Analogue approach to quantum problems

Although the field of quantum chemistry has been around since the 1960s, in practice it is often hampered by available technology.

The discipline, sometimes called molecular mechanics, allows researchers to determine and predict the properties of molecules – a matter of increasing importance as nano-scale technologies develop.

However, the sheer complexity of possible molecular reactions above a certain level makes it impossible to model even on the most muscular computers. To get around this obstacle, scientists sometimes use “semi-empirical” methods, which match results to the behaviours of previously mapped “reference molecules”.

For many applications, however, such methods are simply not reliable enough. Another approach is to combine quantum chemistry and quantum mechanical calculations. This promises more accurate results, although is still considered a compromise and requires enormous amounts of computing grunt to achieve.

One quality that all these approaches share is that they are digital in nature. Now, researchers led by Javier Argüello-Luengo from Germany’s Max-Planck-Institut für Quantenoptik propose an alternative strategy – using analogue simulations.

In a Letter published in the journal Nature the researchers describe combining two existing technologies to create highly sensitive models of molecular reactions.

The first technology involves ultracold atoms organised into frameworks known as optical lattices – an approach devised more than a decade ago to produce powerful models of quantum many-body systems.

Argüello-Luengo and colleagues combine this with a second quantum-level technology, known as cavity quantum electrodynamics. This exploits light trapped in atomic-scale reflective holes and measures the behaviour of captive photons – one of the theoretical approaches to building a quantum computer.

Combining the two strategies, the researchers report, produces a quantum chemical simulator, in which the ultracold atoms “play the role of electrons”. The set-up also produces analogues for nuclear attraction and electrostatic forces.

The result, they say, is “a well-controlled quantum system” which can be engineered according to experimental need.

“This approach,” they write, “has already been used to address questions that the most advanced classical computers cannot resolve.”

However, the researchers concede that at present its use above certain modest scales is impractical.

“We emphasise that some of the elements and conditions required in this approach are beyond the capabilities of the current experimental state of the art,” they write.

“However, the rapid progress of analogue quantum simulation may lead to the realisation of the present ideas in the near future.”