Quantum supremacy?

A Wafer of the D-Wave Quantum Computer.
By Steve Jurvetson from Menlo Park, USA

We have been speaking about quantum computers for a few decades. These computers would work with qubits (quantum bits) instead of bits, and would perform certain operations much faster than ordinary computers.

It has been known since the thirties that quantum computers cannot solve problems that cannot be solved by ordinary computers. Those problems are called non-computable. What they would do, in principle, is solve certain problems (not all) much faster than ordinary computers. That higher speed, which in some cases should be enormous, is called quantum supremacy.

Let's give an example: we know that the decomposition of a composite number into its prime factors can be difficult. It’s trivial if the factors are small, but if the composite number is the result of multiplying two prime numbers of 100 digits each (for example) it is almost impossible to break it down, if we don’t know ay least one of the prime numbers.

The three laws of Robotics

Isaac Asimov

Isaac Asimov was a prolific science fiction and popular science writer who published in the 40s a series of stories about robots, later compiled in the I, Robot collection. In these stories he invented a word that has become a part of the technological vocabulary, as the name of a discipline: Robotics. He also formulated the three famous laws of Robotics, which in his opinion should be implemented in every robot to make secure our interactions with these machines that, when Asimov formulated the laws, were simple future forecasts.

The three laws of Robotics are the following:

First Law: A robot may not harm a human being, or through inaction allow a human being to come to harm.

Second Law: A robot must obey any order given by a human being, except those that conflict with the first law.

Third Law: A robot must protect its own existence as long as such protection does not conflict with the first two laws.

The limits of quantum computing

Alan Turing

In an interview in a major Spanish newspaper (La Vanguardia) published on July 27, 2019, David Pérez García, a researcher in quantum physics, says this: We are just in the beginning of some technologies that we still don’t know how far they will go. He is right, because the future is hardly predictable, but when it comes to quantum computing we tend to think that these computers, if they are viable, will let us solve problems quite different from those that can be addressed by the traditional computers to which we are used. In this context, however, mathematics can help us distinguish between what can be done, and what is completely impossible.

Although quantum computing is a fairly modern concept, its theoretical foundation was established by Alan Turing during the 1930s. Let us review a little of what he showed, for in this way we can correct a few optimistic ideas spread by the media, often driven by experts who approach the issue from very different points of view, compared to Turing.

Travelling to the past?

S.Augustin, by Louis Comfort Tiffany
Lightner Museum

In his Confessions (Book XI, chapter 14), St. Augustine wrote these words, still valid today:

What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.

In the current situation of our scientific and philosophical knowledge, we still don’t know what time is.

·         For classical philosophy and Newton’s science, time is a property of the universe. Therefore, time would be absolute.

·         For Kant, time is an a priori form of human sensibility (i.e. a kind of mental container to which our sensory experiences adapt).

·         For Einstein, time is relative to the state of repose or movement of each physical object. There is, therefore, no absolute time.

·         For the standard cosmological theory, there is the possibility to define an absolute cosmic time for every physical object, measuring the time distance since the Big Bang to the present.

·         For the A theory of time (using J. McTaggart’s terminology) the flow of time is part of reality. The past no longer exists. The future does not yet exist. There is only the present. If the A theory is correct, travel to the past is impossible, because you cannot travel to what does not exist.

·         For the B theory of time, the flow of time is an illusion. Past, present and future exist simultaneously, but for each of us the past is no longer directly accessible, and the future is not yet accessible. Einstein adopted the B philosophy of time. In a condolence letter written to someone who had lost a beloved person, he wrote the following:

The distinction between past, present and future is only a stubbornly persistent illusion.

A few clarifications on the quantum revolution

First of all, let as clarify a few concepts, for recent news published in the media tends to encourage confusion.

·         Qubit: the quantum unit of information. While classical information is expressed in bits, which may take only the values 0 or 1, qubits are formed by superposition (or linear combination) of two quantum states, |0> and |1> (i.e. the horizontal or vertical polarization of a photon) and its value is: α|0>+β|1>, where α and β are two complex numbers called probability amplitudes.

·         Quantum Cryptography: It encrypts information through a protocol that takes advantage of the quantum properties of matter. The procedures devised so far can be deciphered by opponents using quantum procedures, but it is known (or believed) that they are impossible to decipher by classical means. The first quantum cryptography protocol, BB84, was proposed in 1984 by Charles Bennett and Gilles Brassard of IBM.