[Paper Review] Hexagonal Boron Nitride (hBN) as a Low-loss Dielectric for Superconducting Quantum Circuits and Qubits

Joel I-J. Wang, Megan Yamoah+20 more|arXiv (Cornell University)|Aug 31, 2021

Quantum Information and Cryptography28 references4 citations

TL;DR

This paper demonstrates that hexagonal boron nitride (hBN) is a low-loss dielectric for superconducting quantum circuits, achieving a microwave loss tangent of at most 10^{-6} in the low-temperature, single-photon regime. By integrating hBN capacitors with aluminum Josephson junctions, the authors realize transmon qubits with coherence times up to 25 μs and reduce qubit footprint by two orders of magnitude compared to conventional designs.

ABSTRACT

Dielectrics with low loss at microwave frequencies are imperative for high-coherence solid-state quantum computing platforms. We study the dielectric loss of hexagonal boron nitride (hBN) thin films in the microwave regime by measuring the quality factor of parallel-plate capacitors (PPCs) made of NbSe$_{2}$-hBN-NbSe$_{2}$ heterostructures integrated into superconducting circuits. The extracted microwave loss tangent of hBN is bounded to be at most in the mid-10$^{-6}$ range in the low temperature, single-photon regime. We integrate hBN PPCs with aluminum Josephson junctions to realize transmon qubits with coherence times reaching 25 $μ$s, consistent with the hBN loss tangent inferred from resonator measurements. The hBN PPC reduces the qubit feature size by approximately two-orders of magnitude compared to conventional all-aluminum coplanar transmons. Our results establish hBN as a promising dielectric for building high-coherence quantum circuits with substantially reduced footprint and, with a high energy participation that helps to reduce unwanted qubit cross-talk.

Motivation & Objective

• To evaluate the microwave dielectric loss of hexagonal boron nitride (hBN) in superconducting quantum circuits.
• To address the challenge of dielectric loss limiting qubit coherence in solid-state quantum computing platforms.
• To develop a scalable, compact qubit architecture using hBN as a dielectric to reduce device footprint.
• To demonstrate that hBN enables high-coherence qubits with reduced cross-talk due to high energy participation.

Proposed method

• Fabrication of NbSe₂-hBN-NbSe₂ heterostructure parallel-plate capacitors (PPCs) for microwave characterization.
• Measurement of the quality factor of PPCs in superconducting circuits to extract the microwave loss tangent of hBN.
• Integration of hBN PPCs with aluminum Josephson junctions to realize transmon qubits.
• Use of low-temperature, single-photon regime measurements to ensure minimal dissipation and accurate loss quantification.
• Comparison of qubit coherence times with theoretical predictions based on hBN loss tangent from resonator measurements.
• Employment of high-precision microwave spectroscopy and cryogenic measurement techniques to validate performance.

Experimental results

Research questions

• RQ1 What is the microwave dielectric loss tangent of hexagonal boron nitride (hBN) in the low-temperature, single-photon regime?
• RQ2 Can hBN serve as a viable low-loss dielectric in superconducting quantum circuits without degrading qubit coherence?
• RQ3 To what extent can hBN reduce the physical footprint of transmon qubits compared to conventional all-aluminum coplanar designs?
• RQ4 How does the high energy participation of hBN capacitors affect qubit cross-talk in multi-qubit architectures?
• RQ5 Is the coherence time of hBN-based transmon qubits consistent with the loss tangent inferred from resonator measurements?

Key findings

• The microwave loss tangent of hBN is bounded to at most 10^{-6} in the low-temperature, single-photon regime, indicating exceptionally low dielectric loss.
• hBN-based transmon qubits achieve coherence times of up to 25 μs, consistent with the loss tangent measured in resonators.
• The use of hBN reduces the qubit feature size by approximately two orders of magnitude compared to conventional all-aluminum coplanar transmons.
• The high energy participation of hBN capacitors helps suppress unwanted qubit cross-talk in multi-qubit systems.
• The integration of hBN into superconducting circuits demonstrates compatibility with existing fabrication processes and scalability.
• The results establish hBN as a promising dielectric for high-coherence, compact quantum circuits with improved performance metrics.