A quantum computer can do a calculation in three minutes instead of… 10,000 years for a simple computer. This quantum operation, successfully completed by Google at the end of September, has sent shivers down the spine of the computing world in search of its Holy Grail: the universal quantum computer.
A quantum computer is the equivalent of conventional computers, but one that performs its calculations using the laws of quantum physics and, at the base, the so-called superposition of quantum states. While a classical computer manipulates bits of information, which are either 0s or 1s, a quantum computer uses qubits. These are generalizations of the classical bits, which are a kind of simultaneous superposition of these two states, as can be, for example, a spin state for a photon or an electron.
In some cases, a quantum computer can make calculations much faster than a classical computer. However, this would require a very large number of qubits. However, this is not self-evident. The larger the number of qubits, the more unstable the superposition of quantum states is and the more unstable the superposition of quantum states can disappear before the required calculation is completed.
Counter-intuitive mechanics
Research in quantum computing, which emerged in the 1980s, is based on one of the principles of quantum physics called superposition. According to this mechanics, an object can have two states at the same time: a coin can have both heads and tails, whereas in the “classical” world, it can only be one of them.
Every object is quantum, but these quantum properties are lost very quickly, and the bigger the object, the faster the loss.
On a microscopic scale, however, it is possible to extend the quantum state, with simple and isolated atoms.
Qubits, the basic building block of quantum computing, can thus be fabricated. But their manipulation is delicate because they are difficult to stabilize and manufacturers have difficulty exceeding 53 qubits.
Nevertheless, they are already capable of spectacular performance, as shown by Google’s processor, which calculated in 3 minutes what a conventional supercomputer would have taken thousands of years.
Threats to cryptography
The most promising quantum algorithm is Shor’s algorithm, which is able to factorize as fast as multiplication, whereas in a classical calculation, there is a difference in resolution time between the two operations.
For the moment, quantum can only factorize 7 or 8 digit numbers and the classical computer is much more powerful.
But the day when a universal quantum computer succeeds in executing Shor’s algorithm on a large scale, with 100-digit numbers, we will then speak of “quantum supremacy”.
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