New blueprint to realise novel quantum phase transition

Quantum phase transitions can be more subtle than the transition of ice to water. A new experimental protocol proposed by CQT Fellow Ho Wen Wei could help researchers to look for them. Image credit: Shutterstock.com/Aleksandr Grechanyuk

Ice melts and water boils, but not all phase transitions are so easy to see. CQT Fellow Ho Wen Wei and his collaborators offer hope of a “smoking gun” signature of one of the more elusive types.

One of the aims of many-body physics is to characterise how matter organises itself at the large scale, given some interactions at the microscopic scale. This gives rise to the notion of ‘phases of matter’, and a phase transition denotes a sharp change whereby a material changes from one structure to another distinct structure. Deconfined quantum criticality (DQC) is an unconventional phase transition theorised to exist in quantum systems. Researchers hope to confirm whether nature can indeed support such an exotic type of phase transition. To date, however, there is only indirect experimental evidence of DQC occurring in certain magnetic materials.

Wen Wei and his collaborators hope to change that with a proposal to realise and image DQC unambiguously using quantum simulators of individually trapped Rydberg atoms. “Our blueprint is different from previous ones because our aim was to leverage the control and impressive measurement capabilities of present-day and emerging quantum simulators to provide a smoking gun evidence of DQC,” says Wen Wei, whose co-authors are based in the United States. Their blueprint was published on 22 August in Physical Review Letters.

On the brink of change

Many phase transitions are described within the powerful Landau-Ginzburg-Wilson theory, which holds that a phase transition happens when some symmetry of a system is spontaneously broken. This is typically measured by an abrupt change in some observable property dubbed the ‘order parameter’.

When a magnetic material is magnetised, for example, it goes from having electron spins that are pointing randomly to having spins that are all aligned. In the non-magnetised phase, the magnet had rotational symmetry – no matter the orientation of the magnet, the ensemble of spins looks on average the same. In the magnetised phase however, the alignment breaks this symmetry. Here, the order parameter is magnetisation, or the net direction the spins point in.

DQC lies outside of the conventional symmetry-breaking paradigm. Quantum effects create new options in the system – for example, allowing phases that are defined by patterns of entanglement between particles or arise from emergent degrees of freedom.

A new blueprint

Most attempts at seeking evidence of DQC have so far been in condensed matter experiments, specifically, magnetic materials. However, these experiments can only measure coarse-grained, macroscopic properties of the system such as the net magnetisation of the sample. “This may be used to detect the presence of a phase transition, but does not provide direct evidence of the underlying nature of the critical point where the transition happens,” says Wen Wei. “To truly verify DQC would require much more fine-grained probes, such as of entanglement or symmetries.”

In their experimental protocol, the researchers propose an array of neutral atoms trapped in optical tweezers and arranged in a zigzag structure. These atoms would be pumped by lasers to highly excited Rydberg states. Once in these states, the atoms would interact with each other.

This platform has appealing features. Firstly, each atom is individually addressable, giving the researchers the ability to carefully engineer the interactions and conditions necessary for DQC to emerge. More importantly, the researchers can ascertain the precise state of each atom with individual measurement snapshots, akin to taking many photographs of the underlying system with a camera.

This measurement capability distinguishes their proposal from previous experiments. It allows the researchers to measure the joint distribution of measurement outcomes belonging to the order parameters of the flanking phases at the phase transition. This is akin to peering microscopically to see the orientation of the underlying spins of a magnetic material, instead of simply detecting if the material attracts iron fillings or not. With this, the presence of DQC can then be imaged directly. Wen Wei says, “A smoking gun of DQC is the prediction that there is an emergent rotational symmetry present in the shape of this joint order parameter distribution.”

In the paper, the team report numerical simulations which show that quantum simulators based on individually trapped neutral Rydberg atoms is a promising platform to realise DQC. They are now talking to experimental groups about implementing the proposal.