Neophytes build a MOT

Bruce Thompson∗

Department of Physics, Ithaca College, Ithaca NY 14850

Judith OlsonNIST and University of Colorado, Boulder, Colorado

(Dated: July 13, 2012)

Having built a Magneto-Optical Trap (MOT) without the benefit of previous experiencein optics, we thought it would be useful for others, who are considering a build of their own,to read about our experiences. The following is a description of the design decisions andprocess that resulted in an operational MOT using the resources and time available at anundergraduate institution. By building many components and purchasing others, we wereable to complete the MOT in about two years. This account is intentionally conversationaland not meant to be a complete description of a MOT. References are contained herein forreaders to begin their own exploration. Recommendations are given for a phased build of aMOT.This report can be downloaded at: http://faculty.ithaca.edu/bthompso/docs/.

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I. GENESIS OF THE PROJECT

In 2008, we (student JO and faculty member BT) were infected with the desire to builda Magneto-Optical Trap (MOT) even though (or, maybe, because) neither of us had anyexperience in optics. (Note: a glossary of terms is in Appendix A.) Alumnus Evan Salim1

(ES) had returned to Ithaca College to give a talk on the work he was doing in graduateschool on Bose-Einstein Condensates (BEC). The MOT is a first step in cooling atoms tothen make a BEC. Our contagion was fostered by our amazement that light could be used toslow atoms down to speeds equivalent to microKelvin temperatures and by the rich varietyof physics that is used to build the MOT and to describe its operation. The prospectiveproject also provided an opportunity for both of us to strike out into unknown territory -always an exciting notion. Further, the project fit into BT’s desire to offer experimentalresearch experiences and new advanced undergraduate laboratory experiments for physicsmajors.From academic work and work in industry, BT garnered a good bit of experience in

electronic circuit design and construction. BT was also familiar with making parts in amachine shop but did not have a background in optics or in vacuum systems. In addition,BT had been away from thinking about atomic physics since undergraduate school havingstudied geophysical problems in the academic world and in industry.JO began the project with two years of undergraduate physics courses and an academic

year of experience working in another laboratory. In 2009, JO received a summer intern-ship award from the college to work on the MOT. We worked together in that summerand through the following school year. Much to the benefit of the project, JO spent thesubsequent summer at an optical cooling and trapping laboratory in Germany2 and broughtback invaluable practical experience. During academic year 2010-2011, we continued withthe assembly and tuning of the apparatus while JO wrote her senior thesis3 on the MOT.Much to our frustration, we did not succeed in obtaining a cold cloud of atoms (also calleda MOT) before graduation, although BT was successful several months later.This report is conversational in style with the purpose of relating our design decisions

and building experiences. A more complete description of various components can be foundin Olson3. There are many variations that can be made in building the system. Thereferences listed in Appendix B give a starting point for exploration. In this paper, wemention specific manufacturers of components not as endorsements but as examples. Manytimes other manufacturers offer suitable alternatives.

II. OVERALL MOT DESIGN AND OUR APPROACH

As mentioned in section I, the operation of a MOT touches on a variety of physicalconcepts and, thus, provides undergraduates an opportunity to integrate many parts oftheir academic background and to expand their understanding in other areas. Table I listsmost of the physical principles involved and where they are applied.The main components of our MOT are shown in Figure 1. This is a standard MOT layout

as described in Wieman4. Appendix B lists additional references. The beamline begins withan Extended Cavity Diode Laser (EDCL #1), which provides the laser light that cools andtraps the Rubidium atoms in the MOT cell. The laser light is steered, expanded and polar-ized by optical components. The primary beam is split into three perpendicular beams thatpass through the cell and then are reflected back through the cell from the other direction.A small portion of the original beam is split off to Saturated Absorption Spectrometer (SAS)components, which, together with the SideLock Servo (SLS) electronics, provides feedback

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TABLE I: Some physical principles of a MOT

MOT component or topic Physical principle or concept

Extended cavity diode laser Semiconductor physics

(EDCL) Internal lasing modes

2-D aperture diffraction

Simple lens for beam collimation

Diffraction grating

Extended cavity modes and resonant feedback

Electro-mechanical feedback control of grating position

Electrical feedback control of diode current

Laser light and interactions Stimulated emission and coherence

Linear and circular polarization

Absorption; stimulated and spontaneous emission

Photon momentum

Photon angular momentum

Optical components Partial reflection and beamsplitters

Polarizing beamsplitter

Beam expander optics

Half and quarter waveplates

Rubidium characteristics Shell structure and alkali metals

Vapor pressure

Isotopic composition and nuclear spin

Atomic energy levels, fine and hyperfine splitting

Optical molasses Kinetic gas theory

Photon scattering

Doppler shift

Magneto-optical trap Maxwell (anti-Helmholtz) coil magnetic field

Electron magnetic moment and Zeeman splitting

Mechanics of atom trajectories

MOT cloud characteristics Atom count and number density

Atom capture rate and lifetime in the cloud

Doppler temperature and recoil temperature

Going further Evaporative cooling

Magnetic compression

RF cooling

Bose-Einstien Condensation

to the EDCL to tune and stabilize its wavelength. A second laser (EDCL #2, the repumplaser) is tuned and stabilized in a similar manner and directed into the MOT cell in orderto boost the atoms out of a non-interactive state.

The process of constructing a MOT apparatus spans a variety of techniques in exper-

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Diode Laser with control electronics ECDL #1

Saturated Absorption

Spectrometer SAS #1

SideLock Servo

SLS #1

Optical beam

components

Optical beam

components

MOT cell with magnetic

field coils

Diode Laser with control electronics ECDL #2

Saturated Absorption

Spectrometer SAS #2

SideLock Servo

SLS #2

Power supply for magnetic

coils

FIG. 1: A block diagram of the major components of a Magneto-Optical Trap. ECDL #1is the cooling and trapping laser, which is expanded and split to form 3 beams thatconverge on the MOT cell and, after passing trough the cell, are reflected back into it.EDCL #2 is the repump laser, which boosts atoms out of a non-interacting energy state.The red (solid) lines are light beams and the blue (dashed) ones are electrical connections.

imental physics (Table II). These provide a pathway for undergraduates from simpler tomore complex skills. As we contemplated the task of building the system, we had only avague idea of the time commitment and costs involved. Given our backgrounds, we decidedto build the electronic equipment and fabricate the parts for the EDCL. These appearedto be interesting and reasonable projects that would reduce costs from purchased compo-nents. Without previous experience, we shied away from building the vacuum system andthe MOT cell. Serendipitously, the miniMOT5 came on the market at the same time as wereceived capital funds for the project from the college so we were able to let the mysteriesof vacuum systems remain unexplored. The capital funds were also used to purchase theoptical components needed. We were fortunate to find an excellent used optics table locally,which provided the foundation for our construction.

III. PHASED BUILD AND OPERATION

Due to the modular nature of the system, the MOT can be built as a progression of sub-assemblies with a payoff in experimental measurements after each sub-assembly is complete.Although we did not proceed in such a linear manner, Table III shows how the MOT

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TABLE II: Experimental techniques for building a MOT

Experimental Technique Where applied

Component fabrication in a machine shop ECDL parts

Miscellaneous optical supports

Electronics design, layout and construction Laser diode current controller

Laser diode temperature controller

Sidelock servo for laser stabilization

Peaklock servo for laser stabilization

Photodiode amplifier

Differential photodiode amplifier

Optical elements and optical table layout Saturated absorption spectroscopy (SAS)

Beam shaping and expansion

Beam splitting and polarization

Vacuum systems SAS Rb cell

MOT cell

Computer programming System control for measurements

could be built in three distinct phases. Not listed are miscellaneous laboratory componentssuch as power supplies, signal generators, oscilloscopes and surveillance cameras with videomonitors. For our project many of these were already available. An additional tool, whichis useful for initial tuning of the lasers, is a spectrometer with a resolution of about 1nm inthe near infrared.6

TABLE III: Phased construction of a MOT

Phase Components needed Measurements possible OurCost

I - ECDL Extended Cavity Diode Laser Laser diode characteristics 3000

Temperature control electronics ECDL characteristics (USD)

Current control electronics

Photodiode sensor

II - SAS one working EDCL 3000 +

Rubidium reference cell Doppler-broadened spectrum 2000

Side-Lock Servo electronics Doppler-free spectrum

Differential photodiode circuit

Optical components

III - MOT two working SAS systems 10000 +

MOT cell Create the cold cloud 25000

Magnetic gradient coils Cloud characteristics

Optical components

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A. Phase I - Extended Cavity Diode Laser and Electronics

The first phase of construction is to build the lasers and their temperature and currentcontrollers. There are many designs for extended cavity diode lasers (App. B). We used adesign provided by ES, which we have updated slightly.3,7 Although it is fairly intricate, allmachined parts are aluminum and we found that, with sufficient patience and motivation,an undergraduate could machine the parts in a small college shop.We have found that leaving the laser cavity exposed to the room environmental conditions

has lead to some problems. Temperature fluctuations and air currents affect the temperatureof the laser cavity and thus the length of the extended cavity of the laser. Since the laserdiode is cooled below room temperature, humidity is also a problem in the summer. Wehave found pools of condensed water on the floor of the ECDL cavity in particularly humidweather. To fix these issues, we put the ECDL laser into an hermetically sealed box withdesiccant and placed a second TEC with temperature control between the laser cavity andthe box.Designs for temperature and current controllers for the laser diode are readily available

as well (App. B). Since we built the side-lock servo and peak-lock servo electronics (seePhase II) and had some funds available, we bought temperature and current controllers forthe two lasers8. We have subsequently built our own temperature controllers for secondarytemperature control of the laser cavity. These have proven entirely sufficient for the purpose.One advantage of commercial current controllers is that they have diode protection functions.Perhaps as a consequence, we have not yet blown out any laser diodes. We are presentlybuilding current controllers for additional lasers using the design of Libbrecht9 and hopeour laser diodes survive.One additional electronic construction project for Phase I is to build a photodiode am-

plifier so that that the spectrum can be observed and the laser power measured. Simpledesigns are adequate.10

One of the more fine skilled procedures in this phase is the alignment of the EDCL afterit has been assembled. To activate lasing in the extended cavity, the grating is adjusted sothat the first order diffraction is fed back directly into the laser diode (called the Littrowconfiguration). The procedure for this art is described in Olson3 and also in Azmoun andMetz11. We found later that the initial alignment of the grating is best done as follows:secure the laser to the optics table and adjust the grating alignment screws so that theexiting zero order beam is parallel to the table and at the appropriate horizontal angle suchthat the first order beam returns to the laser diode. For our grating of 1800 lines per mmat 780nm, the angle is 89◦. At this point the grating is close to the correct alignment andonly small adjustments are needed to get the laser to flash, indicating the extended cavityis active.12

Once the ECDL is aligned, the power as a function of diode current can be measured. Byusing a spectrometer6, the wavelength can be measured as a function of temperature andgrating angle. Mode hopping3,11 will be readily seen. Some time spent getting used to howthe laser behaves when adjusting the current, temperature and grating position will pay offlater when trying to fine tune the laser frequency to the trapping and repump frequencies.The goal is to stabilize the laser wavelength at around 780nm. The particular combinationof temperature, current and grating angle depends on both the type of laser diode andthe individual diode obtained. Additionally, the spacial characteristics of the beam can beexplored by expanding it with two lenses and using a pinhole and photometer to map itsshape.For most MOT designs, a second EDCL is needed, which can be built and tested simul-

taneously. Our two lasers have different temperature drift, power noise and mode hopping

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characteristics. The differences may be due to variation in the laser diodes in manufactureand in the construction of the ECDL. We use the best one for the cooling and trapping laserand the other for the repump laser, which has less stringent tuning requirements.

B. Phase II - Saturated Absorption Spectrometer

The second building phase involves construction of the Side-Lock Servo (SLS) electronicsand differential photodiode detector circuit.13 The feedback/locking function of the SLSis not needed until the MOT is implemented but DC offset and sweep amplitude controlsfor the grating PZT facilitate the frequency tuning of the laser. A saturated absorptionspectrometer (SAS) can then be built by adding a Rubidium vapor reference cell, a videosurveillance camera and various optical components. Optical assembly and alignment of theSAS is fairly straight forward as described in various references.3,10

Observation of Rubidum fluorescence in the cell using the video camera is the goal of theinitial tuning of the system. Use your experience in tuning the ECDL to obtain a wavelengthof 780 nm. Then adjust the current and/or grating angle (electrically or mechanically) untilthe fluorescence is seen. Very small adjustments of the grating mount screw have a largeeffect on the laser wavelength so a delicate hand is called for.

Once the fluorescence is seen, the diode current and, more slowly, the diode temperaturecan be adjusted, while observing the photodiode signal of a single beam of the SAS on anoscilloscope, in order to find a Doppler broadened D2 spectral line. More experience with theadjustments should allow the observation of the four Doppler broadened D2 spectral linesof the two naturally occurring isotopes, 85Rb and 87Rb. For our ECDL, we are not able toobserve them all with one sweep of the PZT since the laser diode mode hops before scan ofthe full range is complete. But, the peaks can be found by offset and current adjustmentsat one diode temperature.

Now the Doppler-free hyperfine spectrum of each of the formerly Doppler broadened peakscan be observed by employing the differential SAS system.10 The relative frequencies and linewidths of the spectra can be determined using a Michelson or Fabry-Perot interferometer.Other experiments can be done that are based on the ECDL and Rubidium vapor cell.Appendix B lists some of them.

To prepare for operation of the MOT in Phase III, some practice locking the ECDL usingfeedback to the PZT from the SLS is recommended. Locking by using current feedback isnot needed for the initial formation of a cold cloud. For locking practice, set up the beamconfiguration that will be used for the MOT, that is, use a beamsplitter to pick off a beamfor the SAS from what will be the main trapping beam. A microscope slide coverglassmakes a good beamsplitter for this purpose. The best lock will be obtained by fine tuningthe alignment of the SAS to give the greatest possible hyperfine peak signal.

An important step for locking with a homebuilt side lock servo is to determine whichsetting of the SLS polarity switch will lock on the rising slope of peak (i.e. the low frequencyside). This is critical since the laser must be locked on that side of the cooling and trappingpeak in order for the MOT to function. We accomplished this by manually tuning to thedesired location (frequency) while monitoring the DC signal to the PZT. Then the lock wasenabled. If the DC level moved considerably, then it was locking on the falling slope of thatpeak or an adjacent one so we tried again with the opposite polarity. The DC level of thelock indicates the lock frequency and thus the slope polarity can be inferred.

Since two locked ECDL’s are needed for the MOT, a second SAS system should beassembled, aligned and tested.

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C. Phase III - Magneto-Optical Trap

The third phase adds the MOT chamber, the magnetic gradient field coils and beamlinecomponents to create the trap. As mentioned in section II, we purchased a miniMOT5

for our chamber. The miniMOT provides a rectangular chamber, ion pump and Rubidiumsource in a compact unit that greatly simplifies implementation of the MOT system. Theunit provides optical access on 5 sides and easy attachment of mirrors and magnetic fieldcoils. The vacuum is operationally ready in less than a minute after the power is turned onand there is sufficient rubidium pressure a few minutes later.We wound each of our gradient coils from 40 turns of #20 enameled magnet wire with

an average diameter of 3.5 cm and separation of 4.4 cm. These give a vertical field gradientof 0.17 T/m with 3 A of current, which warms the coils only slightly. A DPDT knifeswitch provides an easy way to manually remove the field and to reverse the direction of thegradient. For automated measurements, we are replacing this switch with an electronic one.We used a Hall effect sensor to test our coils. Be aware that stray magnetic fields can movethe location of the zero of the magnetic field (and thus the MOT). Since the Earth’s fieldis about 50µT, it shifts the zero point by less than 1mm with the field gradients commonlyused.The beamline components are standard and described fully in Olson3. The beamsplitters

we used initially did not allow easy balancing of the power in each trapping beam and sowe changed them. For the first splitter in the beamline we are currently using a polarizingbeamsplitter cube preceded by a half-wave plate so that we can adjust the power balancebetween the beams. Our second splitter is now a 50-50 pellicle beamsplitter. We found thatthese changes also reduced the annoying interference structure in our beams.Our quarter-wave plates are unmounted film14 held in place using homemade clips and

oriented to give the circular polarizations as directed in all depictions of the MOT operation.Being optical neophytes, we were initially confused by the nomenclature used to describe thetrapping interaction.4,15 The photon circular polarizations indicated by the symbols σ+ andσ−, are defined with respect to the positive axis direction not the direction of photon travel.Thus, the oppositely propagating beams should have opposite σ (and angular momenta) butthe same helicity (defined with respect to the direction of photon travel).16 A quarter-waveplate is needed in front of each mirror reflecting a beam back through the chamber to reversethe angular momentum and preserve the helicity.After having all the components in place, we proceeded to align the optical components

and tune the lasers in, what seemed to be, a never ending process. That is, it took severalmonths for us to find the right combination of magnetic field polarity, gradient field current,quarter wave plate angle, beam alignment in the chamber, peak tuning and side locking. Asadvised by Wieman4, our eventual success came from paying closer attention to the lasertuning and locking.17

Like proud parents, we present a picture of our newly born 85Rb MOT cloud in Figure 2.Its fraternal twin, the 87Rb MOT followed within a day.

IV. TIME AND COST

As mentioned at the onset, our primary purpose was to offer undergraduate studentslaboratory research experiences both in building and in using the MOT. As such, the timerequired was not an issue provided that the students were learning. We progressed at thehighly variable undergraduate rate and obtained a cold cloud in about 2 years. In additionto JO’s primary role, four other undergraduates participated in the building project either

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FIG. 2: A young MOT as viewed on a video screen using a small security camera onSeptember 2, 2012. The MOT is the non-circular bright spot in the upper center of thepicture. The chamber wall edges can be seen receding from the foreground. In the lowerhalf, the circular quarter-wave film and the magnetic field coil below it are visible. Theother bright spots are miscellaneous items illuminated by the trapping or repump beams.As if playing peek-a-boo, the MOT disappeared within a few seconds of formation.

in college sponsored summer internships or during the academic year and more continue tobe involved with optimization and measurements.

Cost was more of a concern. In approximate numbers, our cost to build the MOT was40000 USD.18 We estimate that the total cost could be reduced to less than 30000 USDby building the vacuum cell10,19 with an additional time commitment of about one under-graduate year. At the other extreme, by purchasing all the equipment and componentsthe time could be reduced to a year or less of assembly and tuning at a cost of more than70000 USD.20

Our funding came from a variety of sources. We obtained an initial grant of capital fundsfrom the college in support of undergraduate research and for teaching laboratory develop-ment. Smaller capital increments were obtained from the Society of Physics Students21, theIthaca College Humanities and Sciences Educational Grant Initiative and from the depart-ment.

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V. ONWARD

Once a cold cloud is created and after pushing it around a bit with a bar magnet, thereis much to do. The properties that are readily measured are the number of atoms in thecloud (using a photometer), the number density (using a video or still camera), and MOTcloud filling rate/lifetime (using a photometer or video camera).4 Our first cloud had lessthan 106 atoms with a fill rate time constant(and thus lifetime in the trap) of 3 seconds. Wehave succeeded in making a very small cloud of atoms without the rubidium getter active(i.e. using just the residual vapor pressure in the cell) that has a fill-time/lifetime of 60seconds. The temperature of the cloud, perhaps the most exciting property, is more difficultto measure (see references in App.B). Each of these properties can be studied and optimizedas a function of the trapping beam detune frequency, the beam power, the beam size andthe trapping field gradient.

We find that our cold cloud visibly fluctuates due to noise in the laser and side lock servocontrol. Consequently, the amount of detuning from the peak fluctuates as well. Because ofthese difficulties, we decided to work on further stabilization of the laser before undertakingmeasurements and optimizations. We are working on increasing the amplitude of the trap-ping peak, reducing the noise in the electronics and adding a current lock in addition to thegrating position feedback. We also plan to implement an acousto-optic modulator (AOM)that will allow us to lock on a peak using the peak-lock servo and accurately tune the offsetfrequency with an RF driver.22

Beyond the primary measurements, there are other experiments that can be done withthe MOT. Appendix B lists some possibilities. We are presently building a third ECDL inorder to characterize the cold rubidium atoms with an independently tuneable laser.

VI. ACKNOWLEDGEMENTS

Immense thanks to Evan Salim for the inspiration to build the MOT and all his help andencouragement during the process. Having someone who is knowledgeable and patient isinvaluable for the success of a project like this and Evan provided that for us.

Thank you to ColdQuanta for developing and building the miniMOT and for their friendlyhelp and service. Our pruchase of the miniMOT took the place of hours of constructiontime and the unit performs flawlessly.

We could not have begun or completed this project without the generous support andencouragement of the Ithaca College Physics Department and the Ithaca College adminis-tration.

JO thanks the Ithaca College Dana Fellowship Program for her support over the summerof 2009 and RISE (Research Internships in Science and Engineering) for the summer of 2010.

BT thanks JO for her perseverance and skill and undergraduates Ryan, Josh, Drake, Ivan,Kelly who also contributed to the success of the project.

Appendix A: Glossary

Here are some abbreviations that are commonly used in this field.

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TABLE IV: Glossary of terms

Term Item

AOM Acousto-Optic Modulator

BEC Bose-Einstein Condensation

EDCL Extended Cavity Diode Laser

OR External Cavity Diode Laser

MOT Magneto-Optical Trap

OR a cold cloud of atoms in a trap

PLS Peak-Lock Servo

PZT Piezoelectric ceramic material

(Lead Zirconate Titanate)

SAS Saturated Absorption Spectroscopy

SLS Side-Lock Servo

TEC Thermo-Electric Cooler (or heater)

Appendix B: Reading and Resources

What follows is a description of the reading and resources that we turned to most oftenfor information and clarification when building our MOT as a starting point for the reader’sown exploration. We have also included a listing of papers of experiments that look, with aneophyte’s eye, like they could be done with the present equipment as the basis.

The most inspirational papers for building are those published in the American Journalof Physics by MacAdam, Steinbach and Wieman10 for building an ECDL and Wieman,Flowers and Gilbert4 for the MOT. These venerable papers are characterized by succinctyet complete information and by clarity in their writing. They offer overviews of theory,experimental methods and experimental use with an excellent understanding of the appro-priate presentation level for undergraduates and undergraduate college programs. Mellishand Wilson19 provides pyramidal design for the MOT cell. Another readable overview ofMOT construction can be found in Lewandowski et al.,22 which goes on to offer a blueprintfor constructing system to create a Bose-Einstein Condensate, for which Wieman23 gives anoverview. Newbury and Wieman24 published a resource letter on atom trapping in 1996.

A more complete guide to the theory and application of a MOT is found in the veryreadable book by Metcalf and van der Straten.25 Useful exercises to guide theoretical un-derstanding can be found in Preston26 for SAS and Gould27 for the MOT.

There are many resources for mechanical designs for an extended cavity diode laser.MacAdam et al.10 is a start. Wieman and Hollberg28 provide a more in-depth review.Another readable account is by Ricci et al.29 The design we used is shown in Olson3,7. Adetailed overview of ECDL operation is found at the Azmoun and Metz website.11

Early, yet adequate, designs for electronic components are found in MacAdam et al.10

and Wieman and Hollberg28. Again there are many other resources. The website of theOptics Group at University of Melbourne30 has a wide variety of updated electronic designsas well as designs for an ECDL, a wavemeter, and a beam shutter. Erickson et al. and theassociated website9 has an updated current driver design. For sidelock and peaklock circuitswe used JILA designs3,13.

While in the process of constructing the MOT, BT kept an eye out for undergraduate level

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experiments that others have done using the MOT or MOT components. In general, theAmerican Journal of Physics (http://ajp.aapt.org/) provides a wealth of experiments forthe undergraduate. The following is a listing of resources found.Measurements of MOT characteristics:

• C. Wieman, G. Flowers, and S. Gilbert, Inexpensive laser cooling and trappingexperiment for undergraduate laboratories, Am. J. Phys. 63, 317 (1995).

• K. Lindquist, M. Stephens, and C. Wieman, Experimental and theoretical studyof the vapor-cell Zeeman optical trap, Phys Rev. A 46, 4082-4090 (1992).

• University of California at Berkeley Wiki - Atom Trapping lab experimenthttp://labs.physics.berkeley.edu/mediawiki/index.php/Main_Page

• University of Colorado at Boulder - Laser Cooling advanced lab experimenthttp://www.colorado.edu/physics/phys3340/

Measurement of the temperature of the neutral atom cloud:

• P. D. Lett, R. N. Watts, C. I. Westbrook, W. D. Phillips, P. L. Gould, and H. J.Metcalf, Observation of atoms laser cooled below the Doppler limit, Phys.Rev. Lett 61, 169-172 (1988).

• P. D. Lett, W. D. Phillips, S. L. Rolston, C. E. Tanner, R. N. Watts, and C. I.Westerbrook, Optical molasses, J. Opt. Soc. Am. B 6, 2084-2107 (1989).

• D. W. Sesko, T. G. Walker, and C. E. Wieman, Behavior of neutral atoms in aspontaneous force trap, J. Opt. Soc. Am. B 8, 946-958 (1991).

• C. D. Wallace, T. P. Dinneen, K. Y. N. Tan, A. Kumarakrishnan, P. L. Gould, and J.Javanainen,Measurements of temperature and spring constant in a magneto-optical trap, J. Opt. Soc. Am. B 11, 703-711 (1994).

Spectroscopy and Doppler-free SAS:

• K. B. MacAdam, A. Steinbach, and C. Wieman, A narrow-band tunable diodelaser system with grating feedback, and a saturated absorption spectrom-eter for Cs and Rb, Am. J. Phys. 60, 1098 (1992).

• D. W. Preston and C. E. Wieman Doppler-free saturated absorption spec-troscopy: laser spectroscopy see reference #1 of Preston (1996).26

• University of Florida Saturated Absorption Spectroscopy laboratory experimenthttp://www.phys.ufl.edu/courses/phy4803L/group_III/sat_absorbtion\/sat_absorbtion.html

• D. Budker, D. J. Orlando, V. Yashchuck, Nonlinear laser spectroscopy andmegneto-optics, Am. J. Phys. 67, 584-592 (1999) and Berkeley Wiki Non-LinearSpectroscopyhttp://labs.physics.berkeley.edu/mediawiki/index.php/Main_Page

• University of Colorado at Boulder - Laser Spectroscopy advanced lab experimenthttp://www.colorado.edu/physics/phys3340/

• D. A. Smith and I. G. Hughes, The role of hyperfine pumping in multilevelsystems exhibiting saturated absorption, Am. J. Phys. 72, 631-637 (2004).

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• B. E. Sherlock and I. G. Hughes, How weak is a weak probe in spectroscopy?,Am. J. Phys. 77, 111-115 (2009).

Experiments with an ECDL and vapor cell as a basis:

• D. A. Van Baak, Resonant Faraday rotation as a probe of atomic dispersion,Am. J. Phys. 64, 724-735 (1996).

• C. Leahy, J. T. Hastings, and P. M. Wilt, Temperature dependence of Doppler-broadening in rubidium: An undergraduate experiment, Am J. Phys. 65,367-372 (1997).

• K. Razdan and D. A. Van Baak, Demonstrating optical saturation and velocityselection in rubidium vapor, Am. J. Phys. 67, 832-836 (1999).

• K. G. Libbrecht and M. W. Libbrecht, Interferometric measurement of the res-onant absorption and refractive index in rubidium gas, Am. J. Phys. 74,1055-1060 (2006).

• A. J. Olson, E. J. Carlson, and S. K. Mayer, Two-photon spectroscopy of rubid-ium using a grating-feedback diode laser, Am. J. Phys. 74, 218-223 (2006).

• A. J. Olson and S. K. Mayer, Electromagnetically induced transparency inrubidium, Am. J. Phys. 77, 116-119 (2009).

• M. Belcher, E. E. Mikhailov, and I. Novikova, Atomic clocks and coherent popu-lation trapping: Experiments for undergraduate laboratories, Am. J. Phys.77, 988-998 (2009).

• C. H. H. Schulte, G. M. Muller, H. Horn, J. Hubner, and M. Oestreich, Analyzingatomic noise with a consumer sound card, Am. J. Phys. 80, 240-244 (2012).

Experiments with the MOT or MOT components as a basis:

• R. W. Fox, S. L. Gilbert, L. Hollberg, and J. H. Marquart, Optical probing of coldtrapped atoms, Optics Letters 18, 1456-1458 (1993).

• W. Petrich, M. H. Anderson, J. R. Ensher, and E. A. Cornell, Behavior of atoms ina compressedmagneto-optical trap, J. Opt. Soc. Am. B 11, 1332-1335 (1994).

• J. D. Kleykamp, A. J. Hachtel, D. G. Kane, M. D. Marchall, N. J. Souther, P. K.Harnish, and S. Bali, Measurement of sub-natural linewidth AC Stark shiftsin cold atoms: An experiment for an advanced undergraduate laboratory,Am. J. Phys. 79, 1211-1217 (2011).

• C. G. Aminoff, A. M. Steane, P. Bouyer, P. Desbiolles, J. Dalibard, and C. Cohen-Tannoudji, Cesium Atoms Bouncing in a Stable Gravitational Cavity, Phys.Rev. Lett. 71, 3083-3086 (1993)

• A. Peters, K. Y. Chung, and S. Chu, Measurement of gravitational accelerationby dropping atoms, Nature 400, 849-852 (1999).

• A. Millett-Sikking, I. G. Hughes, P. Tierney, and S. L. Cornish, DAVLL lineshapesin atomic rubidium, J. Phys. B: At. Mol. Opt. Phys. 40, 187-198 (2007).

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• J. D. Kleykamp, A. J. Hatchtel, G. G. Kane, M. D. Marshall, J. J. Souther, P. K.Harnish, and S. Bali, Measurement of sub-natural linewidth AC Stark shiftsin cold atoms: An experiment for an advanced undergraduate laboratory,Am. J. Phys. 79, 1211-1217 (2011).

∗ bthompso@ithaca.edu1 Evan Salim, IC Physics 2003, PhD University of Colorado 2011, currently at ColdQuanta, Inc.,Boulder, Colorado.

2 RISE (Research Internships in Science and Engineering), Universitat Dusseldorf, Germany3 Judith Olson, Construction and Characterization of a Magneto-Optical Trap (MOT),Senior Thesis, Department of Physics, Ithaca College, 2011. Available at the following link:http://www.ithaca.edu/hs/depts/physics/docs/theses/2011ThesisOlson/.

4 Carl Wieman, Gwenn Flowers, and Sarah Gilbert, Inexpensive laser cooling and trap-ping experiment for undergraduate laboratories, Am. J. Phys. 63, 317 (1995),DOI:10.1119/1.18072

5 ColdQuanta, Inc., 1600 Range Street, Suite 103, Boulder, CO 80301http://www.coldquanta.com/

6 We purchased a ThorLabs, Inc. http://www.thorlabs.com CCS175 Compact Spectrometer500 − 1000nm with a resolution of 0.6nm for this use. There are many alternatives availableincluding, for example, the Red Tide Emmission Spectrometer (USB650) from Ocean Optics,Inc. or Vernier Software & Technology, LLC.

7 Updated drawings for the ECDLs that we built are available athttp://faculty.ithaca.edu/bthompso/docs/ECDLdrawings/. These have slight changesfrom those shown in Olson3. Solidworks files are available on request.

8 ThorLabs http://www.thorlabs.com ITC502-IEEE Laser Diode Combi Controller. This con-troller is no longer available but other separate or combined current and temperature controlunits are available from ThorLabs and other vendors.

9 K. G. Libbrecht and J. L. Hall, A low-noise high-speed diode laser current con-troller, Rev. Sci. Instrum. 64, 2133-2135 (1993) and C. J. Erickson, M. Van Zi-jll, G. Doermann, and D. S. Durfee, An ultrahigh stability, low-noise laser cur-rent driver with digital control, Rev. Sci. Instrum. 79, 0731078 (2008) and websitehttp://www.physics.byu.edu/faculty/durfee/electronics.php

10 K. B. MacAdam, A. Steinbach, and C. Wieman, A narrow-band tunable diode laser systemwith grating feedback, and a saturated absorption spectrometer for Cs and Rb, Am.J. Phys. 60, 1098 (1992), DOI:10.1119/1.16955

11 Azmoun, B. and Metz, S., Recipe for Locking an Extended Cavity Diode Laser fromthe Ground Up, http://laser.physics.sunysb.edu/~bazmoun/RbSpectroscopy/.

12 The lasers used in a MOT are, in general, classified Class IIIb. Safety assessment and appropriatesafety precautions should be taken.

13 http://jila.colorado.edu/bec/BEC_for_everyone/index.html14 Meadowlark Optics, Inc., 5964 Iris Parkway, Frederick, CO 80530 http://www.meadowlark.com/15 E. L. Raab, M. Prentiss, A. Cable, S. Chu, and D. E. Pritchard, Trapping of Neutral Sodium

Atoms using Radiation Pressure, Phys. Rev. Lett. 59, 2631 (1987).16 K. J. Weatherill, A CO2 Laser Lattice Experiment for Cold Atoms, Doctoral Thesis,

Durham University, 2007. http://massey.dur.ac.uk/resources/kjweatherill/index.html

Page 16 has a diagram of the circular polarization of the trapping beams.17 Note that, if you suspect difficulties with locking and your lasers do not drift quickly, you can

get a MOT by tuning them directly to the trapping and repump frequencies and flattening theramp.

18 This number includes the cost of the optics table.

15

19 A. S. Mellish and A. C. Wilson, A simple laser cooling and trapping apparatus forundergraduate laboratories, Am. J. Phys. 70, 965-971 (2002).

20 ColdQuanta offers a ready-made MOT assembly that includes their miniMOT vacuum cell. TheminiMOT kit needs only two lasers and coupling optics to produce a MOT.

21 National Chapter of the Society of Physics Students Undergraduate Research Award for 2011.22 H. J. Lewandowski, D. M. Harber, D. L. Whitaker, and E. A. Cornell, Simplified System for

Creating a Bose-Einstein Condensate, J. Low Temp. Phys. 132, 309 (2003) and websitehttp://jila.colorado.edu/bec/CornellGroup/JLTP_Lewandowski2003.pdf

23 Carl E. Wieman, The Richtmyer Memorial Lecture: Bose–Einstein Condensation inan Ultracold Gas, Am. J. Phys. 64, 847 (1996).

24 N. R. Newbury and C. Wieman, Resource Letter TNA-1: Trapping of neutral atoms,Am. J. Phys. 64, 18-20 (1996).

25 H. Metcalf and P. van der Straten, Laser Cooling and Trapping, Springer (1999) ISBN 0-387-98728-2. Also see H. Metcalf and P. van der Straten. Laser Cooling and Trapping of Atoms.J. Opt. Soc. Am. B 20, 887 (2003).

26 Daryl W. Preston, Doppler-free saturated absorption: Laser spectroscopy, Am. J. Phys.64, 1432 (1996).

27 Phillip Gould, Laser cooling of atoms to the Doppler limit, Am. J. Phys. 65, 1120 (1997).28 C. E. Wieman and L. Holberg, Using diode lasers for atomic physics, Rev. Sci. Instrum.

62, 1-20 (1991).29 Ricci et al., A compact grating-stabilized diode laser system for atomic physics, Optics

Communication 117, 541-549 (1995).30 University of Melbourne, School of Physics, Optics Group

http://optics.ph.unimelb.edu.au/atomopt/technical.html