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Related Experiment Video

Updated: Jun 21, 2026

Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators

Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators

Published on: May 30, 2014

Quantum time evolution in a qubit readout process with a Josephson bifurcation amplifier.

Hayato Nakano¹, Shiro Saito, Kouichi Semba

¹NTT Basic Research Laboratories, NTT Corporation, Atsugi-shi, Kanagawa 243-0198, Japan. nakano@will.brl.ntt.co.jp

Physical Review Letters

|August 8, 2009

Summary

Quantum mechanical analysis reveals that decoherence enables Josephson bifurcation amplifier (JBA) readout of superconducting qubits. Decoherence allows qubit-probe entanglement to separate, enabling measurement, with waiting time dependent on JBA decoherence strength.

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Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators

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Area of Science:

Quantum computing
Superconducting circuits
Quantum measurement

Background:

The Josephson bifurcation amplifier (JBA) is a key component for reading out superconducting qubits.
Understanding the quantum mechanical dynamics of the JBA readout process is crucial for improving qubit performance.
Purely quantum systems do not exhibit the bifurcation phenomenon observed in JBA measurements.

Purpose of the Study:

To quantum mechanically analyze the Josephson bifurcation amplifier (JBA) readout process for superconducting qubits.
To investigate the role of decoherence in enabling JBA bifurcation and hysteresis.
To understand the dynamics of qubit-probe entanglement during the measurement and projection phases.

Main Methods:

Calculated the dynamics of the density operator for a driven nonlinear oscillator coupled to a qubit.
Modeled the JBA readout process by incorporating decoherence into the quantum system.
Analyzed the evolution of the qubit-probe entangled state during the measurement.

Main Results:

Decoherence is essential to reproduce the bifurcation phenomenon with finite hysteresis in the JBA readout.
Observed the entanglement between the qubit and the JBA, which separates into distinct states upon JBA transition (projection).
The waiting time for macroscopic separation of JBA states, crucial for readout, is determined by the JBA's decoherence strength.

Conclusions:

Decoherence is a critical factor enabling the JBA's bifurcation and facilitating superconducting qubit readout.
The quantum projection of the qubit state onto the JBA states is observed during the measurement process.
Optimizing JBA decoherence is key to controlling readout fidelity and measurement time in quantum computing.