Transmon

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Superconducting qubit implementation

title: "Transmon" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["quantum-information-science", "quantum-electronics", "superconductivity"] description: "Superconducting qubit implementation" topic_path: "engineering" source: "https://en.wikipedia.org/wiki/Transmon" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Superconducting qubit implementation ::

[[File:Transmon EJEc.svg|thumb|upright=0.8|Eigenenergies E_m(first three levels, m = 0, 1, 2) of the qubit Hamiltonian as a function of the effective offset charge n_g for different ratios E_J/E_c. Energies are given in units of the transition energy E_{01}, evaluated at the degeneracy point n_g = 0.5. The zero point of energy is chosen as the bottom of the m = 0 level. The charge qubit (small E_J / E_c, top) is normally operated at the n_g = 0.5 "sweet spot" where fluctuations cause less energy shift and the anharmonicity is maximal. Transmon (large E_J / E_c, bottom) energy levels are insensitive to fluctuations but the anharmonicity is reduced. ]] In quantum computing, and more specifically in superconducting quantum computing, a transmon is a type of superconducting charge qubit designed to have reduced sensitivity to charge noise. The transmon was developed by Jens Koch, Terri M. Yu, Jay Gambetta, Andrew Houck, David Schuster, Johannes Majer, Alexandre Blais, Michel Devoret, Steven M. Girvin, and Robert J. Schoelkopf at Yale University and Université de Sherbrooke in 2007. Its name is an abbreviation of the term transmission line shunted plasma oscillation qubit; one which consists of a Cooper-pair box "where the two superconductors are also [capacitively] shunted in order to decrease the sensitivity to charge noise, while maintaining a sufficient anharmonicity for selective qubit control". ::figure[src="https://upload.wikimedia.org/wikipedia/commons/a/a4/4_Qubit,_4_Bus,4_Resonator_IBM_Device(Jay_M._Gambetta,_Jerry_M._Chow,_and_Matthias_Steffen,_2017).png" caption="arxiv=1510.04375 }}"] ::

The transmon achieves its reduced sensitivity to charge noise by significantly increasing the ratio of the Josephson energy to the charging energy. This is accomplished through the use of a large shunting capacitor. The result is energy level spacings that are approximately independent of offset charge. Planar on-chip transmon qubits have T1 coherence times approximately 30 μs to 40 μs. Recent work has shown significantly improved T1 times as long as 95 μs by replacing the superconducting transmission line cavity with a three-dimensional superconducting cavity, and by replacing niobium with tantalum in the transmon device, T1 is further improved up to 0.3 ms. These results demonstrate that previous T1 times were not limited by Josephson junction losses. Understanding the fundamental limits on the coherence time in superconducting qubits such as the transmon is an active area of research.

Comparison to Cooper-pair box

The transmon design is similar to the first design of the charge qubit known as a "Cooper-pair box"; both are described by the same Hamiltonian, with the only difference being the E_{\rm J}/E_{\rm C} ratio. Here E_{\rm J} is the Josephson energy of the junction, and E_{\rm C} is the charging energy inversely proportional to the total capacitance of the qubit circuit. Transmons typically have E_{\mathrm J}/E_{\mathrm C} \gg 1 (while E_{\mathrm J}/E_{\mathrm C} \lesssim 1 for typical Cooper-pair-box qubits), which is achieved by shunting the Josephson junction with an additional large capacitor.

The benefit of increasing the E_{\rm J}/E_{\rm C} ratio is the insensitivity to charge noise—the energy levels become independent of the offset charge n_g across the junction; thus the dephasing time of the qubit is prolonged. The disadvantage is the reduced anharmonicity \alpha = (E_{21}-E_{10})/E_{10}, where E_{ij} is the energy difference between eigenstates |i\rangle and | j \rangle. Reduced anharmonicity complicates the device operation as a two level system, e.g. exciting the device from the ground state to the first excited state by a resonant pulse also populates the higher excited state. This complication is overcome by complex microwave pulse design, that takes into account the higher energy levels, and prohibits their excitation by destructive interference. Also, while the variation of E_{10}with respect to n_g tend to decrease exponentially with E_{\mathrm J}/ E_{\mathrm C}, the anharmonicity only has a weaker, algebraic dependence on E_{\mathrm J}/E_{\mathrm C} as \sim (E_{\mathrm J}/E_{\mathrm C})^{-1/2}. The significant gain in the coherence time outweigh the decrease in the anharmonicity for controlling the states with high fidelity.

Measurement, control and coupling of transmons is performed by means of microwave resonators with techniques from circuit quantum electrodynamics also applicable to other superconducting qubits. Coupling to the resonators is done by placing a capacitor between the qubit and the resonator, at a point where the resonator electromagnetic field is greatest. For example, in IBM Quantum Experience devices, the resonators are implemented with "quarter wave" coplanar waveguides with maximal field at the signal-ground short at the waveguide end; thus every IBM transmon qubit has a long resonator "tail". The initial proposal included similar transmission line resonators coupled to every transmon, becoming a part of the name. However, charge qubits operated at a similar E_{\rm J}/E_{\rm C} regime, coupled to different kinds of microwave cavities are referred to as transmons as well.

Applications

Superconducting quantum processors

Transmons are the default qubit in most large scale quantum processors, including Google's Willow processor, a chip with 105 physical transmon qubits. Other companies that use transmon qubits include IBM, Rigetti, and IQM.

Bosonic or hybrid quantum memory

While most quantum computation systems are qubit-based, an alternative method is to use harmonic oscillator modes, or bosonic modes, as the logical subspace. In such systems, transmons are used as ancillas for universal control of the cavity's bosonic modes. This allows for physical realizations of oscillator-based error correction codes. These codes are known as 'bosonic codes'.

Transmons as qudits instead of qubits

Transmons have been explored for use as d-dimensional qudits via the additional energy levels that naturally occur above the qubit subspace (the lowest two states). For example, the lowest three levels can be used to make a transmon qutrit; in the early 2020s, researchers have reported realizations of single-qutrit quantum gates on transmons as well as two-qutrit entangling gates. Entangling gates on transmons have also been explored theoretically and in simulations for the general case of qudits of arbitrary d.

References

References

  1. (2007-10-12). "Charge-insensitive qubit design derived from the Cooper pair box". Physical Review A.
  2. (2008-05-12). "Suppressing charge noise decoherence in superconducting charge qubits". American Physical Society.
  3. Fink, Johannes M.. (2010). "Quantum Nonlinearities in Strong Coupling Circuit QED". [[ETH Zurich]].
  4. (2017-01-13). "Building logical qubits in a superconducting quantum computing system". Springer Science and Business Media LLC.
  5. (2013-08-22). "Coherent Josephson Qubit Suitable for Scalable Quantum Integrated Circuits". Physical Review Letters.
  6. (2011-12-05). "Observation of High Coherence in Josephson Junction Qubits Measured in a Three-Dimensional Circuit QED Architecture". Physical Review Letters.
  7. (2012-09-24). "Superconducting qubit in a waveguide cavity with a coherence time approaching 0.1 ms". American Physical Society.
  8. (2021-03-19). "New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds". Nature Communications.
  9. (1998). "Quantum coherence with a single Cooper pair". Physica Scripta.
  10. "Google's quantum breakthrough is 'truly remarkable' - but there's more to do".
  11. Cai, Weizhou. (2021-01-01). "Bosonic quantum error correction codes in superconducting quantum circuits". Fundamental Research.
  12. (2022). "Bosonic code". The Error Correction Zoo.
  13. Ma, Wen-Long. (2021-09-15). "Quantum control of bosonic modes with superconducting circuits". Science Bulletin.
  14. (2020-10-27). "Implementation of a Walsh-Hadamard Gate in a Superconducting Qutrit". Physical Review Letters.
  15. (2021-05-27). "Qutrit Randomized Benchmarking". Physical Review Letters.
  16. (2022-12-05). "High-fidelity qutrit entangling gates for superconducting circuits". Nature Communications.
  17. (2023-08-28). "Universal Qudit Gate Synthesis for Transmons". PRX Quantum.

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quantum-information-sciencequantum-electronicssuperconductivity