# Building Quantum Bomb Testers (And Other Thought Experiments) with Quantum Computers

*By Maria Violaris, PhD student at the University of Oxford*

Imagine someone gives you a box, and you can’t see what’s inside. All you know is that it’s either empty or it contains a highly sensitive bomb. If this bomb interacts with anything, if it is hit by even a single *photon* of light, it will explode. Your challenge is to work out whether there is a bomb in the box without exploding it. Classically, this challenge is impossible. The laws of classical physics do not allow you to build a “bomb tester” that can check the contents of the box without potentially causing it to explode. Quantumly, however, things get a little more interesting.

The above scenario is an example of a thought experiment, also known as a gedankenexperiment. These kinds of scenarios have played a crucial role in the evolution of quantum theory since its inception, with physicists since Albert Einstein using them to craft our understanding of quantum’s counterintuitive principles. Even today, combining those thought experiments with the tools of modern quantum computing can give both quantum novices and experts alike a whole new understanding of some of the most fundamental, perplexing, and seemingly paradoxical concepts in quantum mechanics.

I’m Maria, a PhD student researching the foundations of quantum information at the University of Oxford and an intern at IBM Quantum. I was in high school when I first realized that playing with imaginary scenarios like a quantum bomb tester can lead to new discoveries in physics. Since then, I’ve written about thought experiments for Physics World magazine, written a poem about two characters stuck in a thought experiment for a competition, and completed an undergraduate research project on a thought experiment involving a quantum demon. Today, I work on understanding puzzling quantum thought experiments as part of my PhD. Now, through a new video series on the Qiskit YouTube channel and accompanying blog posts, I hope to take you deep into the world of quantum thought experiments.

So, what use are these fictional scenarios to our understanding of a core physics theory?

**Why Quantum Thought Experiments Matter**

The rules and logic of the quantum world can seem completely nonsensical, since we are used to experiencing the universe at the macro scale. Particles act like waves—sitting in multiple places and states at once or obeying statistics not possible to achieve through regular probability theory — but then snap back into following classical laws once we observe them. These unfamiliar behaviors have long required some creativity for our brains to capture their importance and their implications.

Perhaps the most important year for quantum thought experiments was 1935, which brought us Schrödinger’s cat and the Einstein-Podolsky-Rosen (EPR) paradox. In the former, Erwin Schrödinger pictured a cat trapped in a box with a radioactive atom and a vial of poison. If the atom decays, it causes a hammer to smash open the vial of poison and kill the cat. If the atom does not decay, the cat stays alive. According to quantum theory, the atom enters a superposition of decaying and not decaying. Following entanglement with the atom, the cat should enter a superposition of being dead and alive — but surely cats can’t be dead and alive at the same time!

Meanwhile Albert Einstein, Boris Podolsky and Nathan Rosen pictured two particles interacting to become correlated, which then fly apart. But if you measure one particle, the other one instantly takes on a related value seemingly without any information exchanged between the two. To Einstein, this meant that quantum theory implies some “spooky action at a distance” between the particles, now known as “non-locality” between entangled particles.

Some might consider these paradoxes to be examples of physicists thinking too classically about innately quantum scenarios. But these thought experiments have proved to be tremendously influential in our understanding of quantum theory and its implications. They continue to serve as central topics of theory, experimentation, and philosophical debate. Take, for example, the bomb tester scenario I laid out in the first paragraph. This is a real thought experiment first developed by physicists Avshalom Elitzur and Lev Vaidman in 1993. Their thought experiment introduced the concept of a quantum bomb tester that should be able to check the contents of a box without detonating the bomb.

In principle, it is very much possible to make a real quantum bomb tester; we can use a photon source to fire a photon at a beamsplitter, a component designed to both transmit and reflect photons at the same time like a half-silvered mirror. When a photon hits the beamsplitter, it enters an equal superposition of being reflected and transmitted. Then, we use mirrors to guide the reflected and transmitted photons to meet again at a second beamsplitter, where they interfere to merge into one transmitted photon.

Now, what if there is a bomb in one of the paths? The bomb’s presence measures the path of the photon, so the photon will collapse from the superposition to either the reflected or transmitted path. If it collapses into the path with the bomb, then the bomb will explode. But if it collapses into the other path, it will then reach the second beamsplitter, and split into a superposition of being reflected and transmitted. So, if we place detectors after the second beamsplitter and find the photon was reflected, we know for SURE that the bomb is there, without setting it off!

Now, in this setup, there is only a quarter chance that we detect the bomb is there without setting it off. Reassuringly, other bomb testers have been proposed that can detect the bomb with up to 100% probability.

The quantum bomb tester has real-world implications. We can use such a setup to image delicate objects like biological cells without physical photons hitting them to cause damage. We can perform counterfactual quantum computing — where we infer the results of a quantum computation even without running the algorithm. Counterfactual quantum communication protocols have been proposed that could lead to unhackable messages.

The bomb tester and its applications also have important implications for our core understanding of quantum mechanics, and their full interpretation is still under debate. In Elitzur and Vaidman’s original proposal, they even suggested that the many-worlds interpretation of quantum mechanics is needed to explain the experiment.

**Using Quantum Computers to Think**

Creating a real quantum bomb tester poses a practical problem — it has a bomb in it (though thankfully, physicists can build setups that swap the bomb for something safer). However, we’re also armed with an even easier tool that allows us to take these experiments out of our heads and watch how they might play out without requiring any explosions: quantum computers. These devices allow us to create and control quantum states in order to run quantum circuits — the quantum version of classical computing’s logic circuits — that employ superposition and entanglement to run new kinds of algorithms.

One of the nice things about working with quantum circuits is that you don’t need to think about cats or any other clever scientific parables when you’re creating entangled quantum superpositions. If you want to put a qubit in the ground |0> state into a superposition of |0> and |1>, you can simply apply a Hadamard gate (H). To entangle it with another qubit, you add a controlled-not gate (C-NOT) between them.

To better understand the quantum bomb tester, I’ve mapped this experiment onto quantum circuits, and used these circuits to create a game of Quantum Minesweeper that can run on a quantum computer. This is inspired by the classic video game Minesweeper, where the player uses clues to try and work out whether there is a bomb hidden in a box. In my game, the bomb-tester is a quantum circuit, and the clue is the result of measuring a qubit, which tells us whether to predict that a bomb is present or not.

Now if thinking about the bomb tester makes you feel like your brain is being split, superposed and merged again through a series of beamsplitters, not to worry. If you haven’t done so already, be sure to watch the first video in our new Quantum Paradoxes video series (see below), which may help you get a better understanding of these concepts, especially if you’re more of a visual learner. Once you’re done with that, take a look at the Jupyter Notebook linked here, where I’ll show you how to present this controversial bomb tester as a simple quantum circuit, which you can experiment with using Qiskit.

Of course, the quantum bomb tester is far from the only paradoxical thought experiment that you can encode on a quantum computer. If you find this exercise helps you gain a better understanding of concepts like superposition and entanglement, then be sure to keep an eye out for upcoming videos in the Quantum Paradoxes video series.