1. In three lines / what to know first / where this sits in the series

• A superconducting qubit is a small electrical circuit made of superconductors (about 1 mm across) whose "lowest two energy steps" are used as 0 and 1. Because it is drawn on a chip, it is also called an "artificial atom"[Ref. 1]Sourced
• An ordinary circuit cannot be a qubit as it is. Its energy steps are evenly spaced, so we cannot aim at "just 0 and 1." Adding a Josephson junction makes the gaps uneven, so a microwave can move just the two chosen steps[Ref. 1, 2]Sourced
• Control uses microwave pulses; readout uses a small shift in a resonator's response. It must run at a very low temperature, about 20 mK. It is fast, but qubit lifetimes are relatively short, and wiring, cooling and fabrication variation are the challenges. Lifetime changes a lot with the materials (tantalum, aluminum, substrate, oxide layers)[Ref. 1, 3]Sourced

2. Recap: a qubit is an "arrow"

In this series, a qubit's state is drawn as an arrow from the center of a sphere to its surface. Pointing at the north pole means "0," pointing at the south pole means "1," and pointing in between is a superposition. When the arrow is tilted, a measurement always gives 0 or 1, and the chance of each is set by the tilt. After the measurement the arrow is fixed at a pole, and the old tilt does not come back.

3. Why can a circuit be a qubit? (1) The evenly spaced ladder

(1) A familiar example: a swing

A swing takes about the same time for one back-and-forth trip whether you swing it a little or a lot. If you push in rhythm, the swing grows larger and larger. The same thing happens in an electrical circuit. Connect a coil (L) and a capacitor (C), and the electricity sloshes back and forth. This is an LC resonator, a relative of the tuning circuit in a radio. In a tiny system such as an atom or a circuit, the energy of this oscillation comes in "discrete steps." Like a staircase, you can only be on step 1, step 2, step 3, and so on. The steps of an LC resonator are all equally spaced (an evenly spaced ladder)[Ref. 1]Sourced.

(2) The problem: with even spacing we cannot use "just 0 and 1"

A qubit needs just two steps, 0 and 1. In an evenly spaced ladder, though, the gaps 0→1, 1→2 and 2→3 all correspond to the same frequency. A microwave at that frequency moves not only 0→1 but also 1→2 and 2→3, so we cannot aim at "just 0 and 1"[Ref. 1].

(3) The fix: make the spacing uneven

When a Josephson junction (next chapter) is added, the circuit stops acting like a plain coil, and the steps become unevenly spaced. If the 1→2 gap is a little smaller than the 0→1 gap, a microwave at the 0→1 frequency moves only 0 and 1, and 1→2 does not respond. The real difference is about 100–300 MHz when the qubit frequency is 3–6 GHz (a few percent)[Ref. 1]Sourced. Even a tiny difference works, because a frequency can be tuned very precisely.

4. Why can a circuit be a qubit? (2) The Josephson junction and the transmon

(1) What is a Josephson junction?

Superconductivity is the state in which a metal, when cooled, has zero electrical resistance so current flows without loss. A Josephson junction is a part in which a very thin non-conducting layer is sandwiched between two superconductors[Ref. 4]Sourced. Pairs of superconducting electrons pass through the thin layer by the tunnel effect (a quantum phenomenon of slipping through a wall). Commentary: the junction acts like "a coil without resistance," but unlike an ordinary coil, its behavior changes as the current grows (it is nonlinear), and that is the crux. The review by Krantz et al. explains that the Josephson junction is the key ingredient that makes the oscillator anharmonic and so lets us pick out a uniquely addressable pair of levels[Ref. 1]Sourced.

(2) The transmon: the form used most today

A circuit in which the junction is wired in parallel with a large capacitor is a transmon, proposed by Koch et al. in 2007[Ref. 2]Sourced. A large capacitor makes the circuit insensitive to fluctuations of nearby charge (noise). In exchange, the size of the unevenness shrinks a little, but enough remains to be useful. Koch et al. showed that when the ratio of the junction's energy to the capacitor's energy (E_J/E_C) is made large, the sensitivity to charge noise falls exponentially, while the loss of unevenness falls only as a gentle power law[Ref. 2]Sourced. Real transmons are designed with E_J/E_C of 50 or more[Ref. 1]Sourced.

5. Why cool it to such a low temperature?

Superconducting qubits are operated at about 20 mK (millikelvin; 0.02 degrees above absolute zero)[Ref. 1]Sourced. It is easy to think "we cool it to make it superconducting," but that alone does not explain it. For example, tantalum becomes superconducting at 4.2 K[Ref. 3]Sourced. We cool it more than two orders of magnitude below that so that heat does not move the qubit by itself.

The energy difference between 0 and 1 is that of one 5 GHz microwave photon (hf). Unless the surrounding thermal energy (kT) is well below this, heat randomly excites the qubit to "1." So we compute the ratio hf/kT (Our calculation). At 20 mK, hf/kT = 12.0, and the chance of being in the heat-excited state is about 6×10⁻⁶. At 100 mK, hf/kT = 2.4 and the chance is about 8%; at 1 K it is about 44%, almost half and half. At room temperature (300 K), hf/kT = 0.0008 and the excited-state probability is about 50%, so 0 and 1 cannot be told apart at all. The average number of "thermal photons" at 5 GHz is about 6×10⁻⁶ at 20 mK and about 1,250 at 300 K.

A dilution refrigerator cools in many stages from room temperature to about 20 mK. In one example, the cooling power of the bottom stage is only 19 μW (0.019 mW) at 20 mK[Ref. 6]Sourced. So the heat flowing in through the wires to the chip and the heat of the control signals together must not exceed a few tens of μW. This becomes a big wall when qubits are scaled to thousands or tens of thousands (see our explainer on cryogenics and control).

6. Control and readout

(1) One-qubit control: turning the arrow

A microwave pulse tuned to the qubit's frequency turns the arrow. The pulse's length and strength set the angle. For example, a half-strength pulse (π/2 pulse) turns it from the north pole to the equator, and a full-strength pulse (π pulse) turns it from north pole to south pole (see the figure). Rotations about the Z axis can be done just by shifting the phase of the following pulses (a "virtual Z gate"). There is also a trick of shaping the pulse to suppress leakage to unwanted levels (DRAG)[Ref. 1]Sourced.

(2) Two-qubit control: connecting for a moment

When there are two or more qubits, a coupling is made between two of them so they interact for a moment. The review by Krantz et al. describes a CPHASE gate that shifts frequencies to bring two states close (an example by Barends et al.: 43 ns, fidelity 0.9944) and a cross-resonance gate that drives one qubit at the other's frequency (an example shifting a phase by π in about 200 ns)[Ref. 1]Sourced. IBM's current processors (Heron) combine fixed-frequency qubits with couplers whose connection can be tuned on and off (tunable couplers)[Ref. 7]Sourced.

(3) Readout: listening to a resonator's "pitch"

Next to the qubit sits another resonator, the readout resonator. The qubit and the resonator are weakly coupled, and the resonator's resonance frequency (its pitch) shifts slightly depending on whether the qubit is 0 or 1 (dispersive readout). So we send microwaves to the resonator and look at the strength or phase of the returned signal to tell 0 from 1[Ref. 8, 1]Sourced. An experiment that strongly couples a qubit to a resonator at the level of single microwave photons was reported by Wallraff et al. in 2004 (circuit quantum electrodynamics)[Ref. 9]Sourced. The returned signal is extremely faint, so "quantum-limited amplifiers," which add only the minimum noise quantum mechanics allows, are used[Ref. 7]Sourced.

7. The key points on a common "card," and a side-by-side with the other three approaches

Here is everything so far as a "card" that answers the series' five shared questions (what it is made of, what 0 and 1 are, how it is controlled, how it is read, strengths and weaknesses). The other three articles use the same card.

Reading the four approaches in the same format

The arrangement is commentary; the numbers and facts in each cell come from the references (checked 16 October 2026). The numbers are different quantities under different conditions, so please do not rank the approaches by them.

8. Materials and manufacturing: what decides the lifetime

(1) Lifetimes are far shorter than the materials' "ideal values"

Place et al. point out that because the dielectric loss of high-purity sapphire (the fraction of microwave energy turned into heat) is below 10⁻⁹, T1 could ideally exceed 30 ms, yet the lifetimes of real two-dimensional transmons are much shorter[Ref. 5]Sourced. The gap is thought to come from defects at surfaces and interfaces, and contaminants. The review by Müller et al. explains that circuits made of aluminum or niobium are fundamentally limited by loss from two-level-system (TLS) defects in the surrounding amorphous oxide layers[Ref. 31]Sourced. A TLS is a tiny defect whose energy can take only two values; it soaks up energy from the qubit.

(2) From niobium to tantalum, from sapphire to silicon

Place et al. replaced the widely used niobium with tantalum for the superconductor of the transmon's capacitor and resonator, and obtained devices whose lifetime and whose phase-retention time (with pulses that cancel phase disturbances) both exceeded 0.3 ms. They confirmed the improved lifetime in 17 devices[Ref. 5]Sourced. They sputtered tantalum (ejecting metal onto the surface) onto a sapphire substrate heated to about 500 °C, shaped it by photolithography and wet etching, and formed the junction from aluminum and its oxide by electron-beam lithography and evaporation. The native oxide on tantalum (Ta₂O₅) is about 2 nm thick[Ref. 5]Sourced. The authors hypothesize that the complicated oxides on niobium create extra loss and that tantalum's oxide reduces it (a hypothesis). Wang et al. compared tantalum, niobium and aluminum films with the same design and the same junction process, and reported tantalum best, aluminum next, niobium last (best T1 of 503 μs)[Ref. 32]Sourced.Bland et al. then replaced the sapphire substrate with high-resistivity silicon, which greatly reduced the loss of the substrate itself, and reported T1 up to 1.68 ms (a maximum quality factor Q of 2.5×10⁷). The tantalum film was grown at 600–650 °C, its superconducting transition temperature was 4.2 K, and no oxide layer was seen at the tantalum–silicon interface. With the loss lowered, decoherence due to the junction became visible, and with low-contamination junction deposition they obtained devices whose phase-retention time (Hahn-echo method) exceeded T1[Ref. 3]Sourced.

(3) Fabrication variation: a small difference in the junction becomes a difference in frequency

A qubit's frequency is set by the junction's properties. In IBM's study, lining up many fixed-frequency transmons required tuning their relative frequencies precisely, which was a difficult step in scaling. Zhang et al. showed a way to heat junctions locally with laser light (laser annealing) and tune frequencies afterwards. Statistics over hundreds of qubits gave a tuning precision of 18.5 MHz, no measurable effect on lifetime, and a median two-qubit gate fidelity of 98.7% on a 65-qubit processor. The authors state that the precision of the junction resistance expressed as frequency (4.7 MHz) is enough for high-yield scaling beyond the 1,000-qubit level (from the abstract of the arXiv version)[Ref. 29]Sourced. In Google's study, because TLS frequencies change with time, the team forecast the TLS frequencies' motion from past lifetime data and set qubit frequencies to avoid them[Ref. 10]Sourced.

9. Where things stand (checked 16 October 2026) and plans

(1) Google's error-correction experiment (a peer-reviewed paper by a company research team)

Google Quantum AI used a 105-qubit processor with the surface code, an error-correcting code, and showed that each time the code size (distance) goes up by two, the logical error rate falls by about a factor of 2.14 (Λ = 2.14 ± 0.02). The logical error rate of the distance-7 code (using 101 qubits) is 0.143% ± 0.003% per error-correction cycle, and a cycle takes 1.1 μs. This logical-qubit memory outlived the best physical qubit by a factor of 2.4 (it went beyond break-even). The processor's qubits are reported to have a mean T1 of 68 μs[Ref. 10]Sourced.How to read it: this experiment stores a logical qubit's information (a memory). It does not mean logical operations were run to do useful computation. It is also an experiment by a company's research team; the paper is peer-reviewed (Nature), but we have not confirmed a reproduction of the same experiment by another laboratory (could not confirm).

(2) IBM (statements on the company's official pages)

According to IBM's official page (checked 16 October 2026), the processors currently offered are Heron r1 (133 qubits), Heron r2 and r3 (156 qubits, both with tunable couplers), and Nighthawk (120 qubits on a square lattice, each qubit coupled to four others). The page shows the "quality (EPLG)" of Heron and Nighthawk as 3.7×10⁻³ and says Nighthawk r2 is 25 times faster (values shown by the company; we could not confirm the measurement conditions)[Ref. 7]Sourced.

(3) Japan: Fujitsu and RIKEN

On 22 April 2025, Fujitsu and RIKEN announced a 256-qubit superconducting quantum computer (four times the 64-qubit machine of 2023)[Ref. 33]Sourced. The same announcement said a 1,000-qubit machine is scheduled to be installed in a new building at Fujitsu's site in 2026[Ref. 33]Not yet confirmed. As of the check date we could not confirm an official announcement that the 1,000-qubit machine was completed.

(4) Plans (not yet confirmed): IBM's roadmap

IBM's official roadmap states at the outset that it "represents IBM's current intent" and consists of goals that may change or be withdrawn[Ref. 34]Not yet confirmed. 2026: run circuits of 7,500 gates on up to three 120-qubit Nighthawk modules; prototype an error-correction decoder and demonstrate a single Kookaburra module that combines a logical processing unit with a quantum memory. 2028: 15,000 gates on up to 1,080 qubits. 2029: Starling ("200 qubits," 100 million gates). 2033 and after: Blue Jay ("2000 qubits," 1 billion gates). The page says only "qubits," so whether they are physical or logical qubits cannot be told from the page's text (could not confirm).

10. Limits, what we could not confirm, and a summary

(1) Full texts of standards

We did not read the full text of ISO/IEC 4879:2024, which is paid. For IEEE P7130 and P7131 we could not retrieve the IEEE pages and confirmed only the existence of the projects through the StandICT.eu standards repository.

(2) Values shown by companies

We have not confirmed measurement conditions or third-party verification for IBM's EPLG and speed figures or for Fujitsu/RIKEN's plans, which are statements on official pages and in a press release.

(3) Reproduction elsewhere

We have not confirmed reproductions by other laboratories of Google's error-correction result or of Bland et al.'s lifetime result.

(4) Approaches not covered

We do not cover superconducting qubits other than the transmon, such as fluxonium and bosonic (cat-state) qubits.

(5) Progress on plans

Whether the Fujitsu/RIKEN 1,000-qubit machine and each year's IBM roadmap goals had been achieved by the check date is stated only as far as official reports of achievement could be confirmed.

11. How to read evaluations and standards

A superconducting qubit's performance is read by four yardsticks: lifetime (T1, T2), gate error rate, quality of the whole machine, and how far errors are suppressed by error correction. Lifetime says how long a qubit holds its state; the error rate says how often one operation goes wrong. Each number, however, changes with what was measured and how. For standards, vocabulary has come first, while standards for how to measure performance are still at the project stage (checked 16 October 2026).

The arrangement of the table is Our calculation (organization). Because the standards' full texts are paid, we give only titles and status (checked 16 October 2026).

Cautions in reading (for materials engineers)

• "How many qubits" alone does not tell performance. Look at number, quality, connectivity and speed together, and always check physical or logical[Ref. 10, 7]
• A company's announcement and third-party verification are different. IBM's figures are the company's own display; Google's result is a peer-reviewed paper by a company team. We have confirmed a reproduction by another laboratory for neither
• Check whether a standard is a recommendation or a requirement, and whether it is legally binding. ISO/IEC 4879 is a vocabulary standard and does not guarantee performance[Ref. 43]. ISO/IEC standards generally gain binding force only when cited in law or contracts (commentary)
• Do not mix plans with achievements. The roadmap's numbers are Not yet confirmed[Ref. 34]

12. Glossary

13. References (primary sources)

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14. Claim-to-source audit

Last updated: 16 October 2026. Sources are limited to primary ones (peer-reviewed papers, arXiv papers, and official pages of IBM, Fujitsu/RIKEN, the Nobel Foundation and ISO). The standards' full texts are paid and were not read. The temperature and size examples are our own calculations, not the performance of any real machine. We could not confirm reproductions by other laboratories, the IEEE official pages, completion of the 1,000-qubit machine, or the measurement conditions of IBM's figures, so we do not state them. All figures are explanatory concept diagrams or drawings that include our own calculations; Figures 1 and 11 are ImageGen-generated conceptual illustrations.