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Thursday, June 11, 2026

The Qubit: Both Answers at Once, and Hard to Hold

A classical bit is a coin lying flat: heads or tails, 0 or 1. A qubitThe basic unit of a quantum computer. Like a 'bit' in a normal computer, but instead of being only 0 or 1 it can be 0, 1, or a blend of both at once. is the coin while it is still spinning.

What a qubit actually is

The thing a qubit is, is the basic unit of quantum information, a name coined back in 1995.1 A normal bit is always either 0 or 1. A qubit can be both at once, in what physicists call a superpositionThe quantum ability of a qubit to be in a combination of 0 and 1 simultaneously, which gives quantum computers their power.. While it sits in that state it is not secretly one or the other. It genuinely holds both, with definite weights.2

A normal bit is always 0 or 1. A qubit can be both at once.

That lasts only until you look. The moment you measure a qubit, the superposition collapsesThe instant a measurement forces a qubit from a blend of possibilities into one definite classical value. and you get back a plain 0 or 1, at random, with odds set by those weights.2 So a qubit hides everything until you read it, and reading throws most of it away. Physicists draw the in-between state as a point on a sphere, the Bloch sphereA geometric way to picture a single qubit's state as a point on the surface of a sphere., but the idea that matters is simpler: until measured, a qubit is a blend, not a choice.

A qubit drawn on the Bloch sphere. The north pole is state 0, the south pole is state 1, and an arrow points to a spot on the surface in between, a superposition that is a blend of 0 and 1. A measure arrow shows that reading the qubit gives 0 or 1 at random.
Figure 1: A single qubit pictured on the Bloch sphere. The poles are the plain 0 and 1; any point in between is a superposition. Measuring the qubit collapses it to 0 or 1, at random.

Where the power comes from

One qubitThe basic unit of a quantum computer. Like a 'bit' in a normal computer, but instead of being only 0 or 1 it can be 0, 1, or a blend of both at once. is a curiosity. The power shows up when you put many of them together and let them share a single linked state, an effect called entanglementA quantum link where two qubits' states become tied together, so acting on or measuring one affects the other. It is a key resource for quantum computing., so the group can no longer be described one qubit at a time. Here is the fact that makes the whole field worth the trouble. Add qubits and the states they can represent do not add up, they double. Ten qubits span over a thousand states at once; three hundred span more than there are atoms in the visible universe. That exponential room is the source of the promised advantage.2

Three hundred qubits span more states than there are atoms in the visible universe.

None of this comes for free. For a physical system to work as a qubit at all, it has to clear a short, brutal list known as the DiVincenzo criteriaThe short checklist a physical system must meet to work as a qubit: it must scale, be controllable by a few operations, and let errors be removed.: it must scale up without an exponential bill in space or energy, it must be steerable by a small set of operations, and it must let you pull errors back out.3 Most candidate systems fail at least one.

Why a qubit is hard to keep

The deepest problem is that a qubit only behaves quantumly while it stays isolated. Contact with the outside world leaks its information away, a process called decoherenceThe loss of a qubit's quantum information as it interacts with the outside world. The main reason qubits are fragile.. It comes in two forms. T1 is the qubit slowly losing energy to its surroundings. T2 is its phase quietly scrambling, and it is the more dangerous, because it needs no energy at all and so strikes faster.2

I will be honest about how fragile this is. A decade ago, coherence timesHow long a qubit holds its delicate quantum state before noise scrambles it. Longer coherence means more time to compute. ran from a few hundred nanoseconds for early superconducting qubitsA qubit made from tiny electrical circuits chilled to near absolute zero, where they lose all electrical resistance. to whole seconds for trapped ionsA qubit made from a single electrically charged atom held in place by electromagnetic fields and controlled with lasers., with single-operation error rates from a fraction of a percent to tens of percent.2 The hardware has improved a great deal since.4 But the shape of the problem has not changed: once a qubit holds its state long enough, the limit becomes how accurately you can steer it, measured as gate fidelityA score (for example 99.9%) for how accurately a quantum gate does what it is supposed to do. Higher means fewer errors..

This is why qubit countThe raw number of physical qubits in a machine. A weak measure of power on its own, since quality and connectivity matter more. alone tells you little. Because every qubit is leaky, a useful machine has to spend most of itself on error correctionTechniques that combine many shaky physical qubits into fewer reliable ones, so a long calculation stays correct., and most of the qubits in a computer built for fault toleranceThe milestone where a quantum computer can run long calculations correctly despite ongoing errors. It is the field's holy grail. will be there for housekeeping, not for the calculation.2 The qubit is the unit. Keeping it still is the whole game.

Sources

  1. Schumacher, B. “Quantum Coding.” Physical Review A 51, 2738–2747 (1995). DOI: 10.1103/PhysRevA.51.2738. The paper that coined the word “qubit.”
  2. Ladd, T. D., Jelezko, F., Laflamme, R., Nakamura, Y., Monroe, C. & O’Brien, J. L. “Quantum Computers.” Nature 464, 45–53 (2010). DOI: 10.1038/nature08812. Preprint: arXiv:1009.2267.
  3. DiVincenzo, D. P. “The Physical Implementation of Quantum Computation.” Fortschritte der Physik 48, 771–783 (2000). DOI: 10.1002/1521-3978(200009)48:9/11<771::AID-PROP771>3.0.CO;2-E. Preprint: arXiv:quant-ph/0002077.
  4. Kjaergaard, M., Schwartz, M. E., Braumüller, J., Krantz, P., Wang, J. I.-J., Gustavsson, S. & Oliver, W. D. “Superconducting Qubits: Current State of Play.” Annual Review of Condensed Matter Physics 11, 369–395 (2020). DOI: 10.1146/annurev-conmatphys-031119-050605. Preprint: arXiv:1905.13641.