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Conventional wisdom holds that two wrongs don’t make a right. However counter intuitive this may seem, many wrongs could be the key to improving the performance of noisy quantum computers today.
Computers that rely on the power of quantum mechanical phenomena to perform calculations are also extremely susceptible to “noise” from their environment – which leads to errors in the computation. Even at the extreme cold temperatures of a dilution refrigerator where the quantum processors operate, our physical computing elements, superconducting qubits, have coherence times on the order of a few hundred microseconds at best, which sets the timescales over which quantum information is lost. While a major challenge to advancing quantum computers today involves increasing these qubit coherence times, the end goal is to build a fully fault tolerant quantum computer capable of detecting and correcting errors. However, these architectures are likely several years away.
A near-term solution
Single qubit trajectories measured at different noise levels (red, green) are used to estimate the error-mitigated trajectory (blue). Figure modified from A. Kandala et al Nature, 2019
In a new research paper, published in
“The error mitigation technique dubbed ‘zero-noise extrapolation’ is readily accessible for existing quantum computers since it doesn’t require any additional hardware modifications.”
Computations on noisy quantum hardware are limited by the competition between decoherence and circuit depth, a measure of the number of sequential operations performed on the processor. Increasing circuit depth can help create more complex quantum states, and in the context of chemistry simulation, this may allow for a better representation of the energy states of the molecules considered. However, increasing circuit depth on a noisy quantum computer typically implies increased errors from decoherence. However, with the technique developed in this work, our ability to mitigate the effect of decoherence enables us to access more complex and accurate computations that benefit from increased circuit depth.
Our method is fairly broad in its applicability and can be used to improve any quantum computation that relies on expectation values. For example, in this work, we use it to demonstrate improvements in the accuracy of quantum simulations initially considered in our 2017
The path ahead
While our technique enabled computational accuracies that were otherwise inaccessible to the hardware, it is important to note that the improvements are not indefinite and are ultimately limited by the coherence properties of the processor. As we march towards systems with increasing
Error mitigation extends the computational reach of a noisy quantum processor
Abhinav Kandala, Kristan Temme, Antonio D. Corcoles, Antonio Mezzacapo, Jerry M. Chow, Jay M. Gambetta