Abstract
Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation. However, coherence times of quantum circuits made from conventional aluminum-based Josephson junctions are limited by the presence of microscopic two-level systems in the amorphous aluminum oxide tunnel barriers. Here, we have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on silicon substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation, resulting in fewer two-level fluctuators, feasibility for epitaxial tunnel barriers that reduce energy relaxation and dephasing, and a larger superconducting gap of ~5.2 meV for NbN, compared to ~0.3 meV for aluminum, which suppresses the excitation of quasiparticles. By replacing conventional MgO by a silicon substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time {T}_{1}=16.3;{{upmu }}{{{{{rm{s}}}}}} and a spin-echo dephasing time {T}_{2}=21.5;{{upmu }}{{{{{rm{s}}}}}}. These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to the previously reported {T}_{1}approx {T}_{2}approx 0.5;{{upmu }}{{{{{rm{s}}}}}} of NbN-based qubits on MgO substrates. These results are an important step towards constructing a new platform for superconducting quantum hardware.
Highlights
Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation
Alternative materials for the Josephson junctions (JJs) of the qubits have not been studied adequately, even though it has been pointed out that the coherence times of superconducting quantum circuits made from conventional Al-based JJs are limited by energy or phase relaxation due to microscopic two-level systems (TLSs) in the amorphous aluminum oxide (AlOx) tunnel barriers[16,17,18]
It has been characterized using a direct current (DC) readout so that the phase qubit is quite insensitive to the dielectric loss of substrate, which explains the relatively long coherence times obtained in that work
Summary
Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation. Alternative materials for the Josephson junctions (JJs) of the qubits have not been studied adequately, even though it has been pointed out that the coherence times of superconducting quantum circuits made from conventional Al-based JJs are limited by energy or phase relaxation due to microscopic two-level systems (TLSs) in the amorphous aluminum oxide (AlOx) tunnel barriers[16,17,18].
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