BUT DO WE HAVE AN IDEA OF WHAT QUANTUM COMPUTERS ARE? WHAT ABOUT SCHROEDINGER'S CAT?
The Nobel Prize awarded to Aspect, Clauser and Zeilinger gives us the opportunity to try to explore a scientific topic whose foundations were laid almost a century ago.
QUANTUM MECHANICS
The quantum computer is based on laws of quantum mechanics, as opposed to the classical computer we know which is based on the principles of traditional (Newtonian / Einsteinian) physics.
The laws of quantum mechanics apply to elementary particles and in general to very small objects, while the laws of classical physics apply to relatively large objects, which are not affected by the quantum effects that instead characterize the behavior of particles.
In particular, there are three quantum properties that affect quantum computation:
- the overlap,
- correlation (entanglement),
- interference.
All three of these properties of a quantum system are children of the particle / wave duality that characterizes them.
La overlap
It consists in the fact that particles can have two states at the same time (0 and 1, spin up and spin down, for example) until I observe them: the famous Schroedinger's cat which is both dead and alive, until we open the box in which it is closed, decoherentising the state of superposition and making the cat either alive or dead.
This refers to the wave function of the particle which attributes different states to the particle itself with different degrees of probability.
La correlation
It establishes a close mutual dependence between the state of two particles so that, for example, if one has a clockwise spin the other will have an anti-clockwise one; if I change the spin of one of the two particles, the other will immediately reverse its own, regardless of the distance between the two.
This non-locality that the properties of particles are transmitted immediately beyond the speed of light is what made it define ad Einstein "spectral action at a distance", and, to tell the truth, despite having been empirically proven by infinite experiments, it still remains quite mysterious.
L'interference
It asserts that a particle can not only be in two different places, but that its wave can create a phenomenon whereby it also interferes with itself or with other waves by creating peaks where the peaks of the wave add up and of the depressions where the troughs meet the troughs (just like a wave of water).
The peaks represent the points where the particle is most likely to be found, and the troughs where it is least likely.
Quantum computers take advantage of all these properties of particles.
BIT and QUBIT
If the elementary unit of the classical computer is the bit which can be represented by any physical substrate that can have two states: on off, 1 and 0, high and low and so on, the elementary unit of the quantum computer is the quibit which has two states at the same time: 1 + 0.
So a computer with 10 qubits can represent 2 to 10 states (1024), while with 10 bits a classical computer can represent 10 to the second states (100).
This gap grows exponentially with the number of qubits I have available.
To get an idea of the computing power that can be achieved with quantum computers, just consider that with 70 qubits it is possible to represent all digital data on all devices on the planet existing today, reaching quantum supremacy, i.e. the possibility of performing calculations that cannot be performed with classical computers.
With more than 300 appropriately correlated qubits there would be more states than there are particles in the known universe, (which opens the door to not insignificant ontological issues).
At the present state of the art, computers up to about 50 qubits have been built.
Evidently the implementation of quantum machines presents enormous difficulties both on the hardware and on the software side.
The Hardware in fact it must be perfectly separated from the users and the environment to avoid the interference of the system with the environment and the consequent loss of its quantum properties (decoherence). Furthermore, far more physical qubits than logical qubits are now required for system error correction: up to 1000 physical qubits per error-free logical qubit (systems redundancy).
On the side of the Software there is the issue of the implementation of algorithms that exploit the characteristics of quantum systems, which are not, it is worth remembering again, a simple evolution of classical computers.
QUANTUM COMPUTER APPLICATIONS
The first to talk about quantum computers was Richard Feynman, but the interest in their realization only really ignited after the discovery by Peter Shor of the algorithm that bears his name and which demonstrates how with a quantum computer of sufficient power it is possible to solve the problem of the factorization of the primes of an integer (given an integer which are the prime numbers that multiplied together give that number).
This problem is substantially intractable for large numbers by a classical computer, while with a quantum computer it is relatively easy to solve.
It will take many more years (probably over 10), and the ability to implement a HW of millions of related qubits, before quantum computers will be able to find the primes of a 2048-bit number, but obviously the theoretical possibility of doing so. it has aroused the interest of both those who have to protect data and those who want to break the code, thus contributing to the development of research.
miniaturization of circuits and their components is now so high (Moore's Law) that will soon begin to develop quantum behaviors (in this case unwanted) that will raise the number of errors of classical computers themselves.
What other applications of quantum computers are currently under development besides pure computing?
Surely one of the most promising applications is the simulation of systems whose characteristics depend on quantum mechanics, so it is study of innovative materials, bio-medical, simulation of micro components, the development of more powerful quantum computers, the cryptographic systems and, finally, the optimization of the logistics , search in databases.
We thank for the contribution Paolo Riccardo Felicioli