Figure 1: Potential wells of a 2-D optical lattice with atoms “captured” in place [3].
I recently attended a fascinating physics colloquium at Cornell by Professor Ana Maria Rey from the University of Colorado Boulder. Professor Rey discussed two fundamental ideas in condense matter physics and quantum physics. In the first part of her talk, she introduced a major component of quantum physics: the study of individual atoms in a system. In particular, she discussed how scientists have utilized ultra-cooled atomic systems to better understand the properties of condensed matter, such as solids and liquids. Although solid objects give the illusion that atoms are at fixed locations in space, in reality, atoms at room temperature vibrate or move at a speed of about 300m/s. At such high speeds, studying properties of single atoms or complex configurations of atoms becomes extremely difficult if not impossible. Scientists have therefore used cooling to slow down the individual particles. Over the last hundred years, scientists have reached several milestones in extreme cooling. Although scientific evidence has confirmed the impossibility of reaching absolute zero (the theoretical temperature when atoms would stop vibrating altogether), scientists have come increasingly close to this absolute minimum. One method used to reach temperatures in the range of microkelvins is through the use of strong lasers, which cool the atoms by applying extreme pressure. A second, even more effective method, involves the formation of a state of matter known as Bose-Einstein condensate. In this new type of matter, atoms reach the lowest possible quantum state and can be cooled to temperatures as low as 10-100 nanokelvin, i.e., an incredibly low temperature of 10-7 to 10-8K. (Using magnetic techniques and cooling just a few nuclei of magnetic materials, even lower temperatures of 100 pK (picokelvin), i.e., 10-10K, have been reached [1].) By cooling atoms to such low temperatures, the atoms slow down significantly to speeds of the order of cm/s, allowing scientists much greater control of atomic systems to help understand the complexity of multi-particle systems and quantum phenomena.
After introducing the concept of ultra-cooled atoms, Rey transitioned into a discussion the applications of this amazing physical phenomenon. Specifically, Rey discussed approaches for understanding quantum physics proposed by Richard Feynman [2]. Feynman proposed two core ideas to break through to the realms of quantum matter: (1) by using quantum computers to simulate quantum equations, and (2) through the use of highly controlled synthetic quantum systems to represent naturally occurring quantum systems. While quantum computing presents a very interesting challenge for the future, Rey’s research and talk focused on the creating synthetic quantum systems with the goal of controlling temperature, internal atom states, atomic interactions, and many other important factors at microscopic scales. In order to achieve such a system, Rey presented a system known as a laser-cooled Optical Lattice. An Optical Lattice utilizes two counter-propagating beams of laser light to form a standing wave that effectively captures atoms in space. See Figure 1. By creating an optical lattice in a super cooled environment, scientists are able to create fully controllable systems similar to those described by Feynman. What is so useful about these systems is that scientists can use them to make inferences about quantum systems that are much harder to control such as actual condensed matter systems. Overall, scientists continue to work to further increase their ability to precisely manipulate such quantum systems by developing even better cooling procedures, more precise measuring tools, and even whole new systems that incorporate other types of atoms, such as Alkaline metals or trapped ions.
In the second part of the talk, Rey shifted gears slightly to discuss the creation and use of atomic clocks. Current atomic clocks have become scarily accurate through the incorporation of alkaline earth metals — nature’s timekeepers. Atomic clocks are dependent on the vibration of atoms, such as cesium, Cs, or strontium, Sr. Scientists can create highly precise lasers by locking the laser into the frequency of the transmitting atoms (or more precisely to the frequency of the microwave signal emitted or absorbed when electrons in the atom change energy levels). In order to achieve very high precision, the observed atoms must be trapped in a lattice structure to prevent Doppler shifts, recoil, and stack shifts. This can be achieved by using the laser cooled optical lattices (Figure 1), as discussed in the first part of the talk. Incredibly, clocks created by observing the Sr atom can reach a resolution or precision of 10-17 seconds. Additionally, these atomic systems have a quality factor of over 15 billion years, meaning that they would remain correct to the second for longer than the age of our current universe.
After presenting the general theories behind atomic clocks, Rey went further into discussing how careful measurements of these clocks can be used to confirm certain predictions from quantum mechanics. Unfortunately, near the end of her talk she began to discuss fairly complex quantum equations and constants that required prior knowledge of quantum mechanics to fully understand. Overall, she stressed that physicists are entering into a new era with the creation of new extremely precise clocks and the ability to create very controlled precise atomic systems to simulate quantum systems that may not even appear in nature. This line of research has the potential to significantly extend our understanding of condensed matter and quantum phenomena.
At the very end of the lecture, a question concerning the practical use of extremely precise atomic clocks proved to be quite illuminating. With extremely precise time measurements, researchers can optimize many everyday systems, such as the GPS. In particular, atomic clock measurements have proved to be an integral part in the ever increasing GPS accuracy (up 2 cm precision is now feasible) by allowing scientists to incorporate the necessary corrections due to gravitational effects as predicted by Einstein in his theory of General Relativity, which quantifies precisely how time slows down in a gravitational field.
Figure 2. Experimental setup on the Delft University campus [5].
Another very interesting application requiring very accurate clocks that I read about recently was in a breakthrough physics experiment at Delft University in the Netherlands [4]. In this experiment, researchers proved one of the most fundamental predictions of quantum mechanics that states that once entangled, particles (e.g., electrons or photons) can instantaneously affect each other’s behaviors over arbitrary distances. Amazingly, Einstein, himself, did not believe that nature would allow for this to happen. In order to validate the quantum prediction, researchers designed an experiment where they entangled two electrons, and moved them 1.3 km apart.
They then measured the spin (orientation) of one of the elecrons and then almost simultaneously they observed the spin of the other. (I will give a rather high-level description of the idea behind the experiment. The full experimental setup is quite complex to eliminate multiple sources of possible errors [5].) See Figure 2. Quantum mechanics tells us that before we measure the spin of a particle there is a 50% likelihood of being up or down when observed. In other words, it is only the measurement that gives the spin a definite direction. What the researchers observed was that with statistical significance the distant particle observed slightly later had its spin entangled (coupled) with that of the first atom. Researchers are only just able to validate this intriguing phenomenon of “effect at a distance” because of the creation of incredibly precise atomic clocks. In order to view the coupling effect, the second observation must be made in a time shorter than the time a light beam could travel the distance between the two measurement locations. By making the interval between the observations significantly shorter than the time light needs to travel between the locations, researchers can be certain that no information could physically travel between the first observation and the second one. Thanks to super precise clocks, these researchers were able to confirm one of quantum physics strangest predictions.
References
[1] Knuuttila, Tauno. Nuclear magnetism and superconductivity in rhodium. Helsinki University of Technology, 2000.
[2] Feynman, Richard P. Simulating physics with computers. International journal of theoretical physics 21, no. 6/7 (1982): 467-488.
[3] Source: http://jila.colorado.edu/yelabs/research/ultracold-strontium
[4] Markov, John. Sorry, Einstein. Quantum Study Suggests ‘Spooky Action’ Is Real. New York Times, Oct. 21, 2015.
[5] Hensen, B., H. Bernien, A. E. Dréau, A. Reiserer, N. Kalb, M. S. Blok, J. Ruitenberg et al. Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometers. Nature 526, no. 7575 (2015): 682-686.
Stumbling into Science and Math 