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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
Thorlabs' Quantum Eraser educational lab kit shows through analogy the quantum-mechanical principle of complementarity and the erasure of path information. Designed to show the fundamental principles of quantum physics, this experiment clearly displays how nature is often counterintuitive on the quantum scale.
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 educational kit includes all the necessary components and a manual that contains both detailed setup instructions and extensive teaching materials. These lab 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 kit. The imperial version 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. The power supplies and other electronic devices in both the metric and the imperial kit accept voltages of 230 VAC and 120 VAC. Some power supplies include a switch for selecting the mains voltage. See the respective manuals for details. Please contact Tech Support if you need a different language or cord style. 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
In the quantum eraser experiment, single photons are emitted into a Mach-Zehnder interferometer. Using linear polarizers, the photons are "marked" as having either a horizontal or vertical polarization state, indicating which side of the interferometer they have traveled through. The interference pattern (wave property) and path information (particle property) cannot be measured simultaneously, since measuring the path information destroys the interference pattern. A third linear polarizer, placed after the beams have been recombined, "erases" the path information, making the photons anonymous again, and thus restoring the interference pattern.
Rather than using single photons, as in the original quantum eraser experiment, this kit uses a green continuous-wave (CW) laser light source that produces a beam that is visible to the eye. While the outcome of the experiment can be explained using classical physics, using a quantum-mechanical description provides a perfect analogy to the single-photon quantum eraser experiment.
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 power supplies and other electronic devices in both the metric and the imperial kit accept voltages of 230 VAC and 120 VAC. Some power supplies include a switch for selecting the mains voltage. See the respective manuals for details. 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.
Imperial Kit: Included Hardware and Screws
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.
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 Ex and Ey. 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
Additional Items Needed (Metric Versions in Parentheses)
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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.
The interference fringes shift as the linear polarizer is rotated.
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:
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 email@example.com; we'd love to hear from you.
Laser Safety and Classification
Safe practices and proper usage of safety equipment should be taken into consideration when operating lasers. The eye is susceptible to injury, even from very low levels of laser light. Thorlabs offers a range of laser safety accessories that can be used to reduce the risk of accidents or injuries. Laser emission in the visible and near infrared spectral ranges has the greatest potential for retinal injury, as the cornea and lens are transparent to those wavelengths, and the lens can focus the laser energy onto the retina.
Safe Practices and Light Safety Accessories
Lasers are categorized into different classes according to their ability to cause eye and other damage. The International Electrotechnical Commission (IEC) is a global organization that prepares and publishes international standards for all electrical, electronic, and related technologies. The IEC document 60825-1 outlines the safety of laser products. A description of each class of laser is given below: