Spin-valley protected Kramers pair in bilayer graphene

Area of Science:

• Quantum computing
• Condensed matter physics
• Materials science

Background:

• Bilayer graphene (BLG) offers a unique platform for semiconductor qubits due to its valley degree of freedom.
• The ground state of a single-carrier quantum dot (QD) in BLG is a twofold degenerate Kramers pair with opposite spin and valley quantum numbers.
• An out-of-plane magnetic field breaks time-reversal symmetry, enabling qubit encoding in the spin-valley subspace.

Purpose of the Study:

• To investigate the potential of BLG as a platform for long-lived qubits.
• To measure the spin-valley relaxation time of Kramers states in BLG QDs.
• To demonstrate high-fidelity qubit readout in BLG.

Main Methods:

• Fabrication of a tunable quantum dot (QD) device in Bernal bilayer graphene.
• Measurement of spin-valley relaxation time at 30 mK.
• Demonstration of single-shot readout utilizing intrinsic Kane-Mele spin-orbit splitting.

Main Results:

• A spin-valley relaxation time of 38 seconds was measured for Kramers states at 30 mK, significantly longer than for spin-blocked states (0.4 seconds).
• High-fidelity (above 99%) single-shot readout of the Kramers doublet was achieved at zero magnetic field.
• The observed long relaxation time highlights the protection of Kramers states against spin- and valley-mixing mechanisms.

Conclusions:

• Electrostatically defined QDs in BLG can host long-lived Kramers states, potentially serving as robust semiconductor qubits.
• The demonstrated long spin-valley relaxation time and high-fidelity readout are crucial steps towards realizing advanced quantum information processing.
• BLG-based qubits may extend beyond the conventional spin qubit paradigm, offering new possibilities in quantum computing.