The two pillars of modern physics are quantum mechanics and Einstein’s special/general relativity. We generally use quantum mechanics to describe how reality behaves at the quantum level (i.e., the level of atoms and subatomic particles). In our everyday world, the macro level, we generally use Einstein’s special or general relativity to describe the behavior of reality. Thus, it appears we need two separate set of “laws” to describe reality, one for the quantum level and one for the macro level. In addition, quantum mechanics is incompatible with general relativity. This suggests that quantum mechanics may not apply at the macro level. However, in 2010, an important experiment suggested that quantum mechanics is applicable to the macro level.
What happened on December 2010? Scientists at the University of Santa Barbara, United States, published a paper “Quantum mechanics applies to the motion of macroscopic objects”. The University of Santa Barbara scientists made a clear demonstration that the theory of quantum mechanics applies to the mechanical movement of an object large enough to be seen with the naked eye. Now, before we go into the detail, just think about the implications and questions this raises if this proves to be true. Do macroscopic objects have a particle-wave duality? Can macroscopic objects be modeled using wave equations, like the Schrödinger equation? Will macroscopic reality behave, under the right circumstances, similar to microscopic reality? To approach an answer, let’s take a look at what the University of Santa Barbara scientists demonstrated.
Our story starts out with Dr. Markus Aspelmeyer, an Austrian quantum physicist, who performed an experiment in 2009 between a photon and a micromechanical resonator, which is a micromechanical system typically created in a silicon chip and sometimes an integrated circuit, which may or may not contain additional electronic integrated circuits. The micromechanical resonator can be caused to resonate, i.e. move up and down much like a plucked guitar string. The interesting part is Dr. Aspelmeyer was able to established an interaction between a photon and a micromechanical resonator, creating the so-called strong coupling (i.e. a strong and noticeable interaction), able to transfer quantum effects to the macroscopic world. This is the first recorded time in history that the quantum world communicated with the macro world.
So what happened at the University of Santa Barbara at 2010? Andrew Cleland and John Martinis at the University of California (UC), Santa Barbara, worked with Ph.D. student Aaron O’Connell. This team became the first to experimentally induce and measure quantum effect in the motion of a human made object. The work, released in March 2010, was voted by Science and AAAS (the publisher of Science Careers) as the 2010 Breakthrough of the Year “in recognition of the conceptual ground their experiment breaks, the ingenuity behind it and its many potential applications,” according to a AAAS press release.
What exactly did they do? They showed that a mechanical resonator (i.e. in this case a small metal strip that can vibrate freely), which was cooled to its ground state energy (ground state), works at the macro level. To understand how profound this is we need to understand the ground state. The ground state is the lowest fundamental level of energy of vibration a physical entity may have according to quantum mechanics. It is not zero energy, but close. A particle with zero energy would violate the Heisenberg uncertainty principle. We would know where it is and how fast it is going simultaneously! The act of putting a system in its ground state energy has never been done before. Their method was brilliant. They first built a mechanical resonator to the frequency of the microwave. Then, the resonator was physically connected to a superconducting qubit (i.e. a quantum system controlled with great precision, used in research of quantum computers). The set was then cooled to near absolute zero. But, how could they be sure that they were at the ground state. They used quantum qubit as a thermometer and demonstrated that the mechanical resonator contained no extra vibration. In other words, it had been cooled to a level equivalent to its ground state energy. At this point, the experiment is already in the history books, but wait an even larger punch line is coming.
The mechanical resonator is as close as possible to being perfectly still, i.e. the ground state. The scientists then added a single quantum of energy, a phonon, the smallest physical unit of mechanical vibration, thus the lowest possible level of excitement. What they observed next is astounding. When the mechanical resonator absorbed this fundamental unit of energy, the qubit and resonator became entangled quantum mechanically (i.e., quantum entanglement). This means that any change in the quantum state of one of them will be immediately cause a change in the state of the other.
Measurements of the vibrational energy showed that the results exactly followed the predictions of the quantum mechanics. The quantum level and the macro level, given the appropriate physical circumstances, follow the same natural laws. This one experiment may have put us one step closer to a unified theory of everything, the “holy grail” of physics.
