PROJECT INDEX

Measurements of Hyperfine Splittings and Isotope Shifts ______________________

Stark Shift Measurements and Polarizabilities ____________________

Laser Cooling and Atom Trapping ____________________

 Fine and  Hyperfine Structure of Li+ ________________________

Bose Einstein Condensation and Quantum Information _____________________

Climate Change Studies

 

 
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Bose Einstein Condensation and Quantum Information

Bose Einstein Condensation (BEC) was first proposed by S. Bose and A. Einstein in 1924 who predicted that near absolute zero, bosons (particles with integral spin) would all occupy the same state. This prediction did not immediately attract experimental interest because no one had any idea how to achieve such ultralow temperatures. It was not until the mid 1990s after novel methods to cool atoms using lasers and store them in magnetic traps had been developed, that BEC was first observed. The technology to create a BEC is far from straight forward and only about a dozen large well funded experimental groups worldwide have succeeded.

Ours was the first (and so far only) group in Canada to achieve BEC. It consists of 7.8 x 105  87Rb atoms at a temperature of 75 nK with a phase density of 20.The atoms are first laser cooled in a vapour cell magneto-optical trap (MOT) and subsequently transferred to an ultralow pressure MOT.The atoms are loaded into a QUIC trap consisting of a pair of quadrupole coils and a Ioffe coil that generates a small finite magnetic field at the trap energy minimum to suppress Majorana transitions.Evaporation induced by an RF field lowers the temperature permitting the transition to BEC to be observed by monitoring the free expansion of the atoms after the trapping fields have been switched off.

A number of exciting applications can be done using BECs. For example, BEC has enabled so called atom interference experiments to where the atoms behave like two interfering water waves. An experiment was also done to stop light with a BEC. It will be interesting to explore whether the extremely low temperatures can be exploited to generate extremely tightly collimated beam. The latter would be of great interest for nanolithography, a newly created field that seeks to create circuits/devices having dimensions of nanometers. The long range goal would be to use smaller faster semiconductor chips to create faster computers. 
Apparatus: Below is photograph showing the upper and lower vacuum chambers. Atoms are first cooled in the upper vapour cell magneto-optical trap. A laser then pushes the atoms into the lower chamber which is at a pressure of about 2 x 1011 torr. The atoms are then trapped in a second magneto-optical trap and subsequently loaded into a so called QUIC trap. Evaporative cooling is finally used to achieve Bose Einstein Condensation.

 
Temperature Determination of Ultra Cold Atoms: The atom temperature is determined by switching off the magnetic fields and observing the expansion of the atom cloud. A hotter sample of atoms expands faster than a colder sample. The data below show actual pictures (not modeled data!!!) of such an expanding cloud whose temperature is found to be 50 microkelvins.

 

 

 

Evaporative Cooling: The final stage of atom cooling is to switch off all laser beams and apply a rf signal using a small 1 loop antenna. The purpose of this frequency is to flip the spins of the hot atoms which are then expelled from the trap resulting in a cooler collection of trapped atoms. The rf frequency is swept from 20 MHz to the lower frequencies shown in the figure. The cloud size clearly gets smaller as the lower frequency decreases causing the temperature to decrease accordingly.

 

 

 


 
Transition to BEC: Fig. a) Thermal Cloud with N = 1.9 x 106 atoms at temperature T = 450 nK.  Fig. b) Mixed thermal atom cloud and BEC where        N = 1.8 x 106 atoms and T = 400 nK.  Fig. c) Pure Condensate where N = 4.2 x 105 atoms and T < 60 nK.