You might be wondering why today’s Doodle shows an illustration of quantum superposition. April 14 is World Quantum Day, and this year is also the International Year of Quantum — celebrating 100 years since the discovery of quantum mechanics. And what’s special about quantum mechanics? These are underlying rules for the world around us at the smallest level, including molecules, atoms and subatomic particles. And teams at Google are working on using the same principles to power a computer to advance science and solve problems that are otherwise impossible. So let’s talk more about one of those principles: superposition.

Quantum computers are fundamentally different from classical computers due to the phenomenon of superposition, explains Monica Hansen, Head of Technical Operations at Google Quantum AI. Classical computers use bits (a computer’s smallest unit of data) that can either be 0 or 1. Quantum computers, on the other hand, use qubits, or quantum bits, that can be 0 and 1 at the same time. This ability to be in multiple states at once is a fundamental quantum mechanics principle called “superposition.” So when making a Doodle to celebrate World Quantum Day, Googler Chuck Carlson and his team of Doodle designers wanted to use a fun visual metaphor. “We loved this idea of applying the behaviors of quantum to a Doodle,” Chuck says.

Enter thaumatropes: optical illusions that use movement to combine two distinct images into one. You may remember a children’s toy where you pulled two strings attached to a piece of paper with two different drawings on each side. When you pull the strings, the two separate images create a full image.

What’s most exciting about the Doodle is the chance to help people understand one of the most important things about quantum computing. “We thought thaumatropes were a really clean way of visualizing this idea of superpositions — of explaining something that’s so complex in such a simple form,” Chuck says.

“Quantum computers might not replace classical computers for day-to-day tasks,” Monica says. “But the hope is they will one day excel at solving complex problems that overwhelm traditional classical computers — especially those problems involving the behavior of particles like atoms and molecules.”

To learn more about how we’re advancing the science of quantum computing to solve real-world problems, read our piece celebrating World Quantum Day 2025.