information on the properties of the nanoparticle and its immediate environment. The experimental platform has proven useful as a nanoscopic gauge of viscosity and local temperature, for tracking morphological changes of nanorods and molecular coatings, and as a transducer and probe of photothermal and thermodynamic processes.

Plasmonic gold nanorods can be trapped in liquids and rotated at kHz frequencies using circularly-polarized optical tweezers. Introducing tools for Brownian dynamics analysis and light scatteringspectroscopy leads to a powerful system for research and application in numerous fields of science.

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and ...

In Situ Measurement of Vacuum Window Birefringence using 25Mg+ Fluorescence

Accurate control of the polarization states of laser light is important in precision measurement experiments. In experiments involving the use of a vacuum environment, the stress-induced birefringence effect of the vacuum windows will affect the polarization states of laser light inside the vacuum system, and it is very difficult to measure and optimize the polarization states of the laser light in situ. The purpose of this protocol is to demonstrate how to optimize the polarization states of the laser light based on the fluorescence of ions in the vacuum system, and how to calculate the birefringence of vacuum windows based on azimuthal angles of external wave plates with Mueller matrix. The fluorescence of 25Mg+ ions induced by laser light that is resonant with the transition of |32P3/2,F = 4, mF&#160;= 4<img align="center" alt="Equation 100" src="/files/ftp_upload/61175/61175img1.jpg" style="color: rgb(68, 0, 68); font-size: 18px; font-weight: 700;" />&#160;&#8594;&#160;|32S1/2,F = 3, mF&#160;= 3<img align="center" alt="Equation 100" src="/files/ftp_upload/61175/61175img1.jpg" style="color: rgb(68, 0, 68); font-size: 18px; font-weight: 700;" />&#160;is sensitive to the polarization state of the laser light, and maximum fluorescence will be observed with pure circularly polarized light. A combination of half-wave plate (HWP) and quarter-wave plate (QWP) can achieve arbitrary phase retardation and is used for compensating the birefringence of the vacuum window. In this experiment, the polarization state of the laser light is optimized based on the fluorescence of 25Mg+ ion with a pair of HWP and QWP outside the vacuum chamber. By adjusting the azimuthal angles of the HWP and QWP to obtain maximum ion fluorescence, one can obtain a pure circularly polarized light inside the vacuum chamber. With the information on the azimuthal angles of the external HWP and QWP, the birefringence of the vacuum window can be determined.

In Situ Measurement of Vacuum Window Birefringence using 25Mg+ Fluorescence

Presented here is a method to measure the birefringence of vacuum windows by maximizing the fluorescence counts emitted by Doppler cooled 25Mg+ ions in an ion trap. The birefringence of vacuum windows will change the polarization states of the laser, which can be compensated by changing the azimuthal angles of external wave plates.

Accurate control of the polarization states of laser light is important in precision measurement experiments. In experiments involving the use of a vacuum ...

Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

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