Source-linked AI summary
Detecting bit-flip errors in a logical qubit using stabilizer measurements
D. Ristè, S. Poletto, M. -Z. Huang, A. Bruno, V. Vesterinen, O. -P. Saira, L. DiCarlo
TL;DR
The work addresses how to implement stabilizer-based quantum error detection without leaving encoded information exposed during decoding. It combines superconducting-qubit control, flux-mediated gates, multiplexed readout, postselected initialization, and parallelized error-testing procedures, while bounding ancilla-measurement-induced dephasing below 1%. The study also evaluates incoherent errors across all eight three-qubit error/no-error combinations, with intrinsic decoherence remaining a practical constraint.
Problem
Quantum error-correction experiments require control and measurement procedures that preserve encoded information while detecting physical-qubit errors.
Method
The experiment uses superconducting-qubit microwave and flux control, multiplexed readout, postselected initialization, and parallelized stabilizer-gate sequences to implement and test QED.
Results
Ancilla-measurement-induced data-qubit dephasing was bounded below 1%, and incoherent testing covered all eight error/no-error combinations across three data qubits.
Takeaways & Limitations
The methods support stabilizer-based error-detection tests while identifying intrinsic decoherence and measurement timing as practical performance considerations.
Takeaways & Limitations
Intrinsic decoherence dominated data-qubit fidelity loss during ancilla measurements, so data-qubit measurements were advanced to occur simultaneously with ancilla measurements.
Abstract
from arXiv · showhide
Quantum data is susceptible to decoherence induced by the environment and to errors in the hardware processing it. A future fault-tolerant quantum computer will use quantum error correction (QEC) to actively protect against both. In the smallest QEC codes, the information in one logical qubit is encoded in a two-dimensional subspace of a larger Hilbert space of multiple physical qubits. For each code, a set of non-demolition multi-qubit measurements, termed stabilizers, can discretize and signal physical qubit errors without collapsing the encoded information. Experimental demonstrations of QEC to date, using nuclear magnetic resonance, trapped ions, photons, superconducting qubits, and NV centers in diamond, have circumvented stabilizers at the cost of decoding at the end of a QEC cycle. This decoding leaves the quantum information vulnerable to physical qubit errors until re-encoding, violating a basic requirement for fault tolerance. Using a five-qubit superconducting processor, we realize the two parity measurements comprising the stabilizers of the three-qubit repetition code protecting one logical qubit from physical bit-flip errors. We construct these stabilizers as parallelized indirect measurements using ancillary qubits, and evidence their non-demolition character by generating three-qubit entanglement from superposition states. We demonstrate stabilizer-based quantum error detection (QED) by subjecting a logical qubit to coherent and incoherent bit-flip errors on its constituent physical qubits. While increased physical qubit coherence times and shorter QED blocks are required to actively safeguard quantum information, this demonstration is a critical step toward larger codes based on multiple parity measurements.
METHODS
The experiment uses superconducting-qubit control, flux pulses, multiplexed readout, postselected initialization, and parallelized gate sequences to implement stabilizer-based QED. Ancilla-measurement-induced dephasing was bounded below 1%, while incoherent-error tests evaluated all three-qubit error combinations.
- Qubit control: Gaussian DRAG and Wah-Wah microwave pulses control the five qubits while reducing leakage and unwanted frequency-crowding effects.Dt and Ab use Wah-Wah pulses; sideband modulation of three carriers enables coherent control.
- Flux control: Flux pulses implement iSWAP and CPHASE gates, with durations of 12 ns, 40 ns, and 19 ns selected for different interactions.Manual convolution-kernel optimization minimizes distortion from limited flux-control bandwidth.
- Qubit readout: Five-qubit frequency-division multiplexed readout converts digitized, demodulated signals into one integrated voltage per measured qubit.Ancilla readouts are shorter than data-qubit readouts: 600 ns for At and 550 ns for Ab.
- Measurement effects: Ancilla-measurement-induced data-qubit dephasing was bounded below 1%, but intrinsic decoherence motivated simultaneous data and ancilla measurements.This timing choice isolates the ability of stabilizers to detect intentionally added errors.
- Initialization: Postselection initializes four qubits and two buses near their ground states, leaving 1–2% residual excitation in each quantum element.Only 50–60% of runs survive the initialization postselection.
- Gate sequence: Gates are parallelized except where frequency crowding, common-feedline ac Stark shifts, or bus-population ordering require serialization.All other iSWAPs, CPHASE gates, and ancilla measurements are simultaneous.
- Incoherent errors: Incoherent first-round testing applies all eight error/no-error combinations across the three data qubits and weights their fidelities by probability.The analysis calculates both three-qubit fidelity F3Q and logical fidelity FL for each combination.