[Paper Review] Spectroscopy of spin-split Andreev levels in a quantum dot with superconducting leads

Arno Bargerbos, Marta Pita‐Vidal+10 more|arXiv (Cornell University)|Aug 19, 2022

Quantum and electron transport phenomena65 references4 citations

TL;DR

This study demonstrates all-electrical control and spectroscopy of spin-split Andreev bound states in a superconducting quantum dot Josephson junction. By tuning the junction to a spin-1/2 ground state and applying small magnetic fields (<40 mT), the researchers resolve two flux-sensitive spin branches, observe the anomalous Josephson effect, and directly excite spin-flip transitions via microwave-driven electric dipole spin resonance, enabling a path toward robust, electrically controlled Andreev spin qubits with long coherence times.

ABSTRACT

We use a hybrid superconductor-semiconductor transmon device to perform spectroscopy of a quantum dot Josephson junction tuned to be in a spin-1/2 ground state with an unpaired quasiparticle. Due to spin-orbit coupling, we resolve two flux-sensitive branches in the transmon spectrum, depending on the spin of the quasi-particle. A finite magnetic field shifts the two branches in energy, favoring one spin state and resulting in the anomalous Josephson effect. We demonstrate the excitation of the direct spin-flip transition using all-electrical control. Manipulation and control of the spin-flip transition enable the future implementation of charging energy protected Andreev spin qubits.

Motivation & Objective

• To achieve all-electrical control of spin states in a superconducting quantum dot Josephson junction with long-lived quasiparticle states.
• To spectroscopically resolve spin-split Andreev bound states arising from spin-orbit coupling in an InAs/Al nanowire device.
• To demonstrate the anomalous Josephson effect via magnetic field-induced shifts in the energy-phase relation.
• To enable direct microwave-driven spin-flip transitions using electric dipole spin resonance (EDSR) for faster, simpler qubit manipulation.
• To develop a platform for charging energy-protected Andreev spin qubits with tunable, flux- and field-controllable transition frequencies.

Proposed method

• Fabricated a hybrid superconductor-semiconductor transmon device with a gate-tunable quantum dot embedded in an InAs/Al nanowire Josephson junction.
• Employed transmon spectroscopy to probe the joint energy levels of the transmon and quantum dot junction, measuring flux- and magnetic field-dependent transition frequencies.
• Applied external magnetic fields (up to 40 mT) to lift spin degeneracy and tune the energy splitting between spin states, enabling selective population of spin sublevels.
• Used microwave pulses applied to a bottom gate to drive electric dipole spin resonance (EDSR), directly exciting spin-flip transitions between the doublet states.
• Modelled the system using a modified single-impurity Anderson model (SIAM) and compared experimental data to theoretical predictions of spin-split Andreev levels and anomalous Josephson behavior.
• Extracted effective Landé g-factors (g∥ = 11, g⊥ = 3.8) from flux-periodicity and frequency shift measurements, confirming spin-orbit coupling effects.

Experimental results

Research questions

• RQ1 Can spin-split Andreev bound states in a superconducting quantum dot junction be spectroscopically resolved using transmon techniques?
• RQ2 How does an external magnetic field influence the energy splitting and flux dependence of the spin-split Andreev levels?
• RQ3 Can the anomalous Josephson effect—characterized by a non-zero, spin-dependent phase minimum—be observed and controlled in this system?
• RQ4 Is direct electric-dipole-driven spin-flip transition possible in a superconducting hybrid device without auxiliary levels?
• RQ5 To what extent can the transition frequency between spin states be tuned via magnetic field and flux for scalable qubit integration?

Key findings

• The spin-split Andreev bound states were resolved spectroscopically despite energy splitting smaller than the electron temperature, enabled by transmon-based measurement sensitivity.
• Two distinct flux-sensitive branches in the transmon spectrum were observed, corresponding to the spin-up and spin-down states of the doublet, with energy splitting tunable by magnetic field.
• A magnetic field of less than 40 mT induced a measurable shift in the Josephson energy-phase relation, confirming the anomalous Josephson effect with a spin-dependent minimum phase φ₀ ≠ 0, π.
• The effective Landé g-factors were extracted as g∥ = 11 and g⊥ = 3.8, indicating strong spin-orbit coupling and anisotropic spin response to magnetic fields.
• Direct microwave-driven spin-flip transitions were achieved via electric dipole spin resonance (EDSR) using gate voltage pulses, enabling all-electrical control without requiring auxiliary levels.
• The quasiparticle lifetime in the spin-1/2 ground state exceeded 1 ms, attributed to large charging energy suppressing quasiparticle poisoning, enabling stable qubit operation.