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Bomb Tester Demonstration


Bomb Tester Demonstration Kit

  • Designed for Education,Demonstration, and Classroom Use
  • Easy-to-Use Kit Includes All Component Plus Educational Materials

Bomb Tester 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


  • Photons Create an Interference Pattern in a Michelson Interferometer
  • Examines the Principle of Interaction-Free Quantum Measurement
  • "Marking" a Photon with Path Information Destroys the Interference Pattern

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). The kit includes all of the 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.

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. 



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.


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 continously. 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. Next, one arm of the interferometer is blocked to simulate the active bomb and a second measurement is taken. 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 detecteted when one arm of the interferometer is blocked represents the probability of detecting a photon when an active bomb is placed in the setup.


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