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Silicon Metal-oxide-semiconductor Quantum Dots for Single-electron Pumping

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Superconducting qubit with Purcell protection and tunable coupling.

J M Gambetta¹, A A Houck, Alexandre Blais

¹Department of Applied Mathematics, Institute for Quantum Computing, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.

Physical Review Letters

|March 17, 2011

Summary

We developed a tunable superconducting qubit for circuit quantum electrodynamics. This qubit can be tuned to a decoherence-free subspace, suppressing Purcell effect-induced spontaneous emission while enabling qubit measurement.

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Area of Science:

Quantum computing
Superconducting circuits
Quantum information science

Background:

Circuit Quantum Electrodynamics (cQED) architecture is a leading platform for superconducting qubits.
The Purcell effect, caused by spontaneous emission, is a major source of decoherence in cQED systems.
Controlling qubit-resonator coupling is crucial for both qubit operation and mitigating decoherence.

Purpose of the Study:

To introduce a superconducting qubit with tunable coupling to its resonator.
To demonstrate the creation of a decoherence-free subspace by tuning the coupling strength to zero.
To show that qubit measurement is still possible within this decoherence-free subspace.

Main Methods:

Implementation of a superconducting qubit within the cQED architecture.
Precise tuning of the qubit-resonator coupling strength, denoted as 'g'.
Characterization of qubit dynamics and measurement protocols at varying coupling strengths, particularly at g = 0.

Main Results:

The qubit-resonator coupling strength 'g' is tunable from zero to values comparable to other superconducting qubits.
At g = 0, the qubit resides in a decoherence-free subspace, effectively suppressing spontaneous emission due to the Purcell effect.
Qubit state readout is demonstrated within the decoherence-free subspace using dispersive shifts on the resonator's frequency and cycling-type measurements.

Conclusions:

A tunable superconducting qubit architecture is presented that allows access to a decoherence-free subspace.
This approach mitigates Purcell-limited decay while preserving the ability to measure the qubit state.
The tunable coupling offers a new pathway for improving coherence times and control in superconducting quantum processors.