Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2203.04323 · PRX Quantum

Entangling Transmons with Low-Frequency Protected Superconducting Qubits

Andrea Maiani, Morten Kjærgaard, Constantin Schrade

Published Aug 29, 2022• 19 citations
arXivHTMLPDF

Abstract

Novel qubits with intrinsic noise protection constitute a promising route for improving the coherence of quantum information in superconducting circuits.However, many protected superconducting qubits exhibit relatively low transition frequencies, which could make their integration with conventional transmon circuits challenging.In this work, we propose and study a scheme for entangling a tunable transmon with a Cooper-pair parity-protected qubit, a paradigmatic example of a low-frequency protected qubit that stores quantum information in opposite Cooper-pair parity states on a superconducting island.By tuning the external flux on the transmon, we show that noncomputational states can mediate a two-qubit entangling gate that preserves the Cooper-pair parity independent of the detailed pulse sequence.Interestingly, the entangling gate bears similarities to a controlled-phase gate in conventional transmon devices.Hence, our results suggest that standard high-precision gate calibration protocols could be repurposed for operating heterogeneous quantum processors.

DOI
10.1103/prxquantum.3.030329

Related papers

Full Gate-Voltage Control of a Parity-Protected Superconducting Qubit with an Altermagnetic Josephson Junction

Guo-Liang Guo

Parity-protected superconducting qubits offer intrinsically long coherence, but many current implementations require magnetic-flux biasing, which introduces flux noise, control overhead, and limited scalability. Here we propose a parity-protected qubit based on a gate-tunable superconductor-altermagnet-superconductor Josephson junction. Altermagnets are compensated magnets with momentum-dependent spin splitting and zero net magnetization, providing spin-dependent functionality without external magnetic fields. In the proposed junction, the two spin sectors acquire opposite phase shifts, generating two Josephson channels whose interference is controlled electrically by the chemical potential. At the tuned $0$-$π$ transition, the first Josephson harmonic is strongly suppressed while the second harmonic dominates, yielding a double-well potential with two nearly degenerate states of opposite Cooper-pair parity. For realistic gatemon-compatible parameters, we estimate coherence times of up to tens of milliseconds while maintaining fully gate-controlled qubit operations. These results establish altermagnetic Josephson junctions as a promising route toward protected superconducting qubits with local, scalable, and all-electrical control.

arXiv:2607.04097•Jul 5, 2026•0 citations

Parity-Protected Superconductor-Semiconductor Qubit

T. W. Larsen, M. E. Gershenson, Lucas Casparis +7 more

Coherence of superconducting qubits can be improved by implementing designs that protect the parity of Cooper pairs on superconducting islands. Here, we introduce a parity-protected qubit based on voltage-controlled semiconductor nanowire Josephson junctions, taking advantage of the higher harmonic content in the energy-phase relation of few-channel junctions. A symmetric interferometer formed by two such junctions, gate-tuned into balance and frustrated by a half-quantum of applied flux, yields a cos(2φ) Josephson element, reflecting coherent transport of pairs of Cooper pairs. We demonstrate that relaxation of the qubit can be suppressed tenfold by tuning into the protected regime.

arXiv:2004.03975•Physical Review Letters•Jul 30, 2020•102 citations

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

Erwan Roverc'h, Alvise Borgognoni, Marius Villiers +7 more

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.

quant-pharXiv:2603.13114•Mar 16, 2026•0 citations

Moving beyond the Transmon: Noise-Protected Superconducting Quantum Circuits

András Gyenis, Agustín Di Paolo, Jens Koch +3 more

Artificial atoms realized by superconducting circuits offer unique opportunities to store and process quantum information with high fidelity.Among them, implementations of circuits that harness intrinsic noise protection have been rapidly developed in recent years.These noise-protected devices constitute a new class of qubits in which the computational states are largely decoupled from local noise channels.The main challenges in engineering such systems are simultaneously guarding against both bit-and phase-flip errors, and also ensuring high-fidelity qubit control.Although partial noise protection is possible in superconducting circuits relying on a single quantum degree of freedom, the promise of complete protection can only be fulfilled by implementing multimode or hybrid circuits.This Perspective reviews the theoretical principles at the heart of these new qubits, describes recent experiments, and highlights the potential of robust encoding of quantum information in superconducting qubits.

arXiv:2106.10296•PRX Quantum•Sep 2, 2021•117 citations

High-fidelity two-qubit gates with transmon qubits using bipolar flux pulses and tunable couplers

Nikita S. Smirnov, Aleksei R. Matanin, Аnton I. Ivanov +15 more

High-fidelity two-qubit gates are essential for scalable quantum computing. We present a scheme based on superconducting transmon qubits and a control pulse delivery protocol that enables arbitrary controlled-phase gates modulated solely by an independent arbitrary waveform generator pulse. We combined a tunable coupler design with bipolar flux-pulsing to demonstrate a high-fidelity gate with a peak performance of $99.5\%$. Our gates inherit the advantages of both approaches: minimal residual ZZ coupling, built-in echo-like low-frequency noise protection, and time-scale control pulse reproducibility, while remaining easy to calibrate. We optimize the system energy levels to mitigate leakage to the coupler and suppress residual interactions. Numerical simulations of the scheme as three qutrits indicate that an error below $1 \times 10^{-3}$ is achievable. We confirm the scalability potential of the proposed scheme on high-fidelity 4-qubit and 8-qubit quantum processors

arXiv:2509.04965•Physical Review Applied•Jul 13, 2026•0 citations

Construction of new type of CNOT gate using cross-resonance pulse in the transmon-PPQ system

Jeongsoo Kang, Younghun Kwon

The transmon, known for its fast operation time and the coherence time of tens of microseconds, is the most commonly used qubit for superconducting quantum processors. However, it is still necessary to enhance the coherence time and the gate fidelity of superconducting quantum processors for the practical implementation of fault-tolerant quantum computing. Meanwhile, a novel superconducting qubit, which has the ability to protect the Cooper-pair parity on the superconducting island, has been proposed. This new qubit shows better coherence performance than the transmon, but it does not yet have an efficient method for realizing a superconducting hybrid system that harnesses it. In this work, we show how to implement a new type of CNOT gate in a superconducting hybrid system composed of tunable transmon and parity-protected qubit by applying a cross-resonance pulse. First, we provide hardware specifications and pulse parameters to construct a successful two-qubit gate in the hybrid system. Second, we show that our method can supply a CNOT gate of average fidelity with more than 0.998. Therefore, our work implies that the hybrid system may provide a new platform for quantum computers.

arXiv:2501.15218•Jan 25, 2025•0 citations
Novel qubits with intrinsic noise protection constitute a promising route for improving the coherence of quantum information in superconducting circuits.However, many protected superconducting qubits exhibit relatively low transition frequencies, which could make their integration with conventional transmon circuits challenging.In this work, we propose and study a scheme for entangling a tunable transmon with a Cooper-pair parity-protected qubit, a paradigmatic example of a low-frequency protected qubit that stores quantum information in opposite Cooper-pair parity states on a superconducting island.By tuning the external flux on the transmon, we show that noncomputational states can mediate a two-qubit entangling gate that preserves the Cooper-pair parity independent of the detailed pulse sequence.Interestingly, the entangling gate bears similarities to a controlled-phase gate in conventional transmon devices.Hence, our results suggest that standard high-precision gate calibration protocols could be repurposed for operating heterogeneous quantum processors.
Parity-protected superconducting qubits offer intrinsically long coherence, but many current implementations require magnetic-flux biasing, which introduces flux noise, control overhead, and limited scalability. Here we propose a parity-protected qubit based on a gate-tunable superconductor-altermagnet-superconductor Josephson junction. Altermagnets are compensated magnets with momentum-dependent spin splitting and zero net magnetization, providing spin-dependent functionality without external magnetic fields. In the proposed junction, the two spin sectors acquire opposite phase shifts, generating two Josephson channels whose interference is controlled electrically by the chemical potential. At the tuned 000-πππ transition, the first Josephson harmonic is strongly suppressed while the second harmonic dominates, yielding a double-well potential with two nearly degenerate states of opposite Cooper-pair parity. For realistic gatemon-compatible parameters, we estimate coherence times of up to tens of milliseconds while maintaining fully gate-controlled qubit operations. These results establish altermagnetic Josephson junctions as a promising route toward protected superconducting qubits with local, scalable, and all-electrical control.
Coherence of superconducting qubits can be improved by implementing designs that protect the parity of Cooper pairs on superconducting islands. Here, we introduce a parity-protected qubit based on voltage-controlled semiconductor nanowire Josephson junctions, taking advantage of the higher harmonic content in the energy-phase relation of few-channel junctions. A symmetric interferometer formed by two such junctions, gate-tuned into balance and frustrated by a half-quantum of applied flux, yields a cos(2φ) Josephson element, reflecting coherent transport of pairs of Cooper pairs. We demonstrate that relaxation of the qubit can be suppressed tenfold by tuning into the protected regime.
Experimental realization of a cos⁡(2φ)\cos(2\varphi)cos(2φ) transmon qubit
Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The cos⁡(2φ)\cos(2\varphi)cos(2φ) 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 MHz13.6~\mathrm{MHz}13.6 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 100−100-100−fold suppression of the island charge matrix element compared to the unprotected plasmon transition, placing dielectric loss limits above 10 ms10~\mathrm{ms}10 ms. The measured T1=70 μsT_1 = 70~μ\mathrm{s}T1​=70 μs and T2echo=2.5 μsT_2^\mathrm{echo}= 2.5~μ\mathrm{s}T2echo​=2.5 μs are instead limited by 1/f1/f1/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 4e4e4e-tunneling elements.
Artificial atoms realized by superconducting circuits offer unique opportunities to store and process quantum information with high fidelity.Among them, implementations of circuits that harness intrinsic noise protection have been rapidly developed in recent years.These noise-protected devices constitute a new class of qubits in which the computational states are largely decoupled from local noise channels.The main challenges in engineering such systems are simultaneously guarding against both bit-and phase-flip errors, and also ensuring high-fidelity qubit control.Although partial noise protection is possible in superconducting circuits relying on a single quantum degree of freedom, the promise of complete protection can only be fulfilled by implementing multimode or hybrid circuits.This Perspective reviews the theoretical principles at the heart of these new qubits, describes recent experiments, and highlights the potential of robust encoding of quantum information in superconducting qubits.
High-fidelity two-qubit gates are essential for scalable quantum computing. We present a scheme based on superconducting transmon qubits and a control pulse delivery protocol that enables arbitrary controlled-phase gates modulated solely by an independent arbitrary waveform generator pulse. We combined a tunable coupler design with bipolar flux-pulsing to demonstrate a high-fidelity gate with a peak performance of 99.5%99.5\%99.5%. Our gates inherit the advantages of both approaches: minimal residual ZZ coupling, built-in echo-like low-frequency noise protection, and time-scale control pulse reproducibility, while remaining easy to calibrate. We optimize the system energy levels to mitigate leakage to the coupler and suppress residual interactions. Numerical simulations of the scheme as three qutrits indicate that an error below 1×10−31 \times 10^{-3}1×10−3 is achievable. We confirm the scalability potential of the proposed scheme on high-fidelity 4-qubit and 8-qubit quantum processors
The transmon, known for its fast operation time and the coherence time of tens of microseconds, is the most commonly used qubit for superconducting quantum processors. However, it is still necessary to enhance the coherence time and the gate fidelity of superconducting quantum processors for the practical implementation of fault-tolerant quantum computing. Meanwhile, a novel superconducting qubit, which has the ability to protect the Cooper-pair parity on the superconducting island, has been proposed. This new qubit shows better coherence performance than the transmon, but it does not yet have an efficient method for realizing a superconducting hybrid system that harnesses it. In this work, we show how to implement a new type of CNOT gate in a superconducting hybrid system composed of tunable transmon and parity-protected qubit by applying a cross-resonance pulse. First, we provide hardware specifications and pulse parameters to construct a successful two-qubit gate in the hybrid system. Second, we show that our method can supply a CNOT gate of average fidelity with more than 0.998. Therefore, our work implies that the hybrid system may provide a new platform for quantum computers.