Laboratory 2: Single Photon Interference

David Cruz, Nelson Lee, Natalie Pastuszka

University of Rochester Institute of Optics

September 25, 2013

Abstract:

The main focus of this laboratory experiment was to demonstrate the wave-particle

duality of light by observing single photon interference. To accomplish this, a red He-Ne laser

(632.8 nm wavelength, approximately 5 mW power) was attenuated to produce single photons

which were sent through an interferometer onto a detector. The first part of the experiment

demonstrated wave-particle duality by sending the attenuated beam through a Young’s double

slit interferometer and observing the presence of interference patterns detected by an EM-CCD

camera. The second part of this experiment involved using a Mach-Zehnder interferometer to

determine how “which path” information affects interference by splitting the beam through two

different polarized paths and observing the appearance and disappearance of fringes.

Introduction and Background:

Light is comprised of massless particles known as photons. Though these photons are

characterized as particles, they exhibit the properties and behavior of waves as well. This

phenomenon is known as wave-particle duality. The main goal of this experiment is to

demonstrate how these particles interfere with theirselves in the same way that waves would.

One method will be using a Young’s double slit interferometer and observing interference on a

single photon level to show wave-particle duality. The other method uses a Mach-Zehnder

interferometer to observe how having “which-path” information affects interference fringes by

performing a measurement and collapsing the photon’s wave function, thereby causing it to

behave like a particle.

In Young’s double slit experiment, a beam is incident on a screen with two slits. On a

single photon level one would, in theory, expect to be able to know which slit the particle goes

through. The result would be only two intensity patterns on the detector. However, this is not the

case. Instead what we see is a series of light and dark fringes.

Figure 1: Young’s Double Slit Experiment Demonstrating the Wave-Particle Duality of

Photons

This is due to the absence of “which-path” information. By blocking off one of the slits,

thereby knowing the exact path, we would not see fringes. We observed these interference

patterns using an EM-CCD camera, and these fringes are more visible towards the center of the

detector and become less intense as the particles go farther from the central position.

Figure 2: Interference Pattern Observed in Young’s Double Slit Interferometer

Mathematically, to define a contrast of the interference pattern we introduce the following

equation for fringe visibility:

Fringe Visibility = I max – I min

Where Imax and Imin represent the maximum and minimum intensities of the wave. This relation is

observed in the data that was collected and automatically calculated in the software.

In the second portion of the experiment, a Mach-Zehnder interferometer was used to split

the beam, polarize each path vertically or horizontally, recombine the beam, and use a polarizer

to erase “which-path” information. This was done on a beam with a large diameter which can be

seen with the naked eye. The same effect was observed on a single photon level using the EM-

CCD camera. Depending on the angle of the final polarizer, the knowledge of “which-path”

information was destroyed and restored and the disappearance and reappearance of interference

fringes was observed.

Experimental Setup:

I. Young's Double Slit Experimental Setup

The end goal of the experiment was to show the nature of particles as being in accord

with the concept of wave-particle duality. A 5mW He-Ne laser with a 633nm wavelength

supplied the light used in both experiments. Light coming from beam needed to be attenuated so

4 orders of magnitude neutral density filters were used; however, due to classical photon

statistics, we did not achieve antibunching. The attenuated light was sent through slits that were

10 μm wide and 90 μm apart. An EM-CCD camera (observation screen) was used to observe the

interference patterns that we would expect from waves. See the setup below.

(Equation 1)I max + I min

II. Single Photon Interference with Mach-Zehnder Interferometer Experimental

Setup

To achieve single photon interference, we used a Mach-Zehnder interferometer (See

figure below). To allow for the "which-way" information needed to observe wave-like behavior,

we set two polarizers (A and B) positioned at 45 degrees. Again, the neutral density filters were

used to attenuate the beam to average about 1 photon per meter. The light splits into two

separate arms after polarizing beam splitter A and then reunited using a non polarizing beam

splitter. The fringes were then observed using the EM-CCD camera.

Spatial Filter

Double Slit

Neutral Density Filters

Neutral Density Filters

EM-CCDLaser

Figure 3: Setup for Double Slit experiment. Laser beam becomes attenuated to a single

photon level using neutral density filters. A spatial filter was used to remove inhomogeneity

in the laser beam caused by dirt or dust and lens imperfections. The attenuated and

"cleaned" laser passes through the double slit and produces an intensity pattern observed

using the Electron Multiplying Charged Coupled Device (EM-CCD)

Figure 4: Setup for Mach-Zehnder interferometer. The neutral density filters were used

again to attenuate the light coming from the laser and the spatial filter was used to remove

inhomogeneity in the light caused by dust particles and lens imperfections. Photons passed

through the first polarizing beam splitter and separated into two arms (horizontal and

vertical). Both arms were reunited at the non-polarizing beam splitter and passed thru

polarizer B and then observed using the EM-CCD camera.

Non-Polarizing BeamsplitterMirror

Neutral Density and Spatial Filters

Laser

Polarizing Beamsplitter

Polarizer D

EM-CCD

Procedure:

I. Young's Double Slit

Alignment and Preparation

Align the beam in order for light to pass through the slits and for the interference patterns to be

captured by the camera.

Once the apparatus is properly aligned, the laser should be reduced to a single photon level using

neutral density filters.

Capturing Interference Pattern

Adjust visual settings using ImageJ software in order to process the interference patterns.

II. Single Photon Interference with Mach-Zehnder Interferometer

Alignment and Preparation

To align the Mach-Zehnder interferometer, light (bright spots) that should be coming into the

camera should overlap in both the far and near field. The resulting beam should then be aligned

with the camera.

After alignment, neutral density filters should be used to reduce the beam to a single photon

level.

Polarizer B should be rotated between 0 and 360 degrees (relative) and the resulting intensity and

visibility should be recorded.

Capturing Interference Pattern

Once the apparatus is properly aligned and angle is adjusted, use settings on camera (ie. gain,

accumulations and exposure time) in order acquire the best image to see interference patterns.

Results and Analysis:

I. Young’s Double Slit

Using the EM- CCD camera, we were able to verify the phenomena described in the

wave- particle duality theory, where single photons are able to exhibit properties of a wave by

producing interference fringes, which can be observed when single photons travel through two

slits. Different neutral density filters, exposure times, and accumulations were varied to observe

the best possible fringes. To find the order of these neutral density filters we first had to

determine power and wavelength of the light source and use these values calculate how many

photons per meter were being produced using the following equation:

Once this value is calculated, the order of magnitude gives us how many orders of filtration are

needed to achieve a desired transmittance, and in this case single photons.

Figure 5: Interference fringes and analysis under neutral density filter of magnitude 2 at 0.1 second

exposure time.

N(photons/meter)= Power * Wavelength

h * c2

(1)

Figure 6: Interference fringes and plot profile under neutral density filter of magnitude 4 at 0.003 second exposure

time. There is a slight indication of fringes, but they are not clear at all. The plot profile demonstrates a sinusoidal

curve that is less distinguishable and contains a significantly greater amount of noise.

Figure 7: Interference fringes and plot profile under neutral density filter of magnitude 4 at 0.003 second exposure

time and 255 gain. The settings for the acquisition of data are the same in both Figure 6 and Figure 7, with the

exception that Figure 7 has gain. It is important to note the different in visibility of fringes.

Figure 8: Interference fringes and plot profile under neutral density filter of magnitude 6 at 0.01 second exposure

time, 255 gain, and accumulation of 90-10.

Figure 9: Interference fringes and plot profile under neutral density filter of magnitude 6 at 1.00 second exposure

time, 255 gain, and accumulation of 100-10.

The images of the interference fringes were taken using an EM- CCD camera. The plot

profile (left images) of the intensity of the interference fringes through Young’s double slit

experiment were obtained through the program ImageJ. The intensity curve that varies over

distance represents a sinusoidal function with some distortion that is directly correlated to the

images of the fringes. Figures 5, 7, and 9 have the clearest fringes for each neutral density filter

of magnitude order 2, 4, and 6, respectively. There is a distinction in the images when the

acquisition various settings are altered, as is evident when comparing Figures 8 and 9. Both of

the figures have the same order of filters and gain, but were exposed for a different amount of

time and with a slightly different accumulation, thus allowing for better clarity of fringes.

II. Mach- Zehnder Interferometer

Depending on the angle of the final polarizer in the Mach- Zehnder interferometer setup,

we were able to see fringes at different ranges of visibility.

(a) 150 degrees (b) 100 degrees (c) 50 degrees (d) 0 degrees

Figure 10: Images captured by EM- CCD camera of fringe visibility at different angles of polarizer. As the polarizer

changed angles by 45degrees with reference to the recombined beam, the fringes would alternate with appearing

and disappearing. The interference fringes that have the most distinguished bands of minima and maxima (images

(a) and(c)) indicate the absence of the “which- way” path knowledge.

Figure 11: Plot profiles of intensity of images of interference fringes captured by EM- CCD camera. The plot

profile on the left represents the intensity profile of the interference pattern when the maxima and minima bands

were the clearest. The plot profile on the right is of the interference fringes when the polarizer was rotated by 45

degrees with respect to the recombined beam and the image on the left, thus at an image that did not display fringes.

In Figure 11, both figures represent similarly sinusoidal functions, but are out of phase by

180 degrees with respect to each other. The two plot profiles represent the intensity of the

interference fringes at different polarizations of the recombined beams. The plot on the left

shows the plot profile when the interference fringes have the most distinguished bands (Figure

10, plot (a)) and the plot on the right displays the intensity profile of the image that was captured

when the polarizer was set at 45 degrees to the point where the fringes were most visible (Figure

10, plot (b)). According to the “which- path” theory, because we were able to predict which path

the photons would take, no interference fringes were visible. At a point where the polarization

was set to an angle which halted our knowledge of path, fringes appeared.

Conclusion:

The results of our experiments clearly demonstrate the manner in which single photons

behave. Single photons exhibit behavior customary to both waves and particles, as follows the

wave- particle duality theory; single photons create an interference pattern that is explainable

only by the nature of waves as was evident in Young’s Double Slit experiment. The Mach-

Zehnder setup permitted exploration of “which- path” information as the observance of fringes

depended on the angle of the polarizer. By blocking the light through use of the polarizer and

thereby knowing the path of the photons, no fringes were visible. In the situation that the angle

was rotated to a position where there was no predictability of the photon’s path, interference

fringes would appear and confirm the classical quantum properties.

References:

1. Lukishova, Svetlana. Lab 2 Manual, Web. Fall 2013.

http://www.optics.rochester.edu/workgroups/lukishova/QuantumOpticsLab/homepage/

lab_2_manual_oct_08.pdf

2. “A simple experiment for discussion of quantum interference and which-way

measurement”. Schneider, M. B., LaPuma, I.A.

http://www.optics.rochester.edu/workgroups/lukishova/QuantumOpticsLab/homepage/

snyderlapuma.pdf

Contributions:

Under the guidance of Dr. Svetlana Lukishova, all members contributed equally during lab to

calculate values for ND filters; setup equipment and interferometers; and collect and analyze

data.

David Cruz wrote the “Abstract” and “Introduction and Background” sections.

Nelson Lee wrote the “Experimental Setup” and “Procedure” sections.

Natalie Pastuszka wrote the “Results and Analysis” and “Conclusion” sections.