Imagine shining a flashlight in a dark room. You can predict exactly how the light will travel: in a straight line from one point to another. In the world we see, light appears to follow a single, clear path, so this seems obvious.
But quantum mechanics paints a far stranger picture.
Zoom in to the atomic scale, and light does not behave as though it follows only one straight path. Instead, particles of light explore every possible path at once. One path may indeed be a straight line across the room. But others might bounce off a wall, curve through space, or take an incredibly roundabout route before reaching the destination.
In a sense, nature keeps all of these possibilities "alive" at the same time. A single outcome appears only after the light is "observed," that is, after a measurement is made. The path we observe, usually a straight line, is simply the most probable result, the outcome that emerges when all the possible paths interact so that some results become more likely and others less so.
This idea seems almost impossible from the perspective of everyday life, yet it lies at the heart of quantum mechanics. Today, scientists like me are learning how to harness these strange quantum effects to build an entirely new kind of machine: the quantum computer.
Superposition
In everyday life, physics is simple and straightforward. A tennis ball is either on the table or it isn't. A light is either on or off. Even if you were in another room and didn't know which, the object itself would already be in a definite state.
Quantum mechanics doesn't follow these ordinary rules. Under the laws of quantum mechanics, an outcome is not settled until someone observes it.
At extremely small scales, particles don't have a definite state. Rather than being one thing or another, a particle exists in a state that hasn't yet been decided. This isn't because information is missing, but because reality itself hasn't yet been fixed.
Electrons, photons and atoms can exist in a superposition, a mixture of multiple possible states. A particle can be said to be here, there, or somewhere in between. Schrödinger's cat is the famous example: a cat in a closed box connected to a quantum device is both alive and dead until someone checks.
As strange as this may sound, superposition has been confirmed repeatedly by laboratory experiments. But these quantum states are extremely fragile. Interactions with the outside world, such as heat, vibrations, stray electromagnetic fields or accidental contact with surrounding particles, can destroy a delicate superposition and force the system into a single state.
Physicists call this process decoherence. A quantum system behaves a bit like a spinning coin. While the coin is spinning in the air, from our perspective it is neither heads nor tails, and both outcomes remain possible. In quantum mechanics, a particle can exist in a similar kind of mixed state called a superposition, in which multiple possible states coexist at the same time.
Only when a measurement is made does the system "choose" a definite outcome, just as a spinning coin eventually lands on either heads or tails.
Physicists call this process "collapse," but scientists have yet to resolve the fundamental mystery behind this phenomenon. Imagine streaming music. The data is kept in its original form until you launch the streaming service, which is ready to play any song. Pressing the play button turns those possibilities into one actual song. Measuring a quantum particle has a similar effect, collapsing all possible outcomes into a single actual one.
Spooky friendship at a distance
Superposition isn't the only strange feature of quantum mechanics. Another is entanglement, a phenomenon in which two particles interact and their properties become deeply linked, even when they are far apart.
When two particles interact in certain ways, they can become entangled. This means that measuring one instantly tells you something about the other, no matter how far apart they are. Einstein disliked this idea and called it "spooky action at a distance." He believed quantum theory was missing something. But after numerous experiments, scientists have shown that quantum entanglement is real.
A good way to visualize it is to picture two dancers. After plenty of practice, they can match each other's movements even on different stages, without exchanging a word. Their perfect timing comes from a routine they learned together.
From strangeness to a new kind of computer
So what does all this mean for the future of computing?
Ordinary computers, like the one on your desk, use bits. Each bit is either zero or one, much like a light switch. All of your photos, videos and messages are just long strings of these switches. Quantum computers, however, use quantum bits, or qubits. Thanks to superposition, a single qubit can be zero, one, or a mixture of both. Two qubits can encode four possibilities at once, 10 qubits can encode 1,024, and 300 qubits can represent more states than there are atoms in the universe.
Quantum computers aim to perform certain calculations far faster than classical computers, but current systems still face major limitations in speed, stability and computing power.
A standard computer works like a runner who must check every path through a maze one at a time. In a quantum computer, by contrast, quantum states evolve so that useful computational paths reinforce one another, guiding the system toward the correct result.
Quantum computers are not practical for handling email, managing spreadsheets or streaming content, because existing classical computers are perfectly adequate for those tasks. The real potential of quantum computers lies in solving problems too complex even for the world's most powerful classical computers.
These challenges include simulating molecular interactions to help scientists design new drugs, discovering new materials that could improve technologies such as solar cells and batteries, and advancing fields such as cryptography.
The keys and locks of the internet
Nearly all personal information sent online, including bank logins, medical records and private messages, is protected by Rivest-Shamir-Adleman (RSA) encryption, one of the most widely used methods for protecting digital communications. RSA relies on multiplying two large prime numbers. It is easy to multiply them together, but extremely difficult to work backward from the result to figure out which primes were used, and that is what keeps it secure. It is estimated that an ordinary computer would take billions of years to break a strong RSA key by brute-force guessing.
But in 1994, mathematician Peter Shor showed that a sufficiently powerful quantum computer could solve this problem in just a few hours. Today's quantum computers are not yet that large, but with rapid progress, that could become reality within the next 10 to 20 years.
In light of these advances, security experts have warned about a "harvest now, decrypt later" approach. Hackers are already collecting encrypted data in the hope that future quantum computers will make it possible to crack. For example, medical records from 2026 could be exposed in 2040.
Quantum mechanics doesn't just threaten current security systems; it also makes new forms of protection possible. Quantum key distribution could use the delicate nature of quantum states to detect eavesdroppers. If someone tried to intercept a quantum message, doing so would alter the system and leave a clear sign of tampering.
Researchers in China demonstrated quantum key distribution via satellite in 2017, and governments around the world are now working on building quantum-safe networks.
But these advances also raise ethical questions. The same quantum tools that help us understand proteins could also be used to spy on private messages. The country or company that first builds a quantum computer that outperforms classical computers would wield significant power over others.
This shift could prove as significant as the invention of writing or nuclear technology. Technology itself is neither good nor bad; what matters is how people use it.
Where we are now
In 2019, Google announced that its Sycamore processor had performed a calculation in 200 seconds. According to the team's estimate, a typical supercomputer would take 10,000 years to do the same task. Some have questioned this claim, and the calculation itself was not very practical, but it was still a major step and an example of "quantum advantage."
Google's quantum computer Sycamore is trying to solve complex problems faster than classical computers.
Today, IBM, Google, IonQ and many universities are working to build larger and more reliable quantum computers. The biggest problem is that qubits are extremely fragile. Even trivial things such as vibrations, stray light or slight temperature changes can disturb their state. Most quantum computers need to be cooled to temperatures even colder than outer space.
Quantum computers will probably not replace your desktop PC or speed up your everyday computing tasks. But they are part of a technological revolution that could improve medicine, materials science and cybersecurity. The impact will likely arrive gradually. In the near term, quantum computers will remain specialized research tools, but over the next 10 to 20 years, they may begin to play practical roles in a variety of fields.