IBM has launched a new addition to the Qiskit ecosystem called Qiskit Paulice, aimed at improving the reliability of quantum circuits by detecting errors during circuit execution and then filtering out results likely to have been affected by them. The tool relies on quantum checks known as “spacetime Pauli checks,” which are inserted directly into the circuit with a limited increase in hardware and execution requirements.
The add-on is part of efforts to develop error-handling methods for current quantum computers, at a time when fully fault-tolerant quantum computing still requires complex hardware and a large number of qubits. IBM says that currently developed error-handling techniques will help achieve large-scale, fault-tolerant quantum computing, in which errors are corrected during computation, with the goal of reaching this by 2029.
How Does Paulice Work?
Quantum error-handling methods generally fall into three groups: error suppression, error mitigation, and error correction. Error detection is not an independent category; rather, it is a fundamental component of error correction and some mitigation methods, because it determines whether a particular circuit execution has been affected by an error.
In conventional methods, data qubits are used to perform the computation, along with auxiliary qubits known as ancilla qubits that monitor the other qubits and capture indications of errors. However, introducing these qubits and their associated operations can increase circuit depth to the point where the additional operations cause more errors than they detect. Paulice was designed to address this dilemma by placing selected checks within the circuit itself.
Each Pauli check expresses a constraint that is expected to persist during circuit execution. When an error violates this constraint, it appears in a measurement “syndrome,” a sequence of bits indicating that an error has been detected. If the syndrome consists of zeros, no error was detected, while any value of 1 indicates an error according to the check used.
Checks That Account for Space and Time
Spacetime checks differ from conventional checks in that they define constraints not only across qubits located in the hardware layout, but also at specific time points within circuit execution. This makes it possible to monitor errors across extended regions of the computation while avoiding some of the cost associated with measuring high-weight operators, which can increase circuit depth, particularly on devices with limited connectivity between qubits.
Not all checks are equally useful, since each check adds operations that may produce additional noise. Therefore, the qiskit-paulice package uses a noise model and the device’s connectivity constraints to select valid, low-weight, and effective checks for detecting errors. According to IBM, this automated process helps increase detection capability while limiting qubit and circuit-depth costs.
What Does Practical Use Enable?
After running the circuit, syndrome information can be used in various ways. The simplest is to retain executions in which no error was detected and discard executions in which error indicators appeared; this process turns Paulice into a form of postselected error correction. Syndrome information can also be combined with other error-mitigation or error-correction methods.
The tool particularly supports Clifford circuits and circuits dominated by this type, such as circuits built from Hadamard, Phase, and CNOT gates. These circuits are characterized by mathematically understandable behavior and can be efficiently simulated on classical computers, making them suitable for developing and testing error-handling methods.
The add-on includes a tutorial for building a 12-qubit Clifford circuit with four layers of entangling gates, then identifying data and auxiliary qubits according to the device’s connectivity and creating a noise model based on platform data. In the example included in the material, the tool added seven checks to the circuit. The subsequent lesson explains how to run the checked circuit, sample it, examine the syndromes, select error-free samples, and compare the accuracy of the results before and after error detection.
Availability and Future Development
Qiskit Paulice documentation and tutorials are available, along with a GitHub repository for downloading the open-source add-on. IBM says the team plans to add support for non-Clifford systems, analyze noise channels after postselection, and improve the handling of idle noise during check selection. The material also refers to the use of spacetime-check concepts in an experiment by IBM and the University of Chicago involving sampling from large-scale random graph states.