03 Oct

You thought you knew everything about computer science? But have you already heard about quantum information science? This field that uses quantum computing, working thanks to quantum mechanics properties. Before telling you more about this, we should see together some preliminary definitions.

What is quantum mechanics and what is different from classic mechanics ?

Quantum mechanics is focused on phenomena arising at atomic scale or below. The idea of a different mechanics appeared during the XXth century when researchers noticed that it was not possible to explain several problems such as photo-electrical effect (to mention only one of them) only with classic mechanics. The most known of quantum concepts is the wave-particle duality, this law shows that an object can have wave properties but also particle properties. Light, for example can be represented by a wave, this is why it has a wavelength but it is also composed of photons which are particles. Depending on the way of studying it, we have to consider light as a wave or as a particle.

There are other concepts which are unique in quantum physics such as quantum entanglement or quantum superposition. These two concepts are particularly used in quantum information, the rest of the article aims to explain them to you.

Quantum entanglement

This phenomenon is observed in quantum mechanics. It studies the quantum state of 2 objects in a global manner. This means that it is not possible to separate one object from the other even if they are not at the same position. When 2 objects are entangled, the observed physics properties on these objects are correlated, this is not the case when we are studying these object separately. To conclude, if two objects Obj1 and Obj2 are entangled, their properties are not independant anymore, we then have to consider them as a system such as {Obj1+Obj2}. The idea of quantum entanglement comes from an experiment realized by three researchers : Einstein-Podolsky-Rosen. This experiment is known as the EPR paradox. What does this experiment say? The EPR paradox was created to prove that the Copenhague interpretation was wrong. This interpretation says that a quantum state does not exist before we can measure it because we do not have any proof of its existence. But, if two particles have a conservative relationship between their properties, the usual example is : the sum of their spins has to be null (there is then a quantum entanglement between the system state of these particles), if we know the state of the first particle after having measured it, it gives us automatically the information about the other particle without having measured its state. This phenomenon is independant from the distance between the two particles. Nowadays, the existence of entangled states has been observed in laboratories which proved that the EPR paradox was correct.


The quantum superposition principle
In quantum mechanics, the superposition principle allows one quantum state to have several values for a single parameter. This is possible because a quantum state is represented by a vector in a vectorial space called the Hilbert space. This vector have a decomposition which is a linear combination of vectors with a data base. This is the case for any vector. In quantum physics, a parameter corresponds to a data base of the Hilbert space. We have to consider the position of a particle. Its position state is then represented by an infinite sum of vectors with each vector showing a precise position in the space. An interesting property is that the squared length of each of these vectors represents the presence probability of the particle to a given position. If the quantum superposition is right at an atomic scale, it is not possible at a macroscopic scale, because it becomes a non-sense like the Schrödinger cat problem. But it is possible that these superposed states take place in different parallel universes allowing then the particle to take different positions in different universes.


After these several explanations, we can now go further : beginning with quantum computing.

What is a quantum computing?

A quantum computing, is a computer that uses quantum properties when reading algorithms. Small quantum computing already exist since the 90’s and since these years, research is constantly improving. By the way, Google and the NSA are working together on a project aiming to build a big quantum computing. The idea is that if the size of components reach an atomic scale, quantum effects will alter electronic components functions. But, these alterations could, if we achieve to build a big computing, allow us to simulate the universe itself. Unfortunately, this is not for now. Quantum computing will have revolutionary utilities in research because they will allow combinatory calculations (which is not possible with usual computers). They will also create a breakthrough in cryptanalysis allowing to crack codes much faster. How? By interpreting quantum algorithms. How can they read quantum algorithms with bits composed of 0 and 1? They cannot and this is the point, researchers have thus created another unit called “qubit”.

The qubit

bit-pbit-qubitThe qubit : quantum+bit is the smallest quantum information storing unit. It corresponds to a superposition of two states : ket 0 and ket 1. It is a quantum superposition of these two states. In other words, a classic bit is either in the state ket 0 or in ket 1 whereas a qubit is in a superposition of these two states composed of a linear combination of these two states with complex coefficients. However unlike the classic bit, it is impossible to duplicate a qubit. This is not possible because we cannot measure a qubit state without freezing it. Moreover, measuring a qubit does not give any information about its coefficients. Indeed, you know now that a qubit is a linear combination such as a . ket 0 + b . ket 1 with a and b complexes. Yes, but the value observed will be either ket 0 or ket 1. This means that in the first case, a = 0 and in the other case, b = 0… but this is the value after measuring … nothing is said about the values before measuring. However it is possible to teleport a qubit on another one by reinitializing the first one. This allows to keep the content of a qubit but does not give information about its coefficients. Quantum teleportation is a fascinating subjects which will be the subject of another article.

How far is quantum computing nowadays?

In 2001, the CEA of Saclay built a quantum chip. This chip was made with silicium using 3 nano-junctions : two of them were qubits containing spin states in quantum boxes, the third one was used to measure the quantum states.

In 2009, Yale university in the USA used two groups of alluminium atoms acting in two different manners (excited or not) to mimic qubits.

What are the problems to solve?

The main problem in building quantum computing is the fact that when you come to a macroscopic scale, you undergo a quantum decoherence effect, meaning that you are losing quantum properties to join classic mechanics.

What can we expect for the future ?

Quantum computing will allow to realize calculations much more complicated which are not solvable by current computers with classic algorithms. This will provide new tools for research and for fields using a lot of combinatory calculations. Moreover, quantum cryptography is already used by several Swiss banks and allow them, with the non-copy principle of the qubit to get unbreakable keys and to transmit them instantaneously with the quantum teleportation. There will thus be a lot of new features and tools for cryptanalysis and data security.

Thank you for having read this article, do not miss the next article about quantum teleportation !

See you soon, on 2 Steps From Science !


Sources :





Gershenfeld, Neil; Chuang, Isaac L. (June 1998). “Quantum Computing with Molecules”  Scientific American.

Feynman, R. P. (1982). “Simulating physics with computers”. International Journal of Theoretical Physics 21(6): 467–488.

Lai, Richard (2011-04-18). “First light wave quantum teleportation achieved, opens door to ultra fast data transmission”.

DiVincenzo, David P. (2000-04-13). “The Physical Implementation of Quantum Computation”.

NSA seeks to build quantum computer that could crack most types of encryption”. Washington Post


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