Driven by diametrically opposite goals, Google and the U.S. National Security Agency (NSA) are each striving to build full-scale multipurpose quantum computers that can either defend against or attack the other's encryption systems.

Google is being driven by need to prevent the NSA from breaking into its system to access confidential personal data of its millions of users. On the other hand, the NSA is bent on cracking the tough encryption systems Google and other tech firms use to shield their information from them. Quantum computers will attain this aim for both Google and the NSA.

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Google recently said it's gotten closer to building a universal quantum computer. A team of Google researchers in California and Spain has built an experimental prototype of a quantum computer that can solve a wide range of problems and has the potential to be scaled up to larger systems.

The Google prototype combines the two main approaches to quantum computing. One approach constructs the computer's digital circuits using quantum bits or qubits in specific arrangements geared to solve a specific problem. The other approach is called adiabatic quantum computing (AQC).

The first approach is similar to a conventional microprocessor where classical bits are the keys to computation. Most of quantum computing theory is based on this approach that also includes methods for correcting errors. Practical implementation of this approach has been possible only with a handful of qubits.

In the AQC approach, the quantum computer encodes a given problem in the states of a group of qubits. The computer evolves and adjusts the interactions between the qubit group to "shape" their collective quantum state and reach a solution.

Only AQC has provided the first commercial quantum computers. The most recent is one made by the Canadian firm, D-Wave that sells for $15 million. Google owns a D-Wave device.

Google, however, wants to find some way to implement error correction to scale-up AQC. The Google team thinks the first step to achieving that is to combine the AQC method with the digital approach's error-correction capabilities. This new approach should enable a computer with quantum error correction.

"With error correction, our approach becomes a general-purpose algorithm that is, in principle, scalable to an arbitrarily large quantum computer," said Alireza Shabani, a Google team member.

Google believes quantum computers with more than 40 qubits could become a reality in just a few years.

 "At that point, it will become possible to simulate quantum dynamics that is inaccessible on classical hardware, which will mark the advent of 'quantum supremacy'," said Shabani.

It's this quantum supremacy NSA is most concerned about. Large-scale quantum computers are theoretically able to solve certain problems much far quicker than any classical computers like integer factorization using Shor's algorithm.

The NSA's concern is integer factorization, which is the basis of the security of its key cryptographic systems. Integer factorization is impractical using ordinary computers for large integers that are the product of few prime numbers.

A quantum computer, however, can efficiently solve this problem by using Shor's algorithm to find its factors. This ability will allow a quantum computer such as the NSA's to decrypt or read many of the cryptographic systems in use today by leading tech firms.

The reverse, however, will also be true and the race devolves on how to make encryption resistant to a quantum computer attack and no one knows how to do this right now, however. Hence, the headache facing the NSA.

Quantum-resistant encryption algorithms proposed by researchers at Microsoft and other firms haven't been formally proven to be safe against quantum computer attack.

The NSA recently said that advances in quantum computing makes it worry about future-proofing systems being built today to protect vital national infrastructure that will be in service for decades.

TagsGoogle, National Security Agency, quantum computers, adiabatic quantum computing, d-wave