# The big goal: a large quantum computer

Solving the puzzles and challenges posed in Quantum Moves will allow the team of physicists in Aarhus University to take an important step towards building a large-scale quantum computer.

A quantum computer is a computer built using the some of the smallest physical building blocks, such as single atoms. It operates under the rules of quantum physics, making is very different from the normal computers we are used to encountering.

### Why all the fuss about quantum computers?

### There are essentially two answers.

# 1.

The first reason is practical. Moore's law, introduced in 1965, Gordon E. Moore, makes the observation that the computational power of the machines doubles every 18 months. This leads to an ever-decreasing size of fundamental logical units. The fact that this decrease cannot go on indefinitely poses the fundamental challenges that chip manufacturers currently face in achieving more computational power. Very soon the size of a chip will be comparable to a single atom.

Why is this such a challenge? The law's of physics change radically when we enter the world of atoms and other small things. In this world, all physical rules are dictated by quantum mechanics. These rules forbid some things that we are used to, such as certain positions of atoms or electrons. On the other hand, they allow things that are impossible in our everyday life, like being in many places at the same time and walking though walls!

Shrinking a computer to this size means that the computer is not just small, it's also pretty crazy. Scientists have to word hard to make sure that their new minuscule computers do all the calculations they are supposed to do, despite the quantum rules.

# 2.

The second reason to get excited about quantum computers is exactly all the quirkiness of quantum objects! Used right, all the new features of the quantum computer can be harnessed to make totally different types of computing machines that are much more powerful than ordinary computers.

Take the ability of an atom to be in two places simultaneously, or to be in two states at the same time. This is known as the superposition principle and it's pretty much like seeing the head-side and the tail-side of a coin at the same time. It enables the creation of qubits (quantum bits), bits that can be 'zero' and 'one' at the same time.

All computations are based on operations on bit strings, long chains of 'zeros' and 'ones'. If we have a string of qubits, we can code gigantic amounts of information into reasonable short chains. This is because a string of, say, three qubits, can be in a state '000' at the same time as it is in a state '001', '011', '111' and so on - in fact, we can express 2^3=8 bit stings in a 3-qubit string. Four qubits can express 2^4=16 bit strings, 10 qubits express a whopping 1024 strings and if we have a qubit string of just 260 qubits, it can contain more information than there are atoms in the universe. This quantum property allows huge parallel computations and means that just one quantum computer would have more computational power than all conventional computers combined!

Prototypes of quantum computers are currently developed in physics labs all around the world. The problem they face is the immense difficulty of making them big enough to be really interesting. The quantum objects used to create qubits are badly behaved rascals and rarely do the exact things they are told to do. The best available prototypes can tame strings of about 10-20 qubits, which is not yet enough to unleash the full power of quantum computing.

# Our contribution in Aarhus

Quantum Moves is based on an idea of storing single atoms in a very specific trap, where each atom sits in a well like an egg in an egg tray. With such a trap, physicists can store around 300 atoms in a neat configuration. Just imagine being able to use each of these atoms as a qubit! This is, in fact, possible, and with such a configuration we are getting tantalisingly close to having a quantum computer with the superpowers everyone is fussing over.

The missing piece is doing operations and calculations on this large qubit string. Operations are done by picking up individual atoms, moving them around the egg tray trap and merging them with other atoms. This has to be done with great care to preserve the state of the atom (whether it's 'zero, 'one' or a superposition of both) - but also fast, to avoid outside noise and keep calculations efficient.

Sound familiar? This is exactly the challenge posed in the Quantum Moves game! Players move and merge atoms with the goal of ending up in very specific final shapes (which are actually just different states). Our engines take this information and translate it to the way a laser beam is used to pick up and move around an atom in the lab. Different challenges in Quantum Moves correspond to different operations on the qubit strings. Once we have a whole toolbox of operations, we are ready to turn on the 300-qubit quantum computer and start addressing the hugely important and impactful task of working on problems that have been "impossible" to solve for the best present-day supercomputers.