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

Physical Review B

Two-qubit gate based on a multiterminal double-barrier Josephson junction

Serhii E. Shafranjuk

Published Jul 26, 2006• 9 citations
Publisher

Abstract

A multiterminal double-barrier SISIS junction (S and I denote a superconductor and an insulating barrier, respectively) is suggested as a two-qubit gate with tunable intrinsic coupling. Two quantum wells are formed in the vicinities of the left and right SIS subjunctions. This gives two individual qubits, which are intrinsically coupled via the middle S layer due to phase coherence. The interqubit coupling $J$ is tuned by two bias supercurrents ${I}_{1}$ and ${I}_{2}$ across each of the SIS subjunctions independently. Additional coupling is accomplished by transport supercurrents ${I}_{l}^{\mathrm{tr}}$ along adjacent S layers. Using a microscopic model we compute major qubit characteristics and study sources of the intrinsic decoherence. We compute the entanglement of the two-qubit states, leakage and fidelity characteristics versus $J$, and discuss the readout process.

DOI
10.1103/physrevb.74.024521

Related papers

Superconductor-ferromagnet-junction phase qubit

Tae-Wan Noh, H. -S. Sim, Mun Dae Kim

We propose a scheme for a phase qubit in an SIFIS junction, consisting of bulk superconductors (S), a proximity-induced ferromagnet (F), and insulating barriers (I). The qubit state is constituted by the 0 and the π phase states of the junction, in which the charging energy of the junction leads to a superposition of the two states. The qubit is operated by the gate voltage applied to the ferromagnet and is insensitive to the decoherence sources existing in other superconducting qubits. We discuss a scalable scheme for qubit measurement and tunable two-qubit coupling, and a possible experimental realization.

arXiv:0804.0349•Journal of the Korean Physical Society•Jan 1, 2012•4 citations

Superconducting Qubits with Altermagnetic Josephson Junctions

Johanne Bratland Tjernshaugen, Morten Amundsen, Jacob Linder

Identifying a materials platform for creating qubits that are both tunable and resilient towards environmental noise is one of the main hurdles that need to be overcome to realize quantum computation that is practically useful. One pursued avenue to this end is to use superconducting qubits with intrinsic spin-dependent interactions, such as spin-orbit coupling or magnetism. However, the recently discovered class of materials known as altermagnets remains largely unexplored in this context. We here use microscopic calculations to determine how the properties of superconducting qubits are modified when altermagnetic Josephson junctions are included. The key qubit performance parameters, including splitting, anharmonicity, decoherence, and single/coupled-qubit gate operation times, display rich behavior depending on the characteristic properties of the altermagnetic material, such as the strength of the Néel field and the crystallographic orientation of the altermagnetic relative to the interfaces in the system. We focus in particular on the transmon design and show that the qubit is very well protected against decoherence and simultaneously shows superior anharmonicity both near 0-$π$ transition points and when it is in a $φ$-state. We propose that by using strain, the altermagnetic qubit can be moved out of its protected regime to enable faster gate operation times, and then moved back to its protected state. We establish the physical mechanism underlying the behavior of all central qubit metrics, clarifying how real devices interpolate between an altermagnetic double-well regime exhibiting barrier-induced protection and a conventional transmon-like single-well regime. We also discuss how the altermagnetic properties influence flux qubits and fluxonium.

quant-pharXiv:2606.02761•Jun 3, 2026•0 citations

Quantum Two-Level Systems in Josephson Junctions as Naturally Formed Qubits

Alexandre M. Zagoskin, Sahel Ashhab, J. R. Johansson +1 more

The two-level systems (TLSs) naturally occurring in Josephson junctions constitute a major obstacle for the operation of superconducting phase qubits. Since these TLSs can possess remarkably long decoherence times, we show that such TLSs can themselves be used as qubits, allowing for a well controlled initialization, universal sets of quantum gates, and readout. Thus, a single current-biased Josephson junction can be considered as a multiqubit register. It can be coupled to other junctions to allow the application of quantum gates to an arbitrary pair of qubits in the system. Our results indicate an alternative way to realize superconducting quantum information processing.

arXiv:cond-mat/0603753•Physical Review Letters•Aug 14, 2006•118 citations

Supercurrent tuning of the Josephson coupling energy

Maxwell Wisne, Venkat Chandrasekhar

The ability to non-dissipatively tune the Josephson coupling energy of Josephson junctions is a useful tool in frequency-tunable qubits. This is typically done by threading magnetic flux through two junctions connected in a loop, a geometry that exposes the qubit to magnetic environmental noise. In this paper, we show that by biasing a junction with supercurrent from a separate pair of superconducting leads coupled to the device, the Josephson energy can be tuned without the need for a flux loop. Our multiterminal device may enable the realization of a frequency-tunable qubit with greatly reduced susceptibility to flux noise.

arXiv:2507.14357•Physical review. B./Physical review. B•Jan 30, 2026•0 citations

Theory of three-terminal Andreev spin qubits

Aleksandr E. Svetogorov, Kiryl Piasotski, Wolfgang Belzig +1 more

In this Letter, we introduce a next generation of Andreev spin qubits—three-terminal Andreev spin qubits. These qubits outperform two-terminal devices in the speed of two-qubit gates, benefiting from enhanced spin splitting. Our theoretical analysis reveals increased tunability and interqubit coupling mediated by the supercurrent. Furthermore, it provides a comprehensive explanation for the results of a recent experiment on a three-terminal device [M. Coraiola , ]. We identify the optimal operational regime for single- and two-qubit gates. Moreover, our concise theoretical framework for the equilibrium multiterminal Josephson effect offers significant potential for exploring nontrivial physics in the artificial dimensions of superconducting phase differences–—a rapidly growing field of research.

arXiv:2411.11155•Physical review. B./Physical review. B•Sep 15, 2025•1 citation

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
A multiterminal double-barrier SISIS junction (S and I denote a superconductor and an insulating barrier, respectively) is suggested as a two-qubit gate with tunable intrinsic coupling. Two quantum wells are formed in the vicinities of the left and right SIS subjunctions. This gives two individual qubits, which are intrinsically coupled via the middle S layer due to phase coherence. The interqubit coupling JJJ is tuned by two bias supercurrents I1{I}_{1}I1​ and I2{I}_{2}I2​ across each of the SIS subjunctions independently. Additional coupling is accomplished by transport supercurrents Iltr{I}_{l}^{\mathrm{tr}}Iltr​ along adjacent S layers. Using a microscopic model we compute major qubit characteristics and study sources of the intrinsic decoherence. We compute the entanglement of the two-qubit states, leakage and fidelity characteristics versus JJJ, and discuss the readout process.
We propose a scheme for a phase qubit in an SIFIS junction, consisting of bulk superconductors (S), a proximity-induced ferromagnet (F), and insulating barriers (I). The qubit state is constituted by the 0 and the π phase states of the junction, in which the charging energy of the junction leads to a superposition of the two states. The qubit is operated by the gate voltage applied to the ferromagnet and is insensitive to the decoherence sources existing in other superconducting qubits. We discuss a scalable scheme for qubit measurement and tunable two-qubit coupling, and a possible experimental realization.
Identifying a materials platform for creating qubits that are both tunable and resilient towards environmental noise is one of the main hurdles that need to be overcome to realize quantum computation that is practically useful. One pursued avenue to this end is to use superconducting qubits with intrinsic spin-dependent interactions, such as spin-orbit coupling or magnetism. However, the recently discovered class of materials known as altermagnets remains largely unexplored in this context. We here use microscopic calculations to determine how the properties of superconducting qubits are modified when altermagnetic Josephson junctions are included. The key qubit performance parameters, including splitting, anharmonicity, decoherence, and single/coupled-qubit gate operation times, display rich behavior depending on the characteristic properties of the altermagnetic material, such as the strength of the Néel field and the crystallographic orientation of the altermagnetic relative to the interfaces in the system. We focus in particular on the transmon design and show that the qubit is very well protected against decoherence and simultaneously shows superior anharmonicity both near 0-πππ transition points and when it is in a φφφ-state. We propose that by using strain, the altermagnetic qubit can be moved out of its protected regime to enable faster gate operation times, and then moved back to its protected state. We establish the physical mechanism underlying the behavior of all central qubit metrics, clarifying how real devices interpolate between an altermagnetic double-well regime exhibiting barrier-induced protection and a conventional transmon-like single-well regime. We also discuss how the altermagnetic properties influence flux qubits and fluxonium.
The two-level systems (TLSs) naturally occurring in Josephson junctions constitute a major obstacle for the operation of superconducting phase qubits. Since these TLSs can possess remarkably long decoherence times, we show that such TLSs can themselves be used as qubits, allowing for a well controlled initialization, universal sets of quantum gates, and readout. Thus, a single current-biased Josephson junction can be considered as a multiqubit register. It can be coupled to other junctions to allow the application of quantum gates to an arbitrary pair of qubits in the system. Our results indicate an alternative way to realize superconducting quantum information processing.
The ability to non-dissipatively tune the Josephson coupling energy of Josephson junctions is a useful tool in frequency-tunable qubits. This is typically done by threading magnetic flux through two junctions connected in a loop, a geometry that exposes the qubit to magnetic environmental noise. In this paper, we show that by biasing a junction with supercurrent from a separate pair of superconducting leads coupled to the device, the Josephson energy can be tuned without the need for a flux loop. Our multiterminal device may enable the realization of a frequency-tunable qubit with greatly reduced susceptibility to flux noise.
In this Letter, we introduce a next generation of Andreev spin qubits—three-terminal Andreev spin qubits. These qubits outperform two-terminal devices in the speed of two-qubit gates, benefiting from enhanced spin splitting. Our theoretical analysis reveals increased tunability and interqubit coupling mediated by the supercurrent. Furthermore, it provides a comprehensive explanation for the results of a recent experiment on a three-terminal device [M. Coraiola , ]. We identify the optimal operational regime for single- and two-qubit gates. Moreover, our concise theoretical framework for the equilibrium multiterminal Josephson effect offers significant potential for exploring nontrivial physics in the artificial dimensions of superconducting phase differences–—a rapidly growing field of research.
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.