Experimental realization of a $\cos(2\varphi)$ transmon qubit

• URL: http://arxiv.org/abs/2603.13114v1
• Date: Fri, 13 Mar 2026 16:10:42 GMT
• Title: Experimental realization of a $\cos(2\varphi)$ transmon qubit
• Abstract summary: An island is shunted to ground through a tunneling element that selectively transmits pairs of Cooper pairs.<n>We observe a doublet of states of opposite Cooper-pair parity split by $13.6mathrmMHz$.<n>We demonstrate coherent qubit control, single-shot readout, and resolve quantum jumps.
• Score: 30.627193200398082
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The $\cos(2\varphi)$ qubit, consisting of an island shunted to ground through a tunneling element that selectively transmits pairs of Cooper pairs, leverages charge-parity symmetry to protect from charge-induced errors. In this experiment, we observe a doublet of states of opposite Cooper-pair parity split by $13.6~\mathrm{MHz}$. Operating in a soft-transmon regime, this splitting is two orders of magnitude smaller than in previous implementations, pushing charge-induced losses well beyond the measured coherence times. Despite the low transition frequency, we demonstrate coherent qubit control, single-shot readout, and resolve quantum jumps. Charge protection of the qubit is evidenced by a $100-$fold suppression of the island charge matrix element compared to the unprotected plasmon transition, placing dielectric loss limits above $10~\mathrm{ms}$. The measured $T_1 = 70~μ\mathrm{s}$ and $T_2^\mathrm{echo}= 2.5~μ\mathrm{s}$ are instead limited by $1/f$ flux noise in the tunnelling element's loop. This experiment shows that pushing Cooper-pair pairing in the transmon regime sets high limits on charge-induced losses while preserving coherent control and single-shot readout of the low-frequency qubit. We identify flux noise as the dominant remaining limitation, calling for gradiometric designs or novel $4e$-tunneling elements.

Related papers

• The Gain-Engineered Transmon [68.89136228609398]
  We introduce and experimentally realize a noise-biased qubit in a standard transmon-readout resonator circuit.<n>Our results open a path towards using this system as a simple building-block for hardware-efficient quantum error detection and correction schemes.
  arXiv  Detail & Related papers  (2026-08-26T17:00:42Z)
• Coherence limitations of a Fourier-engineered $\cos(2\varphi)$ transmon qubit [1.77745925062582]
  Intrinsically protected superconducting qubits are a promising route toward enhancing coherence times and advancing hardware towards applications in quantum computing.<n>We experimentally realize a $cos(2varphi)$ qubit by Fourier engineering the energy-phase relation in a multi-junction superconducting circuit.<n>A fluxonium qubit with a similar energy spectrum and noise amplitude is less affected by flux noise, highlighting a key challenge for interference-based protection schemes.
  arXiv  Detail & Related papers  (2026-05-07T14:50:52Z)
• Characterization and Comparison of Energy Relaxation in Fluxonium Qubits [0.3735138621055794]
  Fluxonium superconducting qubits have demonstrated long coherence times and high single- and two-qubit gate fidelities.<n>We investigate the dominant limitations to fluxonium qubit energy relaxation time $T_$ using a set of eight planar, aluminum-on-silicon qubits.
  arXiv  Detail & Related papers  (2026-03-24T18:21:42Z)
• Coherence Limits in Interference-Based cos(2$\varphi$) Qubits [0.0]
  parity-protected $cos (2varphi)$ qubits based on interferences between two Josephson elements in a superconducting loop.<n>We show that qubit implementations of a $cos (2varphi)$ potential using a single loop can be described by the same Hamiltonian as two multi-harmonic Josephson junctions in a SQUID geometry.
  arXiv  Detail & Related papers  (2026-01-15T09:18:55Z)
• Charge sensitivity in the transmon regime [0.1794906241363549]
  We show that transmons can be limited by charge-noise, even with $E_textJ/E_textC approx 50$.<n>Our results show that transmons can be limited by charge-noise, even with $E_textJ/E_textC approx 50$.
  arXiv  Detail & Related papers  (2025-08-05T23:33:57Z)
• Distinct Lifetimes for $X$ and $Z$ Loop Measurements in a Majorana Tetron Device [18.14892474471491]
  We present a hardware realization and measurements of a tetron qubit device in a superconductor-semiconductor heterostructure.<n>The device architecture contains two parallel superconducting nanowires, which support four Majorana zero modes (MZMs) when tuned into the topological phase.<n>Two distinct readout interferometers are formed by connecting the superconducting structure to a series of quantum dots.
  arXiv  Detail & Related papers  (2025-07-11T17:55:22Z)
• In-situ mid-circuit qubit measurement and reset in a single-species trapped-ion quantum computing system [34.82692226532414]
  We implement in-situ mid-circuit measurement and reset (MCMR) operations on a trapped-ion quantum computing system.<n>We introduce and compare two methods for isolating data qubits from measured qubits.<n>We experimentally demonstrate both methods on a crystal of two $171textrmYb+$ ions.
  arXiv  Detail & Related papers  (2025-04-17T00:10:35Z)
• Viewing protected superconducting qubits through the lens of the cat qubit [0.4999814847776097]
  We draw analogies between protected superconducting qubits and bosonic qubits by studying the fluxonium Hamiltonian in its Fock basis.<n>We describe cat-qubit-inspired bias-preserving $X$ and $$$ qubits.
  arXiv  Detail & Related papers  (2025-01-27T19:00:01Z)
• High-fidelity gates in a transmon using bath engineering for passive leakage reset [65.46249968484794]
  Leakage, the occupation of any state not used in the computation, is one of the most devastating errors in quantum error correction. We demonstrate a device which reduces the lifetimes of the leakage states in the transmon by three orders of magnitude.
  arXiv  Detail & Related papers  (2024-11-06T18:28:49Z)
• Erasure qubits: Overcoming the $T_1$ limit in superconducting circuits [105.54048699217668]
  amplitude damping time, $T_phi$, has long stood as the major factor limiting quantum fidelity in superconducting circuits. We propose a scheme for overcoming the conventional $T_phi$ limit on fidelity by designing qubits in a way that amplitude damping errors can be detected and converted into erasure errors.
  arXiv  Detail & Related papers  (2022-08-10T17:39:21Z)
• High fidelity two-qubit gates on fluxoniums using a tunable coupler [47.187609203210705]
  Superconducting fluxonium qubits provide a promising alternative to transmons on the path toward large-scale quantum computing. A major challenge for multi-qubit fluxonium devices is the experimental demonstration of a scalable crosstalk-free multi-qubit architecture. Here, we present a two-qubit fluxonium-based quantum processor with a tunable coupler element.
  arXiv  Detail & Related papers  (2022-03-30T13:44:52Z)
• Annihilating Entanglement Between Cones [77.34726150561087]
  We show that Lorentz cones are the only cones with a symmetric base for which a certain stronger version of the resilience property is satisfied. Our proof exploits the symmetries of the Lorentz cones and applies two constructions resembling protocols for entanglement distillation.
  arXiv  Detail & Related papers  (2021-10-22T15:02:39Z)

This list is automatically generated from the titles and abstracts of the papers in this site.

This site does not guarantee the quality of this site (including all information) and is not responsible for any consequences.