Thermal and Residual Excited-State Population in a 3D Transmon Qubit

Area of Science:

• Quantum computing
• Superconducting qubits
• Cryogenic systems

Background:

• Advancements in solid-state qubits have improved coherence and control fidelity.
• Thermalizing cryogenic qubits to milliKelvin environments remains a significant challenge.

Purpose of the Study:

• To systematically investigate the first-excited-state population of a 3D transmon superconducting qubit.
• To determine the thermal equilibrium characteristics of superconducting qubits in a dilution refrigerator.

Main Methods:

• Utilized a modified protocol (Geerlings et al.) to measure the first-excited-state population.
• Employed a variable temperature dilution refrigerator to control the qubit environment.
• Verified results with a flux qubit exhibiting stronger readout resonator coupling.

Main Results:

• The excited-state population followed a Maxwell-Boltzmann distribution between 35-150 mK, indicating thermal equilibrium.
• Below 35 mK, the excited-state population saturated at approximately 0.1%.
• An effective qubit temperature (T(eff)) of 35 mK was determined.

Conclusions:

• Superconducting qubits can reach thermal equilibrium with their milliKelvin environment.
• The observed saturation suggests a lower bound on achievable qubit temperature.
• Inferred qubit lifetime (108 μs) is consistent with measurements (80 μs), assuming hot quasiparticles as the dominant factor.