Effect of Temperature on Magnetic Field MeasurementsDoug Hockey1, Brendan Van Hook1, Ryan Price2

Sponsored by the Department of Physics, University of Maryland, College Park

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Introduction

A Bose-Einstein condensate (BEC) is a collection of atoms which occupy the lowest quantum level when cooled near absolute zero. In order to control the quantum spin state of a BEC, it is necessary to reach a stable resonance frequency. The magnetic field around the BEC is used to tune the frequency. Errors in measurements of the magnetic field will prevent stabilization. Because many electrical components are temperature-sensitive, changes in temperature are capable of affecting the electronics. This could change the current in the electromagnets and thereby change the resonant frequency of the atoms.

Analysis and Conclusion

We set up two different temperature sensors at two different points near the electronics which control and measure the experiment. With the data we have gathered it appears that the temperature could be a cause of the disruption in the field. Even though the data is not as clear as we would like it to be, a correlation is able to be seen between temperature measuring device and the strength of the field. While it may take more data collection to determine if temperature is the sole factor affecting the field, there does appear to be a trend. If that is the case, the next step will be determining how to counteract changes in temperature. The trend line shows a change of about 0.34 milliGauss (mG) per degree Celsius. This can be quite substantial as our resonance is around 5 mG wide and one or two mG is capable of changing the fraction between the two spin states. For scale, the magnetic field of the Earth is around 500 mG.

Background

At our lab Doppler cooling is used to trap and cool rubidium atoms. In Doppler Cooling, light is emitted and then absorbed by the rubidium atoms. The excited rubidium atoms then emit a photon in a random direction and the rubidium loses momentum. Since light can be emitted from nearly every direction the rubidium atom will eventually slow down until it hits it's limit of 150 microkelvin. At this point, the atoms are trapped in an optical dipole trap at around 1 microkelvin. Evaporation techniques then cool the atoms further by allowing more energized atoms to escape. The result is a collection of atoms around 100 nanokelvin which have reached the Bose-Einstein Condensate state of matter.

Figure 1: The basic design of the circuit. Figure 2: The unpopulated circuit board.

1 University of Maryland, 2 Joint Quantum Institute

Circuit Designs and Considerations

We used the AD590, a temperature transducer which outputs 1 µA per Kelvin. Requirements of the circuit were:•Multiple output gains to measure the temperature at different sensitivities.•Single sided printed circuit board (PCB) for ease of construction.•A zero offset close to room temperature.•Output as a voltage between plus and minus 10 volts.

Multiple output gains were required because the level of temperatures drifts was unknown and we needed to fit our output in +- 10 V. We can therefore see drifts on the scale of a few millikelvin or as large as several kelvin. The following gains were chosen:

• 1/3 K per volt for measuring very small changes in temperature.

• 5 K per volt for measuring a wider temperature range.• 1 K per volt for a middle ground between the two.

Our design was quite simple and made use of two operational amplifiers. By varying ratio between two resistors, we are able to amplify our signal to a desired output gain.

Figure 3: The full design of the circuit board.