A Dialogue on Quantum Computing
By: Subhash Kak
Tags: Science, Quantum Computing, AI, Future, Cryptography

We are in the middle of a quantum computing race. China’s investment in this technology is believed to be worth $10 billion, and European Union has launched a €1 billion ($1.1 billion) quantum master plan.

In the United States, President Donald Trump signed the National Quantum Initiative Act in December 2018 to provide $1.2 billion to promote quantum information science over a five-year period.

But within the academic world there is an ongoing debate regarding the future of this technology. After Galileo, let’s call the proponents of the two main opposing positions Simplicio the physicist and Salviati the engineer, with Sagredo as the neutral layperson who has interest in Mullah Nasruddin stories.

Simplicio: Quantum computing will be one of the key technologies that decide which nation gets to be the leading technological power of the twenty-first century. Quantum computers operate in a mode where several pieces of information are simultaneously processed, making them faster than current-day machines. When built, quantum machines will provide an edge in code-breaking and the solution of certain optimization problems.

Juggling pins on top of an inverted pyramid

Salviati:While the mathematics at the basis of quantum algorithms is fine, there are practical engineering reasons why such computers will never be built.

Unlike a classical computer which uses 0s and Is (these could be charges or voltages on different points in a circuit), a quantum computer processes states that may be seen as waves together with their phases. Like waves, quantum bits (qubits) exist in superposition.

The quantum waves interact with the environment and the computer can hopelessly decohere in a fraction of a second and so the computation must be completed in very quick time, placing extreme constraints on the system. Without correction of random errors that are inevitable in any physical system the results will be worthless.

Information is physical [1] and the qubits themselves are evolving. One could say that the loading of the information on the quantum computer is like balancing an inverted pyramid, and this balancing is to be done with objects that are themselves flying about in all different directions. The juggler must mount this pyramid and continue juggling the pins while ensuring he doesn’t fall down.

Furthermore, a computing machine must correct all small random errors for the computation to be correct. But not all qubit errors can be corrected [2]. Thus errors within the encoder, introduced before the encoding entanglements are introduced, cannot be corrected.

Sagredo: Believe in the future of technology. Remember Archimedes said: Give me a lever and a place to stand, and I shall move the Earth.

Simplicio: NASA, Google, IBM, and other organizations are testing quantum computing systems with around 20 qubits. I know that is far behind the milepost of 50 qubits that was set up by DARPA for 2012. But one must be patient.

Salviati:I read this assessment by Michel Dyakonov in IEEE Spectrum [3]: “There is a tremendous gap between the rudimentary but very hard experiments that have been carried out with a few qubits and the extremely developed quantum-computing theory, which relies on manipulating thousands to millions of qubits to calculate anything useful. That gap is not likely to be closed anytime soon.”

He added: “Such a computer would have to be able to manipulate — on a microscopic level and with enormous precision — a physical system characterized by an unimaginably huge set of parameters, each of which can take on a continuous range of values. Could we ever learn to control the more than 10³⁰⁰ continuously variable parameters defining the quantum state of such a system? My answer is simple. No, never.”

Sagredo: Once Mullah Nasruddin promised his king that he can teach his donkey to read. He said all that he needed was 1,000 dinars and ten years. The king agreed but insisted that if the donkey failed the test, Nasrullah would be put to death. When he reached home with the money, his wife was furious. Relax, Nasruddin told his wife, how does anyone know that a donkey cannot be taught to read. Furthermore, even if a donkey can’t be taught remember that ten years is a long time. During that period, the king may be dead, the donkey may be dead, or he may be dead. If they are not dead and if they are unable to teach the donkey, they can flee the kingdom in the tenth year.

Simplicio: Look for the new unexpected research findings that will be obtained as a side result of the quantum computing initiative.

Quantum cryptography

Simplicio: Quantum cryptography is a breakthrough technology. Two users can exchange keys without anyone able to break the system. Surely, this is a fundamental advance of the greatest importance both for civilian communications and military applications [4]. And isn’t it great that such key exchange can be done using wireless between satellites and with naval ships?

Salviati: But quantum cryptography is used only for key distribution (QKD). First, the two parties must be authenticated, which can only be done classically because the world of conscious agents is classical [5]. Since a chain is only as strong as its weakest link, there is no point doing QKD, for the authentication process itself can be used to exchange keys.

Simplicio: We cannot yield the leadership in this technology to competitors who don’t have commitment to democracy and open society.

Sagredo: One day Mullah Nasruddin was in a mischievous mood. He assembled a crowd, and told them a made-up story about gold in the abandoned house beyond the village. When everybody ran to get their hands on the gold, Nasruddin started running with them. When asked by his friends why he was doing so, he said “So many people believe it, that I think it may be true!”

References

[1] R. Landuaer, The physical nature of information. Physics Letters A 217: 188–193, 1996.

[2] S. Kak, General qubit errors cannot be corrected. Information Sciences 152:195–202, 2003.

[3] M. Dyakonov, The case against quantum computing. IEEE Spectrum. 15 November 2018. https://spectrum.ieee.org/computing/hardware/the-case-against-quantum-computing

[4] D. Cardinal, Quantum cryptography demystified: How it works in plain language. Extreme Tech. March 11, 2019.

[5] S. Kak, Epistemic view of quantum communication. In Quantum Foundations, Probability and Information, A. Khrennikov and B. Toni (editors). Springer-Verlag International, pp. 119–128, 2018.