Figure 1: Young's paper

The debate over wave-particle duality started in the 17^th century. Isaac Newton developed the corpuscular hypothesis that light is composed of particles. Later by mathematical approach, Robert Hooke, Christian Huygens, and Augustin-Jean Fresnel refined explanations of wave-like properties of light and in 1803, British scientist Thomas Young, published a paper entitled "Experiments and Calculations Relative to Physical Optics", in which he used a double slit experiment to explain the wave theory of light. While Young’s experiment discredited the corpuscular hypothesis and explained his observations with wave properties of light, further experiments by other researchers has shown that the particle nature of the light could be instead explained by the photoelectric effect, and that Young’s observations actually were explainable by the wave theory of light.

set up

The set-up of the experiment is quite simplistic. As shown in Figure 2, Young’s set up included three parts: a light source (at the time, the sun), a sheet with two closely spaced slits, and an incident sheet. The light was shone against the two slits, and observed on the incident sheet as having an odd pattern that suggested that it must have some hidden properties that might be best described through diffraction.

Figure 2: Set up

phenomenal

From the perspective of light as a wave, we can compare it to water waves. Against a single slit, the wave will spread out and strike the back wall with the most intensity in line with the slit. When the second slit is added, the extrema of the two waves passing through the slits could interfere either constructively or destructively. This phenomenon of bending around an obstacle is called diffraction, a property of waves.  The resulting pattern observed on the back wall is called an interference pattern, which is also a feature of waves.

When we shoot light through a single slit, a band of light in line with the slit should be observed based on the expectations given by the particle theory. And in the double slit case, two parallel bands should be observed. However, the light shoot through a single slit spread out like the water wave, and the interference pattern like the one on the right was always observed. This resulting phenomenon directly supports the wave nature of light.

Figure 3: Diffraction patterns as waves

how could this happen?

At first, people thought probably those light particles collide with each other, thus making the diffraction pattern. Researchers charged trillions of photons one by one against the double slit to see what would happen. After all, the photon has no mass and light travels at a constant speed regardless of Doppler’s effect. Unbelievably, the result did not change at all. The single photon particle seemed to have gone through both slits at once and interfered with itself to give the same observed diffraction pattern as before. Further experiments using electrons and atoms were performed, but the result remained the same.

Figure 4: Probability function distribution

explain

A possible explanation called the Copenhagen Interpretation was given. This is also known as the collapse of the wave function. According to this interpretation, a photonpassing through the double slit does not exist as a particle at all, but has a wave-like property covering the areas of probability where it could be found. Once the photon is observed, the wave function collapses and the photon “becomes” a particle and follows no pattern. This theory also explains the behavior of the particles in the double slit experiment. When we are not “looking at” the particle, the probability wave function of even a single particle is spread out and passes through both slits at the same time and arrives at the detector as a wave showing an interference pattern.

Figure 5: Incidence

how has research grown?

With the discovery of quantum mechanics in the early 20^th century, Young's experiment could be reapplied into new areas. Following Young’s double slit experimental results, scientist Clauss Jönsson mimicked the experiment using electrons. Unlike photons, electrons do have a mass. This however, did not affect the result of the experiment. When the electrons were shot one at a time through the two slits, they too made a linear pattern on the wall. The results from the electron double slit experiment confirmed the wave-particle duality of electron behavior.

Current research explores ways that the double-slit experiment can be refined using more precise lasers in optic treatments. Scientists have discovered a more precise way measure the speed and path of the particles. They are also grasping ways to manipulate the wave and particle characteristics of photons. Scientists have uncovered some ways to apply the quantum mechanic principles behind the double-split experiment to develop interferometric techniques for attosecond science. Researchers are using attosecond science for pulse generation and detection in lasers. With new discoveries in the attosecond science field, optical cameras are being manufactured to have the ability to take quick and frequent shots of electrons. This will be helpful in the material science field, as well as advance research in the optical field.

Researchers found a correlation between phase measurement and its importance during polymer cross-linking and polymerization processes. The wave-particle attributes of photons discovered during the double slit experiment were applied to perform phase measurement in a quantum dot. By limiting the quantum dot to one slit, a measurable phase shift was created. The studies of “laser-induced phase shifts” help support evidence that is useful with polymer cross-linking and polymerization processes.

how do I apply Young's experiment?

Young’s first exploration of Newton’s corpuscular theory has lent itself to many notions of the wave-particle duality of light and its properties at the quantum level. However, research after research, there is still very little application that has come from the exploration of wave-particle duality. Young’s double slit experiment is probably one of the most repeated experiments in science, next to gravity.

It is difficult to pin-point a field of application of this double slit experiment, since it has taken more than 200 years for researchers to begin to be able to understand the quantum properties of light.

Ueda’s group at Tohoku University of Japan experimented the quantum properties of light through the use of a linear Paul trap. Entangled, or unlocalizable without description of a counterpart, pairs of photons were passed through this trap to a double slit, similar to Young’s experiment. Concluding the experiment, Ueda’s group found that they could distinguish between the two entangled photons which could improve the way a surgery is carried out in very sensitive parts of the body.

Howell’s group at the University of Rochester passed photons through a system of cesium vapor, lenses, and an amplitude mask, which is able to shade the intensity of the interference pattern, in turn giving the photons a different ‘natural’ fringe than seen in Young’s experiments and repetitions thereof. Howell’s group was able to control the photons, dimensionalizing the path of the photons into two dimensions and presenting the shape of the letters “UR”.  This control of the diffraction pattern of photons could lead to advancements in medical imaging and information processing.

Shtrikman’s group of the Braun Center for Submicron Research of Israel explored the transport characteristics of waves passing through electronic devices. The “quantum dot” is a semiconductor confined in space whose wavefunction also characterizes the electronic properties of the dot by its phase. By result of double slit interference, Shtrikman’s group was able to measure the phase of an electron traversing a quantum dot and found that there is no observable effect on the phase of an electron in the presence of another. This observation could lead to the isolation of the electron in three dimensional space and challenge the uncertainty theory as to quantize momentum and position of a particle simultaneously.

Carnal, O., and J. Mlynek. "Young’s Double-slit Experiment with Atoms: A Simple Atom Interferometer." Physical Review Letters 66.21 (1991): 2689-694. Print.

Shuster, R., E. Buks, M. Helblum, D. Mahalu, V. Umansky, and Hadas Shrikman. "Phase Measurements in a Quantum Dot via Double Slip Interference Experiment." Nature 385 (1997): 417-19. Web.

Unsworth, J., and J. Sendt. "The Fundamental Importance of Phase Measurement during Crosslinking and Polymerization Processes in Polymers." European Polymer Journal 18.7 (1982): 617-19. Print.

Quantum Theories. The double slit experiment. September 12. 2010. Web. September 25. 2010.