Quantum mechanics governs the world of fundamental particles, where we can see a variety of quantum phenomena that emerge due to the collective behavior of particles like electrons.

These exotic quantum states are unusual, behaving differently from anything we know, and only emerge under extreme conditions like low temperatures or high pressures. Most of these exotic quantum states remain theoretical, as they are hard to produce due to the fragility and delicacy of the quantum world.

Now, researchers from Japan and the US have observed several previously unseen quantum states in a two-dimensional material. These materials join the growing list of what the researchers call a quantum zoo.

In a press release, co-author of the study, Prof. Xiaoyang Zhu from Columbia University, said, “Some of these states have never been seen before. And we didn’t expect to see so many either.”

Several of these quantum states were hidden, requiring the researchers to develop an innovative optical technique. The researchers used this technique to probe the quantum states of twisted molybdenum ditelluride (tMoTe2), a two-dimensional moiré material. 

Topological quantum computer

Moiré materials are made by stacking single-atom-thickness sheets (like graphene) with a slight twist or mismatch between the layers. This slight misalignment creates larger, over-arching patterns known as moiré patterns.

Under certain conditions, moiré materials can exhibit what is known as topological quantum states. These quantum states form uniquely as a result of electron interactions. They are of interest because they could be used to build quantum computers. 

Topological quantum computing stands apart from current approaches by following a fundamentally different strategy. Instead of encoding information in fragile qubits, topological quantum computers would use the global properties of exotic quantum states, making them inherently more stable and less error-prone. 

However, these topological states are often created using external magnetic fields, but they disrupt the qubits in the quantum computer. This means we need a magnetic-free method to create the topological quantum states. 

To do so, the researchers developed their own optical technique. For the material, the researchers chose the twisted moiré material, focusing on the fractional quantum Hall effect. 

Hidden quantum states

In the fractional quantum Hall effect, electrons in a material behave collectively, creating what are known as quasi-particles. These particles have charges that are a fraction of the charge of a single electron.

Scientists call these exotic quasi-particles anyons, and they behave in ways that neither electrons nor photons do.

While this may seem counterintuitive, it actually happens due to quantum mechanics. The catch is that this phenomenon requires strong external magnetic fields, which the researchers wished to avoid.

However, the moiré material tMoTe2 is such that the twist creates an internal magnetic field, allowing the fractional quantum Hall effect to be observed without needing an external magnetic field.

For the optical technique, the researchers use a fast laser pulse that disrupts or melts the quantum states in the material temporarily. Then, a second pulse monitors the recovery of the states.

This method allowed them to study the signature of these hidden quantum states.

The researchers’ optical method, which they named the pump-probe spectroscopy method, revealed around 20 quantum states hidden from other methods. While some of the states were previously observed, several were completely new.

Now, the researchers plan to characterize these new quantum states to determine which could be used for quantum computing applications.

The findings of the study are published in Nature.