What is really going on, and how close are we to building a quantum computer and how can we prepare for the post-quantum era – we asked Julius Ruseckas, a research professor at the Faculty of Physics at VU University.

– Some of the major tech companies (such as Google), many universities, have been working on or putting money into quantum computers for various ends. How close are we to building a working quantum computer? What problems or tasks they can solve now?

– Unfortunately, we are still far away from a proper quantum computer. The current state of the art is the quantum computer that has several tens (around 50) of noisy qubits. (See a good review HERE).

Although theoretically the influence of noise can be made as small as needed using quantum error correction, currently quantum error correction is not achieved in experimental setups.

Presence of noise and the absence of error correction means that the current quantum computer can operate only for a limited short time and perform a limited number of operations.

One may compare the state of quantum computers now to the state of the ordinary, classical computers before World War II: now, we even do not know from which components it is best to build quantum computers.

Most implemented quantum computers currently use superconducting qubits. Other possible schemes that are actively researched are ion traps and quantum dots (for example, so-called NV centers in diamond). Current quantum computers are purely research devices; they do not have any practical applications.

– When do you think we will have a working quantum computer in production? When do you think we will have 100% fault-tolerant quantum computer?

– It is very hard to predict the future. However, trying to extrapolate the current rate of progress (which is, rather, slow), one can conclude that working quantum computers in production will not happen in the near future. As John Preskill, one of the leading scientist in quantum information writes: ‘the era of fault-tolerant quantum computing may still be rather distant. No one really knows how long it will take to get there.’

One can see estimations that quantum computers are 10, 20 or even more years away.

– What about mass production – do you think that could ever happen?

– The majority of physicists think that quantum computers are possible and will be eventually created and produced. However, there is a minority saying that scalable quantum computers are, in principle, impossible.

Currently, we do not know enough about quantum control of large systems to refute this minority opinion definitely.

– What are the main challenges of developing algorithms, software programs, etc. for quantum computers which don’t exist yet?

– Quantum computers can, in principle, do everything that classical computers do. However, there is no sense to use a ‘classical’ algorithm for an expensive quantum computer.

The algorithm for quantum computer should take benefit from quantum properties, most important of which is entanglement. However, there is no general method of how to create such algorithms. Currently, software programs for quantum computers are created by combining known quantum algorithms.

– What kind of skills do you need to program a quantum computer?

– Several organizations (for example, Microsoft) are already creating programming languages for programming of quantum computers. Thus one should expect that the skills needed to program a quantum computer are similar to those required for ‘ordinary’ programming.

However, one peculiarity of quantum computers is that creation of new algorithms that harness their power is challenging. In fact, there is no known general method how to utilize the quantum properties for a given task. There are only several algorithms (for example, Shor’s algorithm for integer factorization) for specific tasks. Creation of new algorithms thus requires deep familiarity with quantum mechanics and mathematics.

– What company or institute is currently winning the race?

– Since the proper quantum computer does not exist yet and will not be created in the near future, one cannot say that some company is winning. Large companies like IBM, Intel, Google, and Microsoft are investing in research of quantum computers.

A significant role is played by the universities such as MIT, ETH Zürich, University of Sydney,  University of Science and Technology of China and many others.

– Companies like IBM warn that quantum computers will instantly break encryption. What do you think will happen?

– Quantum computers using Shor’s algorithm can in principle quickly factorize large integer numbers. Since many encryption schemes currently use the difficulty of factorization, quantum computers can break such encryption.

However, the number of qubits required grows fast with the size of the number that is factored.

It is estimated that 2048-bit RSA keys could be broken on a quantum computer comprising 4000 qubits and 100 million gates. A practical breaking of encryption requires a quantum computer with a large number of qubits. Such a computer will not be created in the near future.

There is time to create and start to use new encryption schemes that are resistant to attacks by quantum computers.

– What would post-quantum encryption be? What are other possible quantum-resistant schemes to secure data?

– Many cryptographers are currently designing new algorithms for classical computers to prepare for a time when quantum computing becomes a threat.

There do exist some public-key algorithms that do not appear to be vulnerable to attack by quantum computers, although none of them is as well-studied as the ones in common use today. For example, many of the public-key systems based upon the lattice reduction problem, such as schemes whose basic elements are polynomials, appear to resist quantum attacks. Which of those schemes will have a significant adoption is hard to guess.

Provided one uses sufficiently large key sizes, the symmetric key cryptographic systems like AES are already resistant to attack by a quantum computer.

– There are many tech companies using encryption (either for their security or for their products). How would you recommend to prepare their security for post-quantum computer era?

– Since large quantum computers are far in the future, for the short-term, it is sufficient to increase the number of bits in the keys, for longer-term one should investigate encryption that is unbreakable by quantum computers: post-quantum cryptography schemes or start to use symmetric key cryptography.

– Which tech fields would be affected the most?

– The most practical benefits of quantum computers I think will be in prediction of properties of large molecules. This task currently is very difficult for ‘classical’ computers. Thus quantum computers should have a large impact in drug creation an search for new materials.

Another task that can be efficiently performed by quantum computers and is required in many areas of technology is search and optimization.

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