Dr. Massine Kelai (QNS)

Here is a story about quantum computers’ qubits for you participating in the 2024 Qubit Art Contest. QNS hopes this article helps you understand and enjoy the wonder of qubits.

The Birth of Quantum Computer

In the 1970s and 80s, the idea of the quantum computer were born in the minds of physicists such as Richard Feynman, Paul Benioff, David Deutsch, and Charles Bennett. Feynman's idea was: "Instead of complaining that the simulation of quantum phenomena requires enormous powers from our current computers, let us use the computing power of quantum phenomena to make them more powerful than our current computers".

The first occurrence of the word qubit is due to Benjamin Schumacher, in 1995, in his famous paper ‘Quantum Coding’ (https://doi.org/10.1103/PhysRevA.51.2738) and it represents the quantum counterpart of a bit (binary digit). In classical computing, this is the basic unit of information and can be represented as two logical levels, commonly called 0 or 1. In contrast to the classical bit, the quantum bit that is so called qubit, obeys quantum mechanics rules. Qubits can be found in the state 0, 1, and ‘any proportion of 0 and 1’. This logic of ‘0 and 1’ can be understood by two quantum phenomena i.e the superposition or the entanglement of the quantum states. Let's decipher the fundamental concepts that can help us understand what qubits are.

The Superposition of Quantum States

Quantum objects can be seen as being simultaneous, with varying probabilities, in several places. Another crucial aspect is the fact that the quantum superposition state is destroyed by the measurement, so the state is said to be projected into a quantum state. A measurement corresponds to the informal idea of ‘looking’ at a quantum object, which immediately collapses the quantum state to one of the two classical states. In other words, when you measure a qubit directly, it loses its superposition of states.

Let's take a simple example to understand all these notions: if we take a 'classical' coin, it has two sides, namely head or tail. It is clear that if we take this coin in our hand and see ‘head’, then the other side is undoubtedly deduced as ‘tail’. However, in the case of a 'quantum coin', this may be in the state of 'head', 'tail' or in a superposition of 'head+tail' states. But as soon as we measure the state of the coin then it will be a specific state of ‘head’ or ‘tail’. The superposition determines the probability for measuring head or tail. As a consequence of the superposition of state, the qubit stores information that is quantitatively and qualitatively different from that of a bit.

The main interest of the quantum computer would be that its parallel processing capabilities are an exponential function of the number of qubits, thus the amount and the time processing of information managed by a qubit is greater than that contained in a bit.

The Entanglement of Quantum States

To understand quantum entanglement, let's take a simple picture. Consider two qubits A and B are separated by a distance as large as one can imagine. We say that A and B are entangled if we cannot describe the quantum state of the system ‘A+B’ as the product of the states A and B separately. In other words, A and B form a single, inseparable whole, and not the sum of two independent entities with quantum states that depend on each other regardless of the distance between them. Such a state is said to be entangled because the interaction naturally creates correlations between the observed physical properties of these separate particles. The fundamental consequence of this quantum effect is that if we measure the quantum state of A, then the information given by the measurement of the quantum state of B is directly and instantaneously given to us.

To illustrate this concept, let's return to our 'quantum coin' example. Let's say there are two 'quantum coins' A and B, such that coin A is on earth and coin B is Mars. We do not know the state of the pieces before the measurement, but by the principle of superposition these two pieces can be in the state 'head', 'tail', or in the superposition of 'head+tail'. These two pieces are said to be entangled if, the measurement of the quantum state of A gives ‘tail’ then irreversibly the state of the second coin B is known in advance (‘head’ for example).

Note that no one knows how this happens, only that it does. Moreover, quantum entanglement has been demonstrated experimentally for various systems (photons, neutrinos, electrons, and molecules). The demonstration of the principle of entanglement by the physicist Alain Aspect, John F. Clauser and Anton Zeilinger earned them the 2022 Nobel Prize in Physics. The phenomenon of quantum entanglement plays an important role in the development of the so-called ‘quantum error correction’ in quantum computing.

The physicist and philosopher of science Etienne Klein gave a romantic metaphor for quantum entanglement: ‘‘Two hearts that have interacted in the past can no longer be considered in the same way as if they had never met. Marked forever by their meeting, they form an inseparable whole.’’

What are Quantum Computers Made of?

Currently, many physical systems are good candidates for quantum computing. We can mention: the use of the polarisation of a photon, trapped ions in electromagnetic cavities, quantum dots generated in semiconductors, Josephson circuits formed by the junction of two superconductors, and the spin of the electrons or the nucleus in the atoms that we will detail in what follows. Indeed, our laboratory is specialized in the use of electron spin in quantum computing.

So, what is spin? Spin is an intrinsic quantum characteristic that all quantum objects exhibit. In the case of electrons, spin can have two values, namely, +1/2 and -1/2 (the equivalent of 0 and 1 for the classical bit) which we call up spin and down spin. By applying an external magnetic field, it is possible to separate the up and down spin states and thus control them to be a specific spin state.

A fundamental aspect of research in quantum computing is to demonstrate quantum advantage. For a given problem, a quantum computer could always solve it while a classical computer could not within a reasonable time limit. The two examples of quantum computers that have made the most headlines in recent years are Google's Sycamore and the University of Science and Technology of China's Jiuzhang.
Sycamore uses qubits implemented by superconducting materials. Google researchers claim that to perform a series of operations that would have taken a supercomputer 10,000 years, Sycamore takes just 200 seconds.
Concerning Jiuzhang, it is the first quantum computer that uses photons and the researchers go even further. They claim that for the series of operations that Jiuzhang performed that took 200 seconds, then it would have taken 2.5 billion years (a little more than half the age of our lovely planet) for a classical computer. It is beyond imagination.

What are the Limitations to Overcome?

The major obstacle in the realization of a quantum computer is what is called decoherence. Indeed, the qubits must be completely isolated from the outside world to avoid entanglement with the environment which destroys the quantum character of the qubits. Moreover, and even if they are well isolated from the outside world, most of the qubits often work at cryogenic temperatures (very close to absolute zero -273.15°C), and going to these temperatures requires large apparatuses and are therefore not suitable for easy use.

Another important drawback is the scalability of qubits: indeed, building large numbers of qubits that keep their coherence is very difficult and this is caused by the interactions between qubits which is also a source of decoherence. Therefore, current research focuses on finding systems with a high number of qubits, operating at room temperature with a long coherence time.

Some Important Years

1994: Peter Shor, a researcher at AT&T, showed that it was possible to factor large numbers in a reasonable time using a quantum computer.

1996: Lov Grover invented an algorithm based on quantum computers allowing one to find an entry in an unsorted database in a smaller steps than a classical computer.

1998: IBM was the first to present a 2-qubit quantum computer.

2001: IBM creates a quantum computer of 7 qubits built around chloroform molecules and factors the number 15 thanks to the algorithm of Shor.

2006: The Institute for Quantum Information Processing at the University of Ulm in Germany presented the first three-dimensional linear European microchip which traps several ionized Ca+ atoms in isolation.

2017: The Center for Quantum Nanoscience (QNS) launched and opened the possibility of using spins as qubits on the atomic scale.

2019: Goggle announced that its quantum computer Sycamore completed a task in 200 seconds that a supercomputer will take 10,000 years.

2020: The University of Science and Technology of China announced that its quantum computer Jiuzhang performed Gaussian boson sampling in 200 seconds that a supercomputer will take 2.5 billion years.