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Bomb Tester Demonstration Kit
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Interference fringes produced by the Michelson interferometer can be observed with the viewing screen.
Bomb Tester Analogy Demonstration Kit
Thorlabs' Bomb Tester Demonstration Kit uses an analogy experiment to demonstrate the principle of "interaction-free quantum measurement" discussed in the "Bomb Tester" thought experiment (published by Elitzur and Vaidman in 19931). This educational kit includes components to build a Michelson Interferometer, a viewing screen to observe the fringes, and a detector to measure changes in intensity of light exiting the interferometer.
Although some of the experiments require the use of a voltmeter, we have not included one with this educational kit since many physics labs are already stocked with them. For customers who would like to purchase a voltmeter to use with the kit, Thorlabs has a digital multimeter available here.
Thorlabs also offers a Michelson Interferometer Demonstration Kit; see the Kit Comparison tab for details.
Thorlabs Educational Products
Thorlabs' line of educational 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 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 and German language manuals/teaching information are available for this product. The imperial educational 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.
1A. Elitzur, L.Vaidman: Quantum mechanical interaction-free measurements, Foundations of Physics 23, 1993, p. 987-997
Bomb Tester Demonstration Kit
The "Bomb Tester" thought experiment demonstrates the principle of interaction-free quantum measurement by proposing a scenario where it is possible to detect the presence of a bomb (triggered by interacting with a photon) without detonating it. Thorlabs' Bomb Tester Demonstration includes a manual which guides students through the thought experiment as well as components and instructions for building an analogy experiment in the classroom.
Schematic of the Bomb Tester Thought Experiment
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Thorlabs' Bomb Tester analogy experiment includes all of the components to assemble a Michelson interferometer. The setup is shown here with the photodetector at the interferometer output. To measure the voltage at the photodetector, a user-supplied voltmeter or multimeter is required.
Bomb Tester Thought Experiment
In a standard Michelson interferometer, light exits a laser and is split into two beams by a beamsplitter. The light then travels down two perpendicular arms of different lengths. Mirrors at the end of each arm reflect the light back to the beamsplitter, where the beams are combined. If the path lengths differ by an integer number of waves, the recombined beams will constructively interfere with each other, creating a bright spot on an observation screen positioned beyond the beamsplitter. Alternatively, if the paths differ by an odd number of half-waves, the two beams will destructively interfere with each other.
Interference experiments can also be performed by passing a single photon through the interferometer, rather than a continuous stream of light. According to quantum mechanics, each photon has two possible states in the interferometer which correspond to the presence of the photon in each arm. The observed interference pattern is created by the superposition of the wavefunctions describing these states. As a result, any photon sent through the system can only reach the screen where the two wave functions do not destructively interfere, i.e. in one of the bright rings observed in the standard experiment when photons pass through the system continuously. If the position on the screen of an individual photon sent through the system is recorded over many trials, the interference pattern will be reproduced.
Finally, consider what will happen to the interference pattern if one of the possible paths the photon can take is "marked." The Heisenberg uncertainty principle predicts that on quantum-mechanical scales, certain pairs of information cannot be known simultaneously. For example, the more precisely the position of a particle is known, the less precisely its momentum can be determined. "Marking" a path through the interferometer provides information about the location of the photon, which will destroy the superposition of states of the wavefunction and erase the interference pattern.
The bomb tester thought experiment examines how these principles can be used to detect the presence of an object without a photon interacting with it. The experiment proposes that there are a certain number of bombs designed to explode after they absorb a photon. Some of the bombs are active, while others are duds. The duds and active bombs cannot be differentiated unless they interact with light. The quantum-mechanics of a "which-path" system can be used to perform this test without detonating all of the active bombs.
To perform the measurement, a Michelson interferometer is aligned so that the interference pattern will have a dark spot at the center and this central minimum is aligned with the input of a detector, instead of a screen. A bomb is placed in one arm of the interferometer and a single photon is passed into the system. If the bomb is a dud, it will not interact with the photon at all. The wavefunctions of the photon in each arm of the interferometer interfere and the photon does not hit the surface of the detector. If the bomb is active, it can interact with the photon. This "marks" the arm of the interferometer containing the bomb, destroying the superposition of states. The photon can be detected by either the bomb or the detector. If the photon interacts with the bomb, then the bomb will detonate. If the photon reaches the detector, we know that the wavefunction collapsed into the arm of the interferometer without the bomb; the absence of detection by the bomb means that the photon must travel in the other arm. Thus, the bombs can be sorted without detonating all of the active bombs.
Thorlabs Analogy Experiment
Thorlabs' "Bomb Tester" analogy experiment uses a Michelson interferometer. The kit includes all of the components to build the interferometer setup, as well as a viewing screen and photodetector. In the analogy experiment, a continuous green laser source is used instead of a single photon source.
The interferometer is set so that destructive interference occurs at the center of the interference pattern and the detector is placed at this location. The scenario with a "dud" is simulated by keeping both of the interferometer arms free from obstruction and the voltage readout from the detector is recorded with a user-supplied voltmeter or multimeter, such as our DVM1 digital multimeter. Next, one arm of the interferometer is blocked to simulate the active bomb and a second measurement is taken. Cut-out figures are included in the manual as visual representations of the "dud" and the active bomb. As in the thought experiment, the light at the detector increases when an object is placed in one arm of the detector since the interference pattern has been destroyed. The fraction of the total power detected when one arm of the interferometer is blocked represents the probability of detecting a photon when an active bomb is placed in the setup.
Bomb Tester Kit Components
Mouse over the photo to see the corresponding components in the table to the right.
The Thorlabs Bomb Tester 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. To perform the experiment using the included photodetector instead of the screen, a user-supplied voltmeter or multimeter, such as our DVM1, is required. We also recommend purchasing our SPW606 spanner wrench for mounting optics into the LMR1 optic mounts included with this kit.
Purchasing Note: English and German 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.
Imperial Kit: Included Hardware and Screws
Metric Kit: Included Hardware and Screws
Interferometer Demonstration Kit Comparison
Thorlabs offers three demonstration kits that make use of an interferometry setup. Each kit is designed to target a different topic and includes accessories to support the experiments described in the kit manual. The EDU-MINT1 interferometer kit is designed to explore the experimental uses of a Michelson Interferometer, while the EDU-BT1 and EDU-QE1 analogy demonstrations are designed to explore concepts in quantum mechanics. Due to the nature of the experiments in the EDU-MINT1 kit, an intrinsically damped breadboard is included to minimize vibrations as students interact with the setup. This added feature supports some of the more challenging experiments, especially the wavelength measurement and white light interference exercises. The other two demonstration kits are built on lighter, aluminum breadboards as the experiments outlined in these kits require less interaction with the setups and vibration is therefore less of an issue. The table below outlines the key features and educational topics addressed by each kit to aid in choosing the best option for your classroom.
Upgrading the EDU-MINT1(/M) to perform like the EDU-BT1(/M) is simple; the only additional components are a photodetector and necessary mounting hardware. See the links below for the list of items required for this upgrade.
We cordially thank Antje Bergmann and Pascal Kuhn (Karlsruhe Institute of Technology) for their prototype of a bomb tester setup.
Do you have ideas for an experiment that you would like to see implemented in an educational kit? Contact us at firstname.lastname@example.org; we'd love to hear from you.