[Paper Review] Coherent spin-qubit photon coupling

Andreas Landig, Jonne Koski+8 more|arXiv (Cornell University)|Nov 6, 2017

Quantum and electron transport phenomena66 citations

TL;DR

The paper demonstrates strong coupling between a single microwave photon in a NbTiN high-impedance resonator and a three-electron spin-qubit in a GaAs triple quantum dot, observing vacuum Rabi mode splitting with g/2π ≈ 31.4 MHz and γ2/2π ≈ 19.6 MHz.

ABSTRACT

Electron spins hold great promise for quantum computation due to their long coherence times. An approach to realize interactions between distant spin-qubits is to use photons as carriers of quantum information. We demonstrate strong coupling between single microwave photons in a NbTiN high impedance cavity and a three-electron spin-qubit in a GaAs triple quantum dot. We resolve the vacuum Rabi mode splitting with a coupling strength of $g/2π\simeq31$ MHz and a qubit decoherence of $γ_2/2π\simeq 20$ MHz. We can tune the decoherence electrostatically and obtain a minimal $γ_2/2π\simeq 10$ MHz for $g/2π\simeq 23$ MHz. The dependence of the qubit-photon coupling strength on the tunable electric dipole moment of the qubit is measured directly using the ac Stark effect. Our demonstration of strong spin-photon interaction is an important step towards coherent long-distance coupling of spin-qubits.

Motivation & Objective

• Motivate long-distance quantum information transfer using photons to connect electron-spin qubits with long coherence times.
• Realize strong coherent coupling between a spin-qubit and a single microwave photon in a circuit QED architecture.
• Characterize how electrostatic tuning affects qubit-photon coupling and qubit decoherence.
• Show that the observed coupling satisfies the strong coupling criterion and map the qubit energy landscape for optimized interaction.

Proposed method

• Use a NbTiN high-impedance resonator to enhance the spin-photon coupling via large Z_r ~ 1.3 kΩ.
• Implement a three-electron spin-qubit in a GaAs triple quantum dot with tunable detuning Δ and asymmetry ε to form qubit states.
• Couple the left quantum dot plunger gate to the resonator to realize electric-dipole coupling.
• Observe vacuum Rabi mode splitting by tuning the spin-qubit into resonance with the resonator and fit with input-output theory to extract g and γ2.
• Calibrate g through ac Stark shift measurements and resonator frequency shifts in the dispersive regime.
• Explore the double sweet spot (DSS) where qubit energy is Less sensitive to ε and Δ fluctuations and map energy contours.

Experimental results

Research questions

• RQ1 Can a spin-qubit in a GaAs triple quantum dot achieve strong coherent coupling to a single microwave photon in a circuit QED setup?
• RQ2 How do detuning Δ and charge admixture (ε, Δ) influence the qubit-photon coupling strength g and qubit decoherence γ2?
• RQ3 Can ac Stark shift measurements provide a direct, calibration-free handle on g in this system?
• RQ4 Does the observed spin-qubit–photon system satisfy the criteria for strong coupling across a range of operating points, including the double sweet spot?
• RQ5 What implications do these results have for scalable, long-distance spin-qubit interconnects in quantum information processing?

Key findings

• Vacuum Rabi mode splitting observed when the spin-qubit is in resonance with the resonator, yielding g/2π = (31.4 ± 0.3) MHz and γ2/2π = (19.6 ± 0.5) MHz.
• Strong coupling regime evidenced by 2g > κ/2 + γ2, with κ/2π = 47.1 MHz at low photon occupation (<1).
• Decoherence γ2 can be electrostatically tuned; minimal γ2/2π ≈ 10 MHz occurs for g/2π ≈ 23 MHz, indicating improved coherence at more spin-like (1,1,1) states.
• Coupling strength g increases with Δ due to greater admixture of charge states, as shown by ac Stark shift calibrations.
• The ac Stark shift enables direct access to g and confirms single-photon regime (average photon number ≈ 0.3 at the Rabi splitting measurement).
• Demonstrates scalable spin-photon coupling platform in materials without relying on ferromagnets or strong spin-orbit coupling, with potential transfer to low-hyperfine hosts.