What is quantum error mitigation?
Quantum error mitigation is a process used to reduce the impact of errors in quantum computing. The process does not eliminate these errors, but it reduces their effect.
Quantum computing can provide massive performance gains in specific settings. But it's not yet a perfect technology, and as such, it still runs into several different errors. One common issue is the abundance of noise. Noise refers to anything that causes a quantum system to behave incorrectly or unpredictably.
The goal of quantum error mitigation is to improve the accuracy of noisy quantum systems, enabling them to produce more reliable and accurate outputs. It does this by practicing redundancy. Multiple versions of a quantum circuit are run, and their outputs are analyzed to estimate a result without errors.
Why is quantum error mitigation important?
Quantum computing uses the unique behaviors of quantum physics to solve problems too complex for traditional classical computing systems.
Classical computing uses binary bits that exist as either a 1 or 0. These act as a stand-in for any singular piece of information and are typically conveyed physically by pulses of electrical voltage or light in fiber optic cables.
Quantum computing, instead, uses quantum bits (qubits). These use the same 1 and 0 states but also add another superposition state, which represents 0, 1 or any position in between. This enables quantum computers to process considerably large amounts of data simultaneously.
Qubits, however, are extremely sensitive to their environment. For example, even something like a small vibration can cause an error. Each occurring error introduces more noise to a quantum computer's computation, skewing its output.
Quantum error mitigation helps reduce this noise without the need for additional processes, such as quantum error correction. Ultimately, mitigation helps produce more accurate information from imperfect systems.
Quantum error mitigation might have its limitations, however, especially in larger quantum computing systems.
What causes quantum error?
There are various errors that quantum computers can experience, including the following:
- Qubit decoherence. Qubits are extremely sensitive to their environment. A small vibration, excess heat or radiation can cause a qubit to lose its quantum state.
- Gate error. An error could also be caused when a quantum operation is misapplied to a qubit.
- Measurement errors. Even if a qubit is in the right state, it could still be misread, negatively affecting the result.
Any of these errors can introduce noise to a quantum computer's computation, resulting in an unreliable or inaccurate output.
What are the different types of quantum errors?
The different causes of quantum errors can be categorized into the following types:
- Phase flip error. This occurs when a qubit loses its phase information and changes its state. This is usually due to the absorption or loss of energy.
- Bit flip error. This error causes a qubit to flip between 0 and 1. The qubit flips between these states due to environmental noise or other external factors.
- Gate operation error. This happens when an operation fails to execute correctly. This is due to hardware imperfections, poor calibrations or additional noise.
- Amplitude damping and leakage error. This occurs when a qubit loses energy or leaks out of a system entirely. This is due to interactions with the environment or hardware imperfections.
Different methods of quantum error mitigation
Multiple error mitigation techniques can be used to reduce the noise level in quantum computers. These mitigation methods include the following:
- Zero-noise extrapolation. This process involves running a quantum circuit at different noise levels. The system then works backward to estimate the outcome without noise.
- Probabilistic error cancellation. This examines detailed knowledge of a system's noise and then inverts it to cancel out noise. It requires many measurements for accurate results.
- Quasi-probability method. This method was developed to cancel out the effects of gate noise by learning how that noise behaves and creating an inverse effect of it. This, in effect, cancels out the noise and creates clean data.
- Virtual distillation method. This technique prepares multiple copies of a noisy quantum state, measures entanglements between them and uses classical post-processing techniques to estimate how a clean version of the data would look.
- Subspace expansion method. This improves the results of quantum algorithms by projecting noisy results into a larger space that includes ideal behaviors to filter out the effects of errors.
How do quantum error suppression, error mitigation and error correction differ?
Quantum error mitigation is also similar in concept to quantum error suppression and correction. However, each has a distinct difference.
Quantum error suppression
Quantum error suppression attempts to prevent errors from happening in the first place. This technique is typically the most basic level of handling errors. Quantum hardware is designed to be more resistant to noise, meaning most errors this method addresses are the closest to the hardware level.
Quantum error mitigation
Quantum error mitigation is a technique that reduces the instances of errors after a quantum circuit has run. This practice runs multiple versions of a quantum circuit for redundancy to analyze their outputs. This enables the system to estimate a result without errors.
Quantum error correction
Quantum error correction is a more advanced technique that attempts to detect and fix errors as they happen. This strategy focuses on redundancy to reduce error rates. Single qubit values are assigned to many qubits so that the system can still recognize and return accurate information if even a few qubits encounter an error.
Quantum error mitigation is a practice that makes quantum computing work somewhat consistently in the short term. Learn more about the near- and long-term outlooks of quantum computing.
