The biggest obstacle to quantum computing is… noise?
We spoke to Jessica Pointing, a world-renowned quantum computing expert and Harvard graduate currently pursuing a Ph.D. in Quantum Computing from Stanford University. Among the interesting insight she shared with us, she explained how various types of noise can affect quantum computations and what can be done to eliminate or avoid the resulting errors.
In the quantum context, ‘noise’ is not an undesirable sound. It is interference that can be caused by internal and external factors. External factors usually include: magnetic fields, variation in temperature, impurities in the qubit material, stray atoms or vibrations. The internal noise could be determined by the interactions between the qubits themselves, because of their entanglement (specific relation between pairs of qubits) and the fact they need to interact in order to perform computations. For more information about entanglement, check out Jessica’s explanation at the 2019 GOTO Conference.
In the meantime, scientists are hard at work discovering ways to detect and cancel noise. As the technology improves, fault tolerance or a specific threshold for noise where quantum computers are considered reliable and sufficiently accurate will increase, making quantum computers reliable enough to work for extended periods of time at full capacity.
Noise and “soft” workarounds
The longer the computer program runs, the more time there is for errors. Despite the error-prone nature of quantum computers, Google ran an experiment on a quantum computer demonstrating that it was able to perform a calculation faster than even the best supercomputers. Google claimed the calculation would take 10,000 years on a supercomputer, but only 200 seconds on their quantum computer. Nevertheless, researchers are exploring applications of this experiment.
The ideal method to reach an error-free computation would be error correction. It has been proven theoretically, and the topic is being studied in various research centers.
“Error correction requires a redundancy of qubits (for every ‘functional’ qubit you would need several others to perform error corrections). Given the fact that the maximum number of qubits available in a quantum computer is currently 53, there is not much room left for both computations and error-correction. A proper and fully reliable quantum computer would require a very large number of qubits”
The technique currently employed in working with NISQs is error mitigation, a way to deal with quantum errors through post-processing with different types of algorithms. According to other scientists, a possible solution could also lie in the software used: “writing quantum software in such a way that errors do as little harm as possible… can work even if we have imperfect knowledge of the nature of the errors, as will certainly be the case in reality” .
Error-free quantum computing, the next step
There is still a long way to go before a fully fault-tolerant quantum computer becomes reality. Up until now, hardware companies have been focusing on building the hardware and creating functional qubits. Now there is an opportunity to explore applying error correction to the quantum devices that have been built.
It is possible that other technologies for developing qubits will be discovered and these will be less prone to errors due to noise. Currently the most common technologies used to build qubits are superconducting circuits, trapped ions, photons, and diamonds.
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Suguru Endo, Simon C. Benjamin, and Ying Li, “Practical Quantum Error Mitigation for Near-Future Applications,” Physical Review X 8, no. 3 (July 26, 2018): 031027, https://doi.org/10.1103/PhysRevX.8.031027.