Physicists build simulator, hope to stand up beautiful Standard Model

Particles, antiparticles and putting meat on bones of theory

Physicists have built a quantum simulator to study the Standard Model of particle physics – a theory concerning the electromagnetic, weak, and strong nuclear interactions, as well as classifying all the subatomic particles known. The simulator includes lasers and four calcium ions, according to new research published in Nature [paywalled].

Gauge theories – which make suppositions about the fundamental forces between particles – are difficult to study using a classical computer. The computation steps needed to simulate each particle interaction increase exponentially as more particles are added – and require too much memory. So physicists are turning to quantum simulators to model the crazy world of quantum mechanics using ions.

Four calcium ions are trapped in an electromagnetic field and are manipulated using lasers. The lasers simulate an electromagnetic field in a vacuum and are directed onto the ions. The ions act like qubits (units of quantum information) and the electrons are excited to different levels when they absorb energy from the laser light.

When the electron is in the ground state, it represents a zero, and when it has been excited and moves up, it represents a one. All the quantum states in-between are a superposition of the zero and one.

Light is emitted when the electron moves back down to its ground state and gives information on each ion's quantum state. The light also depends on the interactions between the chain of calcium ions that are entangled.

"Each pair of ions represents a pair of a particle and an antiparticle," said Esteban A Martinez, lead author and experimental physicist at the University of Innsbruck in Austria.

By studying the ions' light, the researchers measure the brightness of the light, which correlates to quantum fluctuations in the energy and is a simulation of how an antiparticle and particle pair are created. The experiment is a simulation of the Schwinger model that is used in gauge theories, Martinez told The Register.

The simulation must be repeated many times and the conditions can be changed in order to study the mechanism closely.

"This is the first time that the real-time dynamics of gauge theory have been studied experimentally in a lab," said Martinez.

Since the experiment is a simulation, it does not directly probe the nature of particles like CERN (European Organization for Nuclear Research) does, explained Martinez. The quantum simulation allows more physicists to be more flexible with their models, whereas the experiments at CERN are more rigid and only detect the outcomes of the particle interactions.

In the future, Martinez hopes that his team can use the quantum simulator to study confinement – the mysterious process that doesn't allow quarks to be separated.

Quarks are tiny particles hidden within particles. They remain invisible because they can never be isolated and are impossible to see. A strong force keeps them intact. When they are separated, the force grows in strength and the energy stored in the force field creates a quark and antiquark pair – also known as confinement.

The Standard Model is not complete, as it cannot tell us how exactly confinement works yet, said Martinez. ®

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