Measuring Effective Temperatures of Qubits Using Correlations.

Area of Science:

• Quantum Computing
• Quantum Information Science
• Superconducting Circuits

Background:

• Initializing qubits in pure states is crucial for quantum computation.
• Existing methods for evaluating initialization often require high-fidelity readout or involve complex experimental setups.
• Precise measurement of excited state populations is key to assessing and improving qubit initialization protocols.

Purpose of the Study:

• To propose and experimentally validate a new technique for measuring qubit excited state population.
• To develop a method that bypasses the need for high-fidelity readout and external qubit energy levels.
• To enable precise characterization of qubit initialization and decoherence sources.

Main Methods:

• Utilized correlations between two sequential measurements on a qubit.
• Implemented the technique on a circuit Quantum Electrodynamics (QED) platform.
• Compared the proposed method with existing techniques for population measurement.

Main Results:

• Achieved experimental measurement of spurious qubit population with accuracy up to 0.01%.
• Demonstrated a method that does not require high-fidelity readout.
• Showcased the ability to perform "temperature spectroscopy" of the qubit, aiding in the identification of decoherence mechanisms.

Conclusions:

• The proposed technique offers a precise and efficient way to measure qubit excited state populations.
• This method simplifies qubit initialization evaluation by removing stringent readout requirements.
• The high accuracy achieved facilitates deeper understanding of qubit decoherence, paving the way for improved quantum computer performance.