[논문 리뷰] Loopless multiterminal quantum circuits at odd parity

Antonio L. R. Manesco, Anton Akhmerov외 1명|arXiv (Cornell University)|2026. 01. 19.

Topological Materials and Phenomena인용 수 0

한 줄 요약

논문은 시간 반전 대칭을 갖는 홀수 페리티 구역에서 loopless 다중터미널 Andreev 디바이스를 개발·분석하고, 스핀-궤도 상호작용과 커패시티브 샤팅에 의해 이중 우물 에너지-상 풍경과 SU(2) 스핀-상 제어를 가능하게 한다. 전기장 조정을 통해 스핀이 있는 경우의 4차원 저에너지 부분공간을 보편적으로 제어할 수 있음을 보여주고, 자기 플럭스 없이 카이랄성과 스핀 선택적 구동이 가능함을 입증한다.

ABSTRACT

We theoretically investigate loopless multiterminal hybrid superconducting devices at odd fermion parity with time-reversal symmetry. We find that the energy-phase relationship has a double minimum corresponding to opposite windings of the superconducting phases. Spin-orbit coupling adds multi-axial spin splittings, which contrasts with two-terminal devices where spin dependence is uniaxial. Capacitive shunting localizes quantum circuit states in the wells and exponentially suppresses their splitting. For weak spin-orbit strength, the system has a four-dimensional spin-chirality low-energy subspace which can be universally controlled with electric fields only.

연구 동기 및 목표

• Motivate and model multiterminal superconducting weak links in the odd parity sector with time-reversal symmetry.
• Show that a three-terminal, loopless circuit yields a double-well energy-phase relation (SPER) with opposite phase windings.
• Introduce capacitive shunting to localize circuit states and exponentially suppress inter-well splitting.
• Reveal that spin-orbit coupling creates a multi-axial spin dependence in the SPER, extending beyond the two-terminal case.
• Demonstrate universal electric-field control of a four-dimensional spin-chirality subspace and discuss experimental feasibility.

제안 방법

• Construct a minimal two-orbital quantum dot model coupled to three superconducting leads with spin-orbit coupling.
• Derive the spinless SPER as a sum of π-junction terms, forming a symmetric double-well in the three-terminal case.
• Extract the spin-dependent SPER U_SO as V_SO · σ with trajectory-dependent spin axes.
• Add capacitive energy to obtain a quantum phase dynamics Hamiltonian H = T + U and diagonalize numerically.
• Derive an effective 4x4 Hamiltonian for the spinful subspace and analyze drive-induced spin and chirality transitions.
• Provide analytical relations for the coupling terms and illustrate control via charge driving and circularly polarized drives.

Figure 1: Schematics for proximitized single-level dots with two and three terminals in the odd parity sector and its analogous circuits. (a) Two-terminal devices without SOC are typically $\pi$ -junctions. (b) Three-terminal devices without SOC are analogous to a triangle of $\pi$ -junctions, which

실험 결과

연구 질문

• RQ1 How does a multiterminal (three-terminal) weak link in the odd parity sector produce a double-well energy-phase relation without magnetic flux?
• RQ2 How does spin-orbit coupling modify the spin-phase energy relation in loopless multiterminal circuits?
• RQ3 How can capacitive shunting localize circuit states and exponentially suppress inter-well splitting?
• RQ4 What is the structure of the low-energy spin-chirality subspace, and can it be universally controlled by electric fields?
• RQ5 Can the system realize SU(4) control with chirality- and spin-dependent drive, enabling universal qubit manipulation?

주요 결과

• The energy-phase relation in loopless three-terminal devices exhibits a balanced double-well potential with opposite chirality minima in the odd parity sector.
• Spin-orbit coupling introduces a spin-dependent term that spans a full SU(2) structure, unlike the uniaxial spin dependence in two-terminal devices.
• Capacitive shunting localizes the lowest-energy states in the wells and exponentially suppresses their splitting as the charging energy decreases.
• In the spinless regime, reducing E_C leads to exponentially localized wavefunctions in the two wells and exponentially small energy splitting between the two lowest states.
• In the spinful regime, the lowest levels split into spin-orbit doublets, and the derived effective Hamiltonian shows chirality- and spin-dependent couplings that enable resonant, circularly polarized driving to perform selective spin rotations.
• The analysis suggests realistic parameter ranges where the proposed effects are observable, and highlights potential for heavy, flux-free qubits with electric-field control.

Figure 2: (a) Schematic diagram of the microscopic toy model for obtaining the Hamiltonian of the trijunction at odd parity. (b) Schematic diagram of the structure of the dot levels and the tunneling terms that are included in the model. (c) Spin-phase energy relation, $U=U_{0}+U_{SO}$ , for $t=0.1\