## Quantum theory

Very small particles and light behave differently from objects we encounter in normal life, which are described by classical mechanics and classical electrodynamics. The mechanics of light and matter at the atomic and subatomic scale are described by quantum theory, which forms the underlying principles of chemistry and most of physics. An important part of quantum theory was established at the beginning of the 20th century, by people like Erwin Schrödinger, Wolfgang Pauli, Marie Curie, Hendrik Lorentz, Werner Heisenberg, Louis de Broglie, Max Planck and Albert Einstein, all of them present at the 5th Solvay Conference on Quantum Mechanics, 1927 as shown in the picture below. Seventeen of the twenty-nine people shown in the picture were or became Nobel prize winners. In its first centennial of existence quantum theory has already brought us the information age with its disruptive technologies of transistors, lasers, nuclear power, and superconductivity.

But from the point of view of classical physics this theory can be seen as counter-intuitive or even bizarre. Quantum objects appear to be at two places at the same time, electrical currents in a metallic wire can flow clockwise and counter-clockwise at the same time, or an object can sometimes behave as wave and sometimes as particle.

Even a Nobel laureate like Richard Feynman struggled with the implications of quantum mechanics, which led to his famous quote:

I think I can safely say that nobody understands quantum mechanics - Richard Feynman

Although applications of quantum mechanics are not straight forward, this branch of physics opens up an entirely new world of possibilities in science, technology and information processing. One of the most promising ones is the quantum computer.

## Quantum computers

A quantum computer is a device performing quantum computations. It manipulates the quantum states of qubits in a controlled way to perform algorithms. A universal quantum computer is defined as a machine that is able to adopt an arbitrary quantum state from an arbitrary input quantum state. Quantum computers use this principle to accurately compute the behavior of quantum systems or very small particles that follow the laws of quantum mechanics, for example the behaviour of electrons in a hydrogen molecule or more complex systems like how proteins fold. It can also be used to run optimization algorithms very efficiently, execute machine learning algorithms or do pattern recognition much more efficiently then classical (super)computers canj.

The development of a quantum computer is currently in its infancy, systems consist of a few to a few tens of quantum bits (qubits). Main challenges in further development are to make the quantum computer scalable and to make it fault-tolerant. This means that it will be able to perform universal quantum operations using unreliable components.

In the last two decades of the previous century more and more quantum mechanical concepts were brought into information processing, allowing the development of so-called quantum algorithms. One of the early breakthroughs and still one of the strongest arguments for quantum computing to date is Shor’s algorithm for integer factorization into primes. In many ways this algorithm can be seen as a starting signal. Since then the efforts in learning about what is required to build a quantum computer increased manifold.

Parallel to the theoretical efforts also ground-breaking strides were taken on the experimental side. Physicists developed methods to detect and controllably manipulate individual quantum objects such as photons, atoms or electrons. These quantum objects can be used as physical implementations of qubits.

## Challenges and opportunities

There are still many challenges in quantum computers and even more opportunities to explore. Scientists and engineers from QuTech in Delft, the Netherlands, are working hard to make quantum computing a reality. Significant progress is being made. Unlike the quantum computer prototype from Dilbert, our platform is ready to use. The platform consists of an extensive knowledge base, two actual quantum hardware chips, a quantum computer emulator and the editor to run your very real and very own quantum algorithms. Quantum Inspire is here, at your fingertips, to experiment, explore and enjoy!