Go further · Chapter 3 · 3b · A step deeper
Why do qubits lose their information?
The short answer (tap to open)
A qubit holds information only while its surroundings leave it alone. Two things go wrong. It can lose energy and drop to its lower level; engineers call the time this takes T1. Or its delicate in-between state can drift and blur; they call that time T2. Heat, stray fields, flaws in materials and even cosmic rays all cause both.
What are the two ways a qubit fails?
Think of chapter 2's arrow. A qubit's state points in a definite direction, and that direction holds the information.
Energy loss (T1). The arrow falls back to its lowest level, like a ball rolling to the bottom of a bowl. The qubit hands its energy to its surroundings. Engineers measure how long this takes and call it T1.
Dephasing (T2). The arrow doesn't fall, but it wobbles randomly around, so you lose track of exactly where it points. Interference (chapter 4) needs that direction to be precise, so the calculation fails. Engineers call this time T2. It's never longer than twice T1, and usually shorter.
T1 ≈ 100 µs · T2 ≈ 67 µs
The times are illustrative.
What each slider does, in words
- Energy loss: the arrows fall back to the bottom of the dial. The time this takes is T1.
- Wobble: the arrows don't fall, but they fan out, so you lose track of where they point. The time this takes is T2, never longer than twice T1.
What causes it?
- Heat. Every bit of warmth is a stream of tiny kicks, so superconducting qubits run at about a hundredth of a degree above absolute zero.
- Stray fields. Magnetic and electric noise from wires, electronics and even nearby equipment.
- Flaws in materials. Microscopic defects in the chip's surfaces and insulators that act like tiny rogue two-level systems.
- Cosmic rays and radioactivity. In 2021 Google's team showed that high-energy particles from space and natural radioactivity can hit a chip and cause bursts of errors across many qubits at once.
How long do qubits last today?
It depends on the type. The best superconducting qubits keep their state for around a thousandth of a second. Trapped-ion qubits can keep certain states for seconds to minutes, and labs have reported far longer. Long isn't everything, though: what matters is how many operations fit inside that time, and how often each one goes wrong. Chapters 8 to 11 compare them.
Can we beat it?
Partly, with better materials, shielding, and even labs deep underground. The full answer is chapter 8: error correction, which spreads one qubit's information over many so that errors can be spotted and fixed before they spread.
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A qubit fails in two ways: it falls, or it blurs.
Heat, noise, flaws and cosmic rays cause both, and error correction is the long-term answer.