An object held in two places at once can interfere with itself. Then a gas molecule bounces off it and carries away a hint of where it was. A few such hints and the superposition is, for all practical purposes, gone.
Each run starts a fresh superposition. Watch the right-hand panels: every collision shrinks the off-diagonal squares and flattens the fringes a little. Then pump out the gas and see the superposition survive.
Real objects are hit far more often than the toy above. Pick an object and its surroundings. The estimate assumes one gas collision is enough to record which place the object is in, true once the two places are more than about 0.02 nm apart.
A beam splitter puts the object into a superposition of the upper and lower path. While it waits in the chamber, gas particles bounce off it. Each bounce happens in one branch and not the other, so the particle's new direction is a partial record of where the object was.
The 2×2 grid is the density matrix. The diagonal squares are the 50/50 odds of each path and never change. The off-diagonal squares measure how much the two branches can still interfere. The fringe panel shows what the output detectors would record if you recombined the paths now.
Nothing pushed the object or forced it to choose. Each branch is still there. But the two branches are now entangled with different states of the gas, and to see interference you would have to bring every scattered particle back together too. That is hopeless, so the object behaves like an ordinary thing that is simply in one place or the other.
This is why we never see cats in superposition. A cat in air is hit by about 10²⁷ air molecules every second, so its superposition would vanish far faster than light can cross a single atom.
Watched step by step by Serge Haroche's team in Paris in 1996, using a few photons trapped between mirrors. Haroche shared the 2012 Nobel Prize.
Every quantum technology is a race against the leak you just watched. Some measure it to learn about the world; most spend enormous effort slowing it down.
Superconducting qubits keep their coherence for roughly 0.1 to 1 millisecond, and a two-qubit gate takes tens of nanoseconds. That leaves room for thousands of operations before coherence runs out, and today's best two-qubit gates still fail about once in every few hundred to a thousand tries.
In the demo: the decoherence time sets how long you have before the fringes fade.Instead of stopping decoherence, error correction spreads one logical qubit across many physical ones and repeatedly checks for leaks without reading the data. In 2024 Google's Willow chip showed the logical error rate falling as the code grew, the long-sought sign that the approach can scale.
In the demo: each collision is a small error that a code would have to catch.Superconducting quantum processors sit in dilution refrigerators at around 10 millikelvin, inside vacuum cans and magnetic shields. Stray heat, radio noise, even cosmic rays and natural radioactivity have been shown to cause bursts of qubit errors.
In the demo: lower the temperature and pump out the gas.In an MRI scanner, proton spins are put into a superposition and their shared rhythm then fades as each spin is jostled by its neighbours. How fast it fades, called T2, is typically tens to hundreds of milliseconds and differs between tissues, so T2-weighted scans make tumours, swelling and fluid stand out.
In the demo: T2 decay is the off-diagonal squares shrinking.Atomic clocks read time from interference fringes in a superposition of two energy levels. The longer the superposition survives, the narrower the fringes and the better the clock, which is why the best clocks hold atoms or ions in ultra-high vacuum, often for a second or more.
In the demo: fewer collisions means sharper fringes for longer.Vienna groups send big molecules through interferometers to find where quantum behaviour ends. In 2003 they let a little gas into the chamber and watched C₇₀ fringes fade with pressure; in 2004 they heated the molecules until their own glow gave them away. They have since seen interference with molecules of about 2,000 atoms.
In the demo: raise the pressure and watch the fringes go.