Tackling Curious Quantum Properties
Since Max Planck proposed the quantum theory, the field of quantum mechanics has been met with a plethora of responses—disbelief, intrigue, skepticism. What makes quantum mechanics clash with classical physics, however, also makes quantum mechanics counterintuitive. How can light be both a particle and a wave? How can objects be in two places at once? While many believe that quantum mechanics is only a theory for describing things on the microscale, Yale Professor of Physics Jack Harris has shown that quantum mechanical theory can apply to larger objects as well.
Named one of the “Best Brains under 40” by Discover Magazine in 2008, Professor Harris has been marked as a distinguished, promising researcher in the field of quantum physics. In July 2009, Harris received the Young Faculty Award from the Defense Advanced Research Projects Agency (DARPA) for his work on photons that could potentially advance telecommunications and unbreakable encryption applications for the Department of Defense. In November 2009, he also won the Arthur Greer Memorial Prize for Outstanding Scholarly Publication or Research, an award granted to a junior faculty member in the sciences. All of these accolades attest to the significance of Harris’ dedicated, groundbreaking research in quantum mechanics.
Revolution in Perception of Quantum Mechanics
Even a century after the conception of quantum mechanics, the field is still evolving. “There’s been a revolution in how people think about quantum mechanics in the last 20 years,” explains Professor Harris. One particular change in our perspective of quantum mechanics has to deal with its sense of scale. People used to view quantum mechanics as a theory of how our environment behaves on the atomic and subatomic scale. Quantum mechanics was not yet considered to be a theory useful for describing larger objects.
The revolution from that old school of thought, according to Professor Harris, met two barriers. First were heat and friction. We should be able to describe everything using quantum mechanics, but most people don’t think of macroscopic objects as capable of quantum activity. This “has less to do with their size, and more to do with the fact that they experience excessive heat and friction,” Harris explains, “While it’s true that the bigger the object gets, the less quantum-like it gets, the cutoff is much less sharp when you get rid of heat and friction.”
The second barrier is deeper and more philosophical – the consequences of quantum mechanics applied at a large scale are difficult to believe. Historically, people have contrasted quantum mechanics with classical physics. Under quantum mechanics, objects do not have well-defined positions or velocities (a concept known as the Heisenberg Uncertainty Principle) as they do in classical mechanics. Instead, there are only probabilistic descriptions of these properties. This lack of information is unsettling as even solidly grounded mathematics yield unsatisfying answers for classical questions about quantum systems. Harris remarks, “But if you step back and look at the full system description, and think ‘what other questions can we ask?’ we find that the quantum system contains much more information…Quantum effects aren’t really a limitation. They’re a resource to our knowledge.”
Testing the Theory of Quantum Mechanics
With that said, the test of any theory is its ability to predict reality. And quantum mechanics did hold up to many experiments that classical mechanics failed to explain—the famous double-slit experiment that demonstrated the wave-particle duality of photons, for example. It is commonly accepted that in the simplest quantum mechanical system, a hydrogen atom, the electron whizzes around the nucleus and never stops. Even in the ground state, the electron continues to orbit in seemingly perpetual motion. People can accept this by relegating such a result to the strange domain of the atomic scale—where quantum mechanics rule, bizarre outcomes occur. But could macroscopic objects too exhibit such a phenomenon?
In 1983, scientists predicted that electrical circuits would have a net circulating current in the ground state. Much like the electrons whirling around the atomic nucleus nonstop, the electrons in a circuit were believed to circulate without an energy source. Note that this is not superconductivity—this is a normal circuit, with resistance. Yet until Professor Harris’ recent work, it was unclear how to experimentally show that this phenomenon occurred at a macroscopic level—or whether it occurred at all.
Jack Harris and Microcantilevers
The mysteries of quantum mechanics have intrigued so many minds and in the 1980s, Jack Harris succumbed as well. Harris started by posing philosophical questions about quantum mechanics when he read popular science magazines. “How can solid objects be in multiple places at once? How can two things be entangled?” he wondered. “It just seemed so bizarre and fascinating and amazing to me that it was what I was interested in.” And so, fascinated by quantum mechanics, Harris pursued physics.
In the late 1990s, as a graduate student in physics at the University of California Santa Barbara, Harris was making microcantilevers, or as he calls them, “little floppy force detectors.” That was when it first dawned upon him how to experimentally show the existence of this macroscopic circuit current. He ended up pursuing postdoctoral research in a different field, but when Harris started his lab at Yale, he was eager return to this problem.
“The technique is incredibly simple,” he said with a laugh during a recent interview. Given a ring of metal, he wanted to measure the current. Traditionally this could be accomplished by attaching an ammeter; however, in this case, the addition of an ammeter would add too much resistance. Instead, the Harris group used one of the world’s most sensitive force detectors. When a magnetic field was applied to moving charge, such as the predicted persistent current, a magnetic force proportional to the strength of the field and the velocity of the charge was felt. Since the current was so small, they needed to apply a very strong magnetic field. To ensure necessary measurements could be taken, they also used the “world’s most sensitive force detector,” the microcantilever, capable of measuring forces as small as 10E-18 Newtons. They placed the ring on the microcantilever and, when the magnetic field was turned on, they could detect wobbles in the microcantilever, indicating a force was present.
Photons and Mirrors
Professor Harris’s next project was even bigger—literally. He wanted to see the quantum effects in millimeter square mirrors, 1,000 times larger on a side and 1,000,000 times greater in mass than the micron scale circuits that exhibited persistent currents. He is currently attempting to use photons, also known to Harris as “quantum mechanical beasts,” to explore the quantum properties of these large mirrors. “A good way to search for quantum effects is to bang photons into things,” Harris claims. Because photons have some momentum, they give the mirror a push with each hit. One of the basic concepts in quantum mechanics is that upon collision, the two parties share many properties. In the case of the photon and the mirror, the quantum properties of the photon are transferred to the motion of the mirror. Therefore, if photons are superimposed, the mirror “gets kicked” in two ways. For larger mirrors, the friction and heat make such quantum behavior, like a large mirror being in two places at once, disappear almost instantaneously. Thus to see the ghostly quantum properties of the photons transferred to a larger object like a mirror, the Harris group focuses on making the mirror smaller and reducing the friction and heat.
So far they have observed that the mirror has discrete “orbitals” for the entire object. Due to the interaction with photons, the entire mirror takes on discrete energy states. Currently, the Harris group aims to observe the Heisenberg uncertainty principle at work on a large scale so that they can to attempt to measure how the position of the mirror causes the mirror to shake.
Applications of this Research
The observation of quantum properties for macroscopic systems would be more than just a theoretical proof of existence. There are many real applications as well. One fundamental limitation of any light source is its inherent flicker, as the photons are produced by random processes. This flicker can make communications and laser applications less accurate. “It turns out that if you take a flickering light and bounce it off these mirrors, basically the mirror’s motion organizes the photons,” explains Harris. This would allow laser and fiber optics equipment to have much clearer signals.
While Harris’ quantum mechanical research can lead to many improvements in the technology we use day-to-day, his work can also revolutionize our general concept of time and space. As Harris said, “A lot of what people do is circumscribed by their sense of what is possible. Just by taking a millimeter object and showing it can be two places at once knocks down barriers between what’s possible and not possible.”
About the Author:
Sherwin Yu is a sophomore in Morse College majoring in Computer Science and Math and Molecular Biophysics and Biochemistry.
The author would like to thank Professor Jack Harris for his help in writing this article.
Photon Shuttle: Landau-Zener-Stückelberg Dynamics in an Optomechanical System,
Georg Heinrich, J. G. E. Harris, and Florian Marquardt, Physical Review A 81, 011801(R) (2010).
High sensitivity cantilevers for measuring persistent currents in normal metal rings,
A. C. Bleszynski-Jayich, W. E. Shanks, B. R. Ilic, and J. G. E. Harris, Journal of Vacuum Science & Technology B 26, 1412 (2008).
Dispersive optomechanics: a membrane inside a cavity,
A. M. Jayich, J. C. Sankey, B. M. Zwickl, C. Yang, J. D. Thompson, S. M. Girvin, A. A. Clerk, F. Marquardt, and J. G. E. Harris , New Journal of Physics 10, 095008 (2008).
Stable, mode-matched, medium-finesse optical cavity incorporating a micromechanical cantilever,
J. G. E. Harris, B. M. Zwickl, and A. M. Jayich, Review of Scientific Instruments 78, 013107 (2007).
Dynamical multistability in high-finesse micromechanical optical cavities,
Florian Marquardt, J. G. E. Harris, and S. M. Girvin, Physical Review Letters 96, 103901 (2006).