Skip to content
ContentLora

    Tip: press / anywhere to search.

    Explainer

    How qubits work

    A qubit is the quantum version of a bit: it can be 0, 1, or a weighted combination of both, and many qubits can be entangled so their states are correlated.[1][2] Quantum algorithms use interference to make right answers likely, but qubits lose their state easily through decoherence, which is the central engineering problem of the field.[3][4]

    Editor reviewedUpdated Quantum computingComputingPhysics

    From bits to qubits

    A normal computer bit is a switch: off (0) or on (1). A qubit can be 0 or 1 too, but it can also sit in a mixture of both, with a “weight” for each. This is called superposition.[1] When you measure a qubit you always get a plain 0 or 1, with probabilities set by those weights.

    A qubit is a two-level quantum system whose state is a weighted combination of the basis states |0⟩ and |1⟩; measurement returns one basis state, with probabilities set by the weights.[1] Registers of entangled qubits can represent states that classical machines struggle to track, which is the root of the hope that quantum computers can simulate the quantum nature of matter.[2][5]

    Entanglement and interference

    Qubits can be linked so that their results are correlated: measure one and you learn something about the other. This is entanglement.[2] A quantum program then arranges for the possibilities to add up like waves, so that paths to wrong answers cancel and paths to right answers reinforce. IBM calls this interference “the engine of quantum computing”.[3]

    Entanglement correlates the states of qubits so that a measurement on one constrains the others.[2] Algorithms are designed so that quantum waves build on each other at useful outcomes and cancel elsewhere; this interference, not superposition alone, is what IBM identifies as the engine of quantum computing, which is why “trying all answers at once” is a misleading description.[3]

    Why qubits are fragile

    Any interaction with the outside world, including stray heat, light or vibrations, can collapse a qubit’s quantum state. This is decoherence.[4] IBM’s first cloud quantum computer in 2016 used five superconducting qubits held at roughly 15 to 20 millikelvin in a dilution refrigerator to keep them stable.[6] Coherence times vary widely by technology: Google’s 2024 Willow chip reports T1 times approaching 100 microseconds, while a Caltech neutral-atom array kept superposition for about 13 seconds.[7][8]

    Long coherence is not the only measure. Gate fidelity, how often an operation does exactly what it should, matters just as much. Quantinuum reports 99.921% fidelity for two-qubit gates on its Helios system, and IonQ reported 99.99% in 2025.[9][10] Even 99.9% means one error per thousand operations, far too many for long algorithms, which is why error correction is essential.[11]

    Ways to build a qubit

    Physicists have several competing ways to make qubits:

    • Superconducting circuits: tiny electrical circuits on chips, cooled near absolute zero. Used by Google, IBM and USTC.[12][13][14]
    • Trapped ions: charged atoms held in electromagnetic fields and controlled with lasers or microwaves.[15]
    • Neutral atoms: uncharged atoms held in arrays of laser “optical tweezers”.[8]
    • Topological qubits: a proposed design that would store information in exotic states of matter, still unproven.[16]
    • Silicon spin qubits and photonics, which are also among the approaches in DARPA’s benchmarking programme.[17]

    Physical versus logical qubits

    A physical qubit is one piece of hardware. A logical qubit is a group of physical qubits working together under an error-correcting code so that the group behaves like one far more reliable qubit.[18] Headline qubit counts usually refer to physical qubits; the number that matters for useful computing is logical qubits and how many operations they can run, which is how IBM states its 2029 target of 200 logical qubits and 100 million operations.[19]

    Questions readers ask

    Does a qubit try every answer at once?

    Not in a useful sense. A qubit can hold a weighted combination of 0 and 1, but the speed-up comes from interference, which reinforces some outcomes and cancels others.[1][3]

    Why do quantum computers need to be so cold?

    Heat and other disturbances destroy quantum states, a process called decoherence. Superconducting qubits are kept in dilution refrigerators at roughly 15 to 20 millikelvin.[4][6]

    How long does a qubit keep its information?

    It depends on the technology. Google's Willow superconducting qubits reach about 100 microseconds, while a Caltech neutral-atom array held superposition for about 13 seconds.[7][8]

    Sources

    Each numbered claim is a statement we checked against the sources listed with it. Status shows how well established it is.

    1. [1]

      A qubit can act like a classical bit, storing a zero or a one, but it can also exist in a weighted combination of both at once (superposition). confirmedas of 2026-10-10

    2. [2]

      Entanglement is the ability of qubits to correlate their state with other qubits, so that measuring one tells you something about the others. confirmedas of 2026-10-10

    3. [3]

      Interference, in which quantum amplitudes reinforce some outcomes and cancel others, is described by IBM as the engine of quantum computing. confirmedas of 2026-10-10

    4. [4]

      Decoherence is the process by which a quantum state collapses into a non-quantum state, through measurement or disturbance from the environment, causing qubits to fail. confirmedas of 2026-10-10

    5. [5]

      NIST describes quantum computers as a new kind of machine that can, in theory, simulate the quantum nature of matter and tackle certain problems that are currently unsolvable. confirmedas of 2026-10-10

    6. [6]

      IBM put the first quantum computer on the cloud on 4 May 2016, a five-qubit superconducting device kept in a dilution refrigerator at roughly 15 to 20 millikelvin. confirmedas of 2021-05-04

    7. [7]

      Willow's qubits have T1 coherence times approaching 100 microseconds, about five times longer than Google's previous generation. confirmedas of 2024-12-09

    8. [8]

      In September 2025 a Caltech team reported in Nature an array of 6,100 cesium-atom qubits held by optical tweezers (one laser split into 12,000 tweezers), with superposition lasting about 13 seconds and 99.98% accuracy in manipulating individual qubits; entangling the qubits for full computations was the stated next step. confirmedas of 2025-09-24

    9. [9]

      Quantinuum reports Helios two-qubit gate fidelity of 99.921% and single-qubit gate fidelity of 99.9975%, with all 98 qubits fully connected. confirmedas of 2026-10-10

    10. [10]

      In October 2025 IonQ reported 99.99% two-qubit gate fidelity using its Electronic Qubit Control technology, which drives qubits with precision electronics instead of lasers. confirmedas of 2025-10-21

    11. [11]

      Google's below-threshold experiment, published in Nature in 2025, ran a 101-qubit distance-7 surface code with 0.143% logical error per cycle, with errors suppressed by a factor of 2.14 each time the code distance grew by two. confirmedas of 2025-02-01

    12. [12]

      Google's Willow chip, announced on 9 December 2024, has 105 superconducting qubits. confirmedas of 2024-12-09

    13. [13]

      IBM unveiled the Nighthawk processor in November 2025, with 120 qubits linked by 218 tunable couplers, allowing circuits about 30 percent more complex than its previous processors. confirmedas of 2025-11-12

    14. [14]

      The University of Science and Technology of China reported in Physical Review Letters in March 2025 that its 105-qubit Zuchongzhi 3.0 superconducting processor ran an 83-qubit, 32-layer random circuit sampling task it estimated at 10^15 times faster than the most powerful supercomputer. confirmedas of 2025-03-06

    15. [15]

      Trapped-ion quantum computers hold charged atoms (ions) in electromagnetic fields and manipulate and encode information in them with microwave signals and lasers. confirmedas of 2026-10-10

    16. [16]

      The Nature peer-review file for Microsoft's 2025 paper stated that its results do not represent evidence for Majorana zero modes, and the paper itself did not claim a topological qubit. confirmedas of 2025-02-25

    17. [17]

      DARPA lists the QBI Stage B approaches as neutral atoms (Atom Computing, QuEra), trapped ions (IonQ, Quantinuum), superconducting circuits (IBM, and Nord Quantique with bosonic error correction), silicon spin or atom qubits (Diraq, Photonic, Quantum Motion, Silicon Quantum Computing) and photonics (Xanadu). confirmedas of 2025-11-06

    18. [18]

      In the same experiment the distance-7 logical memory outlived the best physical qubit on the chip by a factor of about 2.4. confirmedas of 2025-02-01

    19. [19]

      In June 2025 IBM said it would build IBM Quantum Starling, a fault-tolerant quantum computer with 200 logical qubits able to run 100 million quantum operations, in Poughkeepsie, New York, by 2029. confirmedas of 2025-06-10

    Revision history (1)
    1. Page created.

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Jan 10, 2027.

    Cite this page

    "How qubits work." ContentLora, updated Oct 10, 2026. https://contentlora.com/explain/how-qubits-work

    Spotted an error? Suggest a correction or emailcorrections@contentlora.com.

    Keep exploring

    ☰ Contents