[Paper Review] Single-Shot Readout of Macroscopic Quantum Superposition State in a Superconducting Flux Qubit

Hirotaka Tanaka, Shiro Saito+4 more|arXiv (Cornell University)|Jul 12, 2004

Quantum Information and Cryptography2 references8 citations

TL;DR

This paper demonstrates single-shot readout of a macroscopic quantum superposition state in a superconducting flux qubit using a current-biased dc-SQUID detector. By operating the SQUID in a narrow switching current regime below 100 nA, the researchers achieved distinct, non-overlapping readout of the qubit's energy eigenstates |0⟩ and |1⟩, confirming coherent quantum behavior through microwave-induced transitions and observing a coherence time of T₂* ≈ 5 ns.

ABSTRACT

Single-shot readout experiments were performed on the two lowest-energy states of a superconducting qubit with three Josephson junctions embedded in a superconducting loop. We measured the qubit state via switching current Isw of a current-biased dc-SQUID, a quantum detector surrounding the qubit loop. The qubit signals were measured in a small Isw regime of the SQUID, typically less than 100 nA, where the Isw distribution is particularly narrow. The obtained single-shot data indicate that the qubit state is readout, through the flux generated by the qubit persistent-current, as energy eigenstates rather than current eigenstates.

Motivation & Objective

• To achieve single-shot measurement of a macroscopic quantum superposition state in a superconducting flux qubit.
• To overcome limitations of averaged readout by demonstrating high-fidelity, real-time state detection.
• To confirm the presence of macroscopic quantum coherence through microwave-induced transitions.
• To investigate the quantum-classical boundary using a fabricated, scalable solid-state qubit.

Proposed method

• A superconducting flux qubit with three Josephson junctions was fabricated using suspended bridge and shadow evaporation techniques.
• The qubit was coupled magnetically to a current-biased dc-SQUID detector with mutual inductance M ≈ 7 pH.
• Single-shot readout was performed by measuring the switching current (I_sw) of the SQUID in a regime with narrow I_sw distribution, typically <100 nA.
• The qubit was operated at a flux bias point where the I_sw distribution for |0⟩ and |1⟩ states showed complete separation.
• Microwave irradiation at resonant frequency was applied to probe coherent transitions between the qubit states.
• Thermodynamic and quantum statistical models were used to compare experimental data with canonical distribution predictions.

Experimental results

Research questions

• RQ1 Can a single-shot measurement resolve the two lowest-energy states of a superconducting flux qubit with high fidelity?
• RQ2 Does the observed switching current distribution reflect energy eigenstates rather than current eigenstates?
• RQ3 Is macroscopic quantum coherence present in the system, as evidenced by microwave-induced transitions?
• RQ4 How does the coherence time T₂* of the qubit state relate to the observed linewidth of the resonant transition?
• RQ5 Do the experimental results confirm that a macroscopic quantum object follows canonical statistical mechanics?

Key findings

• The single-shot readout successfully distinguished the |0⟩ and |1⟩ energy eigenstates with no overlap in the switching current distribution.
• The measured coherence time T₂* was approximately 5 ns, derived from the linewidth of the microwave-induced transition.
• Microwave irradiation at resonance induced a measurable transition from the |0⟩ to |1⟩ state, confirming the presence of coherent quantum tunneling.
• The experimental switching current data showed excellent agreement with theoretical predictions based on the canonical distribution at finite temperature.
• The absence of χ-shaped crossing steps in the high-fidelity sample indicated suppressed quantum tunneling, consistent with a high barrier and dominance of thermal excitation.
• The system exhibited behavior consistent with a macroscopic quantum object obeying statistical mechanics, as confirmed by thermal averaging and histogram analysis.