Hardware & Errors
Transmon
The dominant type of superconducting qubit: a Josephson-junction circuit designed to be insensitive to charge noise.
Definition
A transmon is a tiny electrical circuit, made of superconducting metal and a thin insulating gap, that behaves like an artificial atom with two usable energy levels. It is the qubit used by IBM, Google, Rigetti, IQM, and most superconducting quantum computers. Its main virtue is that it keeps its quantum state much longer than earlier superconducting designs, because it is built to ignore stray electric charges.
In depth
The history of superconducting qubits is largely the history of improving coherence. Early charge qubits, such as the Cooper pair box shown by Nakamura and colleagues in 1999, could be placed in superpositions but lost coherence in nanoseconds because fluctuating background charges shifted their energy. Koch, Schoelkopf, and coauthors at Yale introduced the transmon in 2007 by adding a large shunt capacitor, which flattens the dependence on charge exponentially at the cost of only a modest loss of anharmonicity. Coherence times rose from microseconds to hundreds of microseconds within a decade.
Today, transmon performance is limited mostly by materials and fabrication. Defects in surface oxides, tiny two-level systems in the substrate, and quasiparticles all cause energy loss. Switching to tantalum for the capacitor films and high-resistivity silicon substrates has produced devices with energy relaxation times above a millisecond in the lab, though processors with many qubits typically show 100 to 300 microseconds. Gate times are short, about 20 to 50 nanoseconds for single-qubit gates and 30 to 200 nanoseconds for two-qubit gates, which gives thousands of gates per coherence time.
The transmon is controlled with microwave pulses and read out by coupling it to a resonator whose frequency shifts slightly with the qubit state. Neighboring qubits are coupled by capacitors, resonators, or tunable couplers. Variants include fixed-frequency transmons, used by IBM with cross-resonance gates, and flux-tunable transmons, used by Google and Rigetti, which can be tuned into resonance for fast gates. Other designs, such as fluxonium, trade size for higher coherence and are an active research direction.
Technical Definition
A transmon is a Josephson junction shunted by a large capacitor, so that the ratio of the Josephson energy E_J to the charging energy E_C is about 50 or larger. In this regime the energy levels are nearly insensitive to offset charge, with a small anharmonicity, typically minus 200 to minus 300 MHz, that lets the lowest two levels be addressed as a qubit. Transition frequencies are about 4 to 6 GHz and devices operate at around 10 to 20 millikelvin.
Visual Explanation: An Analogy
A swing is a classic oscillator whose energy levels are evenly spaced, so you cannot push it to just one level. The Josephson junction acts like a swing with a slightly warped frame, so the first jump costs a bit more energy than the second. That small difference lets you pick out the lowest two levels as a qubit.
Key facts
| Item | Detail |
|---|---|
| Invented | 2007, by Koch, Schoelkopf, and colleagues at Yale |
| Frequency | About 4 to 6 GHz, operated at about 10 to 20 mK |
| Anharmonicity | Typically minus 200 to minus 300 MHz |
| Coherence | Typically 100 to 300 microseconds in large chips; above 1 ms reported in laboratory devices |
| Gate speed | Tens of nanoseconds for single-qubit gates, tens to hundreds for two-qubit gates |
| Used by | IBM, Google, Rigetti, IQM, Alice and Bob (cat qubits are related), and many labs |
Where the idea comes from
The Cooper pair box of 1999 and the circuit quantum electrodynamics architecture of Blais, Wallraff, and Schoelkopf in 2004 set the stage. The transmon of 2007 became the standard, and the 2025 Nobel Prize in Physics, awarded to Clarke, Devoret, and Martinis, honored the macroscopic quantum effects in superconducting circuits on which it is built.
Real-World Use Cases
- Qubits in processors from IBM, Google, Rigetti, and IQM.
- Experiments in quantum error correction, such as Google's surface code demonstrations.
- Testbeds for new materials and fabrication processes that improve coherence.
- Quantum sensing of microwave photons and in circuit quantum electrodynamics research.
How it differs from related ideas
- Compared with trapped ions, transmons are fast but have shorter coherence, and every device has slightly different parameters.
- Compared with charge qubits, transmons trade a little anharmonicity for far less charge noise.
- Fluxonium qubits have longer coherence but lower frequencies and slower gates.
Common Misconceptions
- A transmon is not an atom. It is a circuit with millions of atoms that behaves like one in its quantum states.
- It does not need a vacuum chamber. It needs extreme cold and good shielding.
- Higher qubit count does not mean higher quality. Coherence and gate fidelity matter more.
Frequently asked questions
What is a transmon qubit?
A superconducting circuit based on a Josephson junction and a large capacitor, designed so that its energy levels are insensitive to charge noise, widely used in quantum computers.
Why must transmons be cooled?
To keep the metal superconducting and to prevent thermal energy from exciting the qubit, they run at about 10 to 20 millikelvin.
How long do transmons stay coherent?
Typically a few hundred microseconds in processors, and more than a millisecond in the best laboratory devices.
Related Terms
Keep exploring
- Algorithm: Quantum Error-Correcting Codes (Shor Code, Surface Code)
- Hardware: IBM Heron (r1, r2, and r3)
- Hardware: Google Willow
- Hardware: IQM Garnet