Quantum information is information stored and processed using the rules of quantum physics rather than classical physics. Where ordinary digital information exists as bits locked into either 0 or 1, quantum information exploits the strange behavior of subatomic particles to exist in richer, more complex states. This distinction isn’t just theoretical. It underpins a growing set of real technologies, from ultra-secure communication links spanning thousands of kilometers to sensors that detect single photons of light.
Bits vs. Qubits
The basic unit of classical information is the bit: a switch that’s either off (0) or on (1). Every file on your computer, every streaming video, every text message breaks down into long strings of these binary values. The basic unit of quantum information is the qubit, and it behaves very differently.
A qubit can be 0, it can be 1, or it can exist in a “superposition” of both at the same time. Think of a coin sitting on a table: it’s heads or tails. A qubit is more like a coin mid-flip, occupying a blend of both possibilities until you look at it. Mathematically, the state of a qubit is described as a combination of its two possible values, with each possibility carrying a specific weight. Those weights determine the probability of getting one outcome or the other when you finally measure it.
This superposition is what gives quantum information its power. A system of two classical bits can be in one of four states at any moment (00, 01, 10, or 11). A system of two qubits can represent all four of those states simultaneously, and a system of 300 qubits can represent more states than there are atoms in the observable universe. That exponential scaling is why even small quantum systems can, in principle, tackle problems that would overwhelm conventional computers.
Entanglement: Correlated Without a Connection
Superposition is only half the story. The other defining feature of quantum information is entanglement, a type of correlation between particles that has no equivalent in everyday experience. When two qubits become entangled, their states are linked so tightly that measuring one instantly tells you something about the other, regardless of the distance between them. Erwin Schrödinger, who coined the term in 1935, called it “the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.”
Here’s the subtle part: entanglement doesn’t let you send a message faster than light. The result you get when measuring one entangled particle looks random on its own. It’s only when you compare the results from both particles that the correlation shows up. What entanglement does provide is a shared resource, a kind of coordinated randomness that two distant parties can use for tasks like encryption and teleportation of quantum states.
Albert Einstein famously objected to entanglement, arguing that each particle should have its own definite properties independent of anything happening far away. But experiments inspired by physicist John Bell’s work have repeatedly confirmed that quantum correlations are real and cannot be explained by any hidden, pre-existing agreement between the particles. The universe genuinely allows this kind of nonlocal connection.
Why Quantum Information Can’t Be Copied
One of the most important rules governing quantum information is the no-cloning theorem: it is physically impossible to make a perfect copy of an unknown quantum state. This is a hard limit, not a matter of needing better technology. The math behind it is surprisingly simple. Copying a quantum state would require a process that works the same way on every possible state, and the equations of quantum mechanics don’t allow such a process to exist.
For classical information, copying is trivial. You can duplicate a file a million times without degrading it. But a qubit in an unknown superposition cannot be duplicated. You can copy a qubit that’s definitely 0 or definitely 1 (those are classical-like states), but the moment you try to clone a qubit in a genuine superposition, the process fails.
This limitation has a silver lining: it makes quantum information naturally suited for secure communication. If an eavesdropper tries to intercept a quantum message, they can’t copy the qubits and pass them along undetected. Any attempt to read the information disturbs it in a measurable way, alerting the sender and receiver to the intrusion.
Measurement Changes Everything
In classical computing, reading a bit doesn’t change its value. You can check whether a switch is on or off without affecting it. Quantum information doesn’t work that way. Measuring a qubit forces it out of superposition and into a definite value, 0 or 1. The rich blend of possibilities collapses into a single outcome, and the original superposition is lost.
This is often called “wave function collapse,” though physicists disagree about what’s actually happening. One interpretation treats the collapse as a real physical event. Another, proposed by Hugh Everett in 1957, holds that collapse is an illusion and all possible outcomes are realized in branching parallel realities. Recent experiments using extremely sensitive particle physics instruments have tested whether collapse is a physical process, but the question remains one of the deepest in physics. What everyone agrees on is the practical consequence: measuring quantum information is irreversible and destructive to the original state.
How Quantum Information Is Measured
Classical information theory uses a concept called Shannon entropy to quantify how much information a source produces. Quantum information has its own version, called Von Neumann entropy, which measures the uncertainty in a quantum system. When you know exactly what state a quantum system is in (a “pure” state), the Von Neumann entropy is zero. The less you know about how the system was prepared, the higher the entropy.
Von Neumann entropy plays a dual role. It tells you the minimum number of qubits you’d need to faithfully encode the output of a quantum source, and it also tells you the maximum amount of classical information (in regular bits) that you could extract from that source by making the best possible measurement. It’s the quantum world’s answer to the question “how much information is here?”
Physical Hardware for Storing Qubits
Quantum information needs a physical home, and researchers are pursuing several approaches. The two leading platforms are trapped ions and superconducting circuits, each with distinct trade-offs.
Trapped ion systems use individual atoms suspended in electromagnetic fields. Their qubits hold their quantum state for remarkably long periods, with dephasing times around half a second and virtually no energy loss over time. They also offer fully connected qubit networks, meaning any qubit can interact directly with any other. The downside is speed: a two-qubit operation takes about 250 microseconds.
Superconducting systems, built from tiny circuits cooled to near absolute zero, are far faster. A two-qubit gate completes in 250 to 450 nanoseconds, roughly a thousand times quicker than trapped ions. But their qubits are more fragile, maintaining coherence for only about 60 microseconds, and they connect in more limited patterns. A study published in the Proceedings of the National Academy of Sciences comparing the two architectures found higher accuracy and coherence times in the trapped-ion system, with higher clock speeds in the superconducting system. Neither platform has a decisive overall advantage yet.
Both approaches share a fundamental challenge: errors. Qubits are extremely sensitive to noise from their environment, and correcting those errors requires combining many physical qubits into a single reliable “logical” qubit. In a recent experiment at ETH Zurich, researchers encoded one logical qubit across 17 physical qubits, and estimated that 41 physical qubits would be needed for a logical qubit that’s stable against a broader range of errors. Scaling up to thousands of logical qubits, which useful quantum computers will need, remains the field’s central engineering problem.
Quantum Communication Over Long Distances
Quantum information isn’t just stored and computed on. It can be transmitted. Quantum communication uses the principles of entanglement and no-cloning to create encryption keys that are theoretically unbreakable, because any interception attempt leaves a detectable trace.
China has built the most extensive quantum communication infrastructure to date, including a 2,000-kilometer fiber-optic quantum network with 32 nodes connecting Beijing to Shanghai. In March 2025, scientists from South Africa and China established a record-breaking quantum satellite link spanning 12,900 kilometers, the longest intercontinental quantum-secured connection ever demonstrated. The link used the Jinan-1 microsatellite in low Earth orbit and marked the first quantum satellite communication in the Southern Hemisphere. An earlier milestone, a 7,600-kilometer link between China and Austria using the Micius satellite, was achieved in 2017.
Quantum Sensing
Beyond computing and communication, quantum information principles are being applied to measurement. Quantum sensors exploit the extreme sensitivity of quantum states to detect physical quantities like magnetic fields, gravity, and light with precision that classical instruments can’t match. According to NASA research, quantum sensors offer higher signal-to-noise ratios, better measurement resolution, and accuracies tied to fundamental physical constants. Single-photon detectors, for example, can register individual particles of light across a range from visible wavelengths to long-wave infrared, enabling measurements at the absolute physical limit of sensitivity.
These sensors have potential applications in Earth observation, navigation, and medical imaging, wherever squeezing out more precision from a measurement translates into better data.