Two-electron States Spotted In Bilayer Graphene Quantum Dots Useful For Qubits

Researchers at the Laboratory for Solid State Physics, ETH Zürich, and the National Institute for Materials Science in Japan have advanced the study of bilayer graphene quantum dots. These structures confine electrons in a space small enough to exhibit both long-lived spin and valley states, crucial for solid-state qubits. The team utilized circuit quantum electrodynamics (cQED) to achieve improved energy resolution in spectroscopy of these few-carrier states, surpassing traditional methods.

The study revealed dispersive features of two and three electron states, enabling the detection of Pauli spin and valley blockade, and characterization of the spin-orbit gap at zero magnetic field, consistently measured at approximately 1.5 meV. This precise measurement is vital for developing solid-state qubits, as it enhances understanding of electron interactions within bilayer graphene quantum dots.

Bilayer graphene is a promising platform for quantum dots due to its unique electronic properties, including long-lived spin and valley states. These properties are essential for robust quantum information storage and processing. The research focused on a double quantum dot system, where multiple carriers are distributed across two adjacent dots, with exchange interactions central to qubit operation.

Traditional methods relied on measuring electrical current through the dots, limited by electronic temperature and phonon interactions. Instead, the team employed a superconducting high-impedance resonator capacitively coupled to the bilayer graphene double quantum dot. This cQED architecture allows for the detection of electric dipole transitions between quantum dot states, providing improved energy resolution.

Finite source-drain bias was introduced to create a non-equilibrium population of electrons, enhancing the visibility of transitions. The observed dispersive features are linked to the interaction between the quantum dot and the microwave resonator. The study explains that the effect of the double quantum dot states on the resonator is condensed into its electric susceptibility, which describes the system’s polarizability in response to the electric field.

Kane-Mele spin-orbit coupling, an intrinsic property of the material, splits the energy levels of electrons based on the alignment of their valley and spin magnetic moments. The team observed a zero-field energy gap consistent with previously characterized values. The ability to detect Pauli blockade via the resonator signal is promising for fast qubit readout, converting quantum information into a measurable electrical signal.

The device was fabricated from a van der Waals heterostructure, with bilayer graphene encapsulated in hexagonal boron nitride. The high-impedance resonator was integrated onto the same chip, allowing for direct capacitive coupling to the double quantum dot. This hybrid architecture enables simultaneous measurement of both transport and microwave properties, providing comprehensive characterization of the quantum dot’s behavior.

Bilayer graphene is rapidly becoming favored for constructing quantum dots capable of hosting both long-lived spin and valley states. This approach allows for detailed examination of states containing just a few carriers, focusing on the behavior of two electrons within the quantum dot structure. The ability to precisely measure these few-carrier states is critical for realizing solid-state qubits in bilayer graphene.

The researchers achieved this by employing a superconducting high-impedance resonator to detect subtle changes in the system’s electric dipole moment. By tuning the energy of the resonator, they observed dispersive features corresponding to transitions between two- and three-electron states. Detecting this blockade with the resonator signal provides a mechanism for selectively probing specific quantum states.

The energy resolution achieved with this cQED technique surpasses traditional transport measurements, allowing characterization of the spin-orbit gap at zero magnetic field. This state-selectivity enables researchers to selectively address and manipulate individual qubits within the system. The findings deepen our understanding of few-carrier spin and valley states in bilayer graphene quantum dots, opening new avenues for exploring the potential of this material in quantum information science.