Large-Scale Quantum Superposition

Scientists have succeeded in creating the first basic quantum machine using a small visible paddle that resonates in a mixed quantum state of moving and not moving. The system works by having the paddle connected to a superconducting electrical circuit, cooling it down, and then carefully setting it to vibrate. In doing so, they have shown that it is possible to induce a quantum-mechanical ground state from a regularly-sized object previously thought to only obey the laws of classical physics. It is currently considered to be the largest object ever placed in a quantum superposition of states artificially as of August, 2009.

In general, the larger an object is, the harder it is for it to maintain a coherent quantum superposition of states. This peculiar idea is commonly illustrated by a famous thought experiment devised by physicist Erwin Schrödinger, which points out the surprising behavior of the quantum world if it could be readily applied to objects on a macroscopic level. In Schrödinger’s cat, a sealed box governs the state of a cat that's inside it through a quantum radioactive process which occurs randomly and controls whether a vial of poison gas within the box is effectively broken or left intact. Since there is no way of knowing the cat's condition without looking inside the box, the cat is proposed to be in a combined state of both alive and dead, just as a quantum object can be in multiple states at once. However, as soon as the box is opened and an observer becomes entangled with a specific outcome, the cat's quantum superposition of states immediately decoheres into one apparent result.

Amazingly, in the Many-Worlds Interpretation of quantum mechanics, developed by physicist Hugh Everett III, the universe itself is thought to exist in a quantum superposition of infinitely many states that each correspond to a different quantum "world," or parallel universe. These many worlds are similar to pocket universes existing within a unified multiverse but instead of them being far away from each other, they appear probabilistically within the same physical space. As a result, any event, no matter how small, may act as a point from which every possible future will diverge and exist within its very own timeline. This view allows both alive and dead states of the cat to persist simultaneously but only within separate realities, regardless of whichever one has been observed after the box is opened. Although moving and not moving states for a macroscopic object would normally be measured independently of one another as well, a small paddle is able to retain its combined state of motion through the use of an experimental setup that substantially delays the onset of decoherence. The achievement can be interpreted as a major step towards showing how the rules of quantum mechanics could also be applied to the movement of everyday objects.

Young's Double-Slit Experiment

At the Physics Department website of the University of Colorado in Boulder, I found a contemporary example of Young's double-slit experiment useful for showing how light behaves when traveling through either one or two slits. This experiment is well-known for first presenting evidence to suggest the wave nature of light in a time when many were only aware of its particle nature. Originally performed by professor Thomas Young in 1801, the outcome played an important role in the general acceptance of a wave theory of light and the natural wave-particle duality of different kinds of particles. The experiment is also thought to be at the heart of all quantum-mechanical weirdness.


A projector and background display setup with two slits (Image: CU).

Basically, light fired at a thin plate with single slit cut in it will travel through and onto a background surface appearing as a one-band pattern. Interestingly, light in the double-slit version of this experiment reaches the two slits and diffracts like a wave, interfering with itself either constructively or destructively to create a background pattern of bright and dark fringes (see above).

Now, if you were to perform the experiment with individual particles of matter such as electrons, it is surprising that a similar interference pattern begins to take shape in the background as well. Classical particles fired one by one would be expected to go through either slit by chance and create a simple pattern of just two bands. According to quantum theory, a fringe interference pattern could only happen if a single electron behaves as a physical wave of potentials and then interferes with itself after going through both slits, maintaining its wave-like nature until it arrives at the background surface as a particle. Other proposed solutions include the electron passing through one slit, the other slit, or even neither slit.

Many thought this explanation deviated too much from the presumed behavior of only going through one slit at a time. However, when scientists place an observing device at the slits in order to see what an approaching electron actually does, its classical particle nature suddenly emerges and changes the background pattern into a mere two bands. It acts as if the information gathered about which way a particle goes through prevents any wave-like behavior from taking effect. This suggests that certain observation techniques will affect how objects interact at a quantum level. A successful attempt to determine the path of a particle, while leaving the fringe interference pattern produced by wave-like behavior intact, was completed in January of 2012 by using entangled photons and a light source with two intensity maxima. This method also allows particle and wave-like natures to be viewed simultaneously.

In the Copenhagen interpretation of quantum mechanics, the act of observation instantly measures and reduces a system's set of possible outcomes to randomly assume one probable value. This phenomenon is referred to as a collapse of the wave function, and it links an object or a system's unobserved state to recognizable properties such as momentum or position. In fact, a quantum entity is thought to exist in all of its theoretically possible states until one of them is observed or naturally evolves with time via physicist Erwin Schrödinger's famous equation. For this case, directly observing a particle as it goes through the slits is enough to break the delicate quantum superposition of states necessary to achieve an interference pattern from wave-like behavior. This experiment is significant for showing light and matter's wave-particle duality and how an observer can have a role in determining the reality of a quantum-mechanical situation.