The first stable quantum gas of molecules with large electric dipole moments has been made by physicists in the US. Unlike other quantum gases, the molecules interact with each other over relatively large distances. This means that the system could be used to study a wide range of quantum phenomena — and perhaps even be used to create robust quantum bits that could be used to store and process information.

Previous attempts at making such a gas had failed because it proved impossible to cool molecules with large dipole moments to the sub-milliKelvin temperatures needed to create a quantum gas. But now a team led by Deborah Jin and Jun Ye at NIST/JILA in Boulder, Colorado have come up with a laser technique that solves this problem (Sciencexpress 10.1126/science.1163861) .

First created in the mid 1990s, a quantum gas is formed when the constituent atoms or molecules are cooled until they have little energy and are sufficiently close together for the overall behaviour of the gas to be governed by quantum, rather than classical, physics. The forces between the gas atoms tend to be very small and act over very short distances. Physicists have learned how to control these forces using magnetic fields to allow the atoms or molecules to get very close together. This has let physicists use ultracold gases as “quantum simulators” for studying more complicated systems such as electrons in superconductors.

Long-range challenge

What researchers had not been able to do is make a quantum gas from atoms or molecules that interact via long-range forces. This could be very useful for simulating many “real-life” systems which involve long-range interactions between electrons.

The NIST/JILA technique starts with a very cold gas that is simply a mixture of potassium and rubidium atoms. The atoms are confined by a laser beam and are subjected to a magnetic field, which creates a weak attraction between potassium-rubidium pairs that binds them into a polar molecule.

At this point the molecules are relatively large and each molecule has a great deal of internal energy — the molecules are both vibrating and rotating. In order to make a dense quantum gas, these molecules must “shrink” in size by losing much of their internal energy. Unfortunately, this expelled energy tends to heat up the gas, making the shrinking process a difficult one.

The team got around this problem by firing near-infrared laser light at two specific wavelengths at the molecules. This causes the molecules to give up their internal energy as photons of red light, which exit the gas without heating it.

Lowest energy states only

This left the majority of the molecules in their lowest vibrational and rotational energy states, resulting in a quantum gas with a temperature of 350 nK and density of 1012 molecules per cubic centimetre.

The team then measured the dipole moment by applying a small electric field to the gas and determining the resultant shift in the molecule’s energy levels using laser spectroscopy. The dipole moment of a molecule in the lowest energy state was 0.566 Debye, which is about one third the dipole moment of a water molecule.

The creation of a stable quantum gas of molecules with large electric dipole moments is of particular interest to physicists who are keen on making quantum bits (or qubits) for quantum computers. This is because qubits made from such molecules would (in principle) be robust to interference from outside influences, but could be manipulated by the simple application of an electric field.

“This is a landmark paper!” said David DeMille — a physicist at Yale University who studies the use of dipolar quantum gases in quantum computing. DeMille told that the work also opens the door to the study of chemistry at ultracold temperatures as well as new tests of fundamental symmetries in physics.