Quantum Eraser Demonstration Kit

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Interference Fringes are only Shown on the Left Viewing Screen, where the Path Information has been "Erased"

Quantum Eraser Analogy Demonstration Kit

• Designed for Educational, Demonstration, and Classroom Use
• Complete Kit Includes All Hardware Plus Extensive Manual and Teaching Materials
• Easy to Assemble and Use
• Choose from Kits Containing Imperial or Metric Components

Thorlabs Educational Products

Thorlabs' educational line of products aims to promote physics, optics, and photonics by covering many classic experiments, as well as emerging fields of research. Each kit includes all the necessary components and a manual that contains both detailed setup instructions and extensive teaching materials. These kits are being offered at the price of the included components, with the educational materials offered for free. Technical support from our educational team is available both before and after purchase.

Purchasing Note: English, German, and Portuguese language manuals/teaching information are available for this product. The imperial kit contains the English manual and US-style power cord. The appropriate manual and power cord will be included in the metric kit based on your shipping location. Please contact Tech Support if you need a different language, cord style, or power supply. As with all products on our website, taxes are not included in the price shown below.

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Quantum Eraser Demonstration Optical Components and Beam Path

The Quantum Eraser Experiment

The quantum eraser experiment demonstrates the quantum-mechanical principles of complementarity, path information, and superposition. An interferometer can be used to demonstrate the wave nature of light, by creating two optical paths that have slightly different path lengths. According to the classical interpretation, light strikes a viewing screen after traveling along the two paths in the interferometer. If the difference in path lengths is equal to an integer number of waves, a bright spot is produced through constructive interference, and if the path length difference is equal to an odd number of half waves, a dark spot is produced through destructive interference. Since all light rays traveling through the system do not travel an equivalent path length due to the lens, a bullseye interference pattern is produced.

Interference experiments can also be performed with light sources that emit a single photon at a time. A classical interpretation would predict that as the single photon passes through the interferometer, it is presented with a choice of which path to follow, and an interference pattern is not created since all of the light travels through one path. However, a classical interpretation cannot be applied to an experiment performed with single photons, and it will not predict accurate results.

According to the quantum-mechanical interpretation of the experiment, the photon has two possible states in the interferometer, corresponding to the photon in the first arm, and the photon in the second arm. The wavefunctions of these states are superimposed and interfere, and the result is that the photon will only strike the screen in a location where bright fringes would be expected if multiple photons were present. A single photon cannot reproduce an entire interference pattern by itself, but if many single photons are recorded over a period of time, the cumulative result is an interference pattern that is identical to one produced by a standard interference experiment.

On a quantum-mechanical scale, certain pairs of information cannot be determined simultaneously. For instance, the Heisenberg uncertainty principle states that the more precisely one knows the position of a particle at a given time, the less precisely one can know the momentum. If information exists (even if it is not measured by a conscious observer) that reveals one property (in this case, which path the photon traveled), then each particle will exhibit no wave interference, as the superpositioning of wavefunctions is destroyed. But if no information exists about one measurement, the wavefunctions of the photon’s different states superimpose, thereby creating the interference pattern. Additionally, if information exists that reveals one property, but then that information is destroyed, or "erased" (so that the path information can no longer be observed), the particles will again exhibit wave interference.

Thus, the result of the single photon interferometer is altered if the experimenter attempts to measure which path the photon followed. If the path measurement is performed, there are no longer two states to interfere, and the photons no longer produce the interference pattern; instead, they create a single, large spot on the screen. However, if the path information is "erased" so that it cannot be measured, the interference pattern returns.

Thorlabs Quantum Eraser Demonstration

The Thorlabs quantum eraser demonstration is an analogy to the original single-photon quantum eraser experiment but uses a continuous beam of photons that is visible to the naked eye. A Mach-Zehnder interferometer and a green laser diode are employed to create an interference pattern that is viewed on two screens, as shown above. A linear polarizer in each of the two signal paths allows either horizontally or vertically polarized light to pass through it, providing information about which path a given photon has traveled. As expected, the interference pattern disappears. A third polarizer, placed after the beams have been recombined, and set at 45° to each of the first two polarizers, serves to "erase" the path information, and the interference pattern reappears. While these results can be explained by classical properties of light polarization, the quantum-mechanical explanation serves as an analogy to the single photon experiment.

Quantum Eraser Kit Components

Mouse over the photo to see the corresponding components in the table to the right.

The Thorlabs Quantum Eraser demonstration kit has been carefully engineered to be easy to set up and to give clear, reliable results. The kit contains many stock Thorlabs components, making it possible to expand the scope of the experiment by purchasing additional components.

Note: English, German, and Portuguese language manuals/teaching information are available for this product. The imperial kit contains the English manual and US-style power cord. The appropriate manual and power cord will be included in the metric kit based on your shipping location. Please contact Tech Support if you need a different language, cord style, or power supply. As with all products on our website, taxes are not included in the price shown below.

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Thorlabs' Educational Kits provide flexibility in the classroom, allowing users to adapt the content to their own teaching needs. With that in mind, we enjoy hearing feedback from our customers with details about how they use the kits in their own classrooms. Use the Contact Us button to the right if you would like to submit your own user-generated content.

Pancharatnam's Phase Demonstration

We cordially thank Professor Surendra Singh, University of Arkansas, for sending us the following modification of the EDU-QE1 to demonstrate Pancharatnam’s phase of light.

Theory
Electromagnetic waves have both a polarization state and a dynamical phase that must be taken into account when considering the effects of propagation through material media. An electromagnetic plane wave of angular frequency ω propagating in the z-direction can be represented by its electric field:

In this equation, the dynamical phase is represented by

where n is the refractive index of the medium and c is the speed of light. φ₀ represents the initial or reference phase. The polarization state of the wave is determined by the relative magnitudes and phase difference of the x and y components of the electric field vector, captured by this part of the equation:

where δ is the relative phase difference between E_x and E_y. If the polarization state of the wave changes during propagation, the wave may acquire an extra contribution to its phase in addition to the change in its dynamical phase. This phase contribution is referred to as Pancharatnam's phase of light (also known as Pancharatnam-Berry phase).¹

Experiment
Pancharatnam's phase can be observed by making some minor modifications to the Mach-Zehnder interferometer in the Quantum Eraser Demonstration Kit. Circular polarization is required, so a linear polarizer and a quarter-wave plate must be added between the laser source and the first lens in the setup. An inexpensive way to achieve this is to use a pair of 3D movie glasses, such as the RealD™ 3D Glasses included in our Polarization and 3D Cinema Technology Kit; versions of these glasses can also be easily found and purchased online.

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Figure 2: The Mach-Zehnder Interferometer in the EDU-QE1 is modified to show Pancharatnam's phase by adding a pair of 3D glasses to circularly polarize the light, an extra mirror, and a linear polarizer.

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Figure 1: A Mach-Zehnder interferometer can be modified to show Pacharatnam's phase by adding an extra mirror to one arm.
M = Mirror, BS = Beamsplitter, Q = Wave Plate, LP = Linear Polarizer
LC = Left Circular Polarization, RC = Right Circular Polarization

When circularly polarized light is reflected off of either a mirror or a beamsplitter, it is converted to the opposite polarization state: at each reflection, left circularly polarized light becomes right circularly polarized or right circularly polarized light becomes left circularly polarized. In a typical Mach-Zehdner interferometer, the light in each arm undergoes the same number of reflections, so the light from each arm of the interferometer is in the same polarization state after it is recombined by the final beamsplitter before Port A (or Port B).

To observe Pancharatnam's phase, an extra mirror is added to one arm to introduce an additional reflection, which corresponds to an additional polarization-state change in that arm. The path lengths along the two arms of the interferometer are set to be exactly equal, removing any contribution from the dynamical phase. The schematic in Figure 1 traces the change of the polarization state at each reflection as light travels through the interferometer. As a result of these reflections, the output light at Port A or Port B is a combination of right circularly polarized light from one arm of the interferometer and left circularly polarized light from the other arm. At this point, no fringes will appear at Port A or Port B since the light from one arm is in a polarization state orthogonal to the light from the other arm.

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Figure 3: The linear polarizer must be hand-held in front of the viewing screen for the experiment to work. If it is mounted on the breadboard, rotating the polarizer will introduce vibrations that obscure the interference pattern.

Placing a linear polarizer in front of Port A or Port B will cause an interference pattern to appear on the screen. The polarizer selects the components of the right circularly and left circularly polarized light that are parallel to the polarizer's transmission axis. If the polarizer is placed with its transmission axis at an angle φ to the x axis, the light from the two arms of the interferometer emerges from the linear polarizer as two linearly polarized coherent beams with the same polarization state and a relative phase difference of 2φ. This generates an interference pattern that can be observed on the screen.

As the linear polarizer is rotated, the fringes will shift even though the path difference between the two arms of the interferometer has not changed, which means that the dynamical phase is not contributing to the change in the interference pattern. The fringe shift is entirely generated by the difference in polarization state between the light from the two arms of the interferometer, which translates into a difference in phase after the light passes through the linear polarizer. This phase difference is an example of Pancharatnam's phase.

Tips for Aligning the Setup:

• When setting up this modified Mach-Zehnder interferometer, recall that the interference pattern becomes smaller in diameter when the path length difference between the interferometer arms increases. Try to have almost equal path lengths in both arms to make the pattern easier to view.
• In practice, the mirror in the upper arm must be placed asymmetrically to introduce as small a difference in path length as possible (i.e. not in 45° with respect to the incident laser beam).
• Interference is only visible when a linear polarizer is placed in front of the screen (as explained above).
• To adjust the alignment of the setup, we recommend using the beam walk method described in Step 10 (c) of Chapter 4.3.3 in the EDU-QE1 manual.
• The linear polarizer used to view the change in the interference pattern must be handheld for the experiment to work. Otherwise, vibration will obscure the interference pattern when the polarizer is rotated.

Professor Singh has written a detailed explanation of the experiment including Jones vector calculations of the phase, which can be downloaded here.

¹S. Pancharatnam, Proc. Ind. Acad. Science A44, 247 (1956), also in Collected Works of S. Pancharatnam (Oxford U. Press, Oxford 1975).

We cordially thank Antje Bergmann (Karlsruhe Institute of Technology) for sharing her design of the quantum eraser setup.

Do you have ideas for an experiment that you would like to see implemented in an educational kit? Contact us at techsupport@thorlabs.com; we'd love to hear from you.