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HALDANE PHASE DISCOVERED IN UNIQUE ATOMIC SPIN CONFIGURATION


Max Planck researchers have conducted an experiment to measure the characteristics of the so-called Haldane phase in a unique configuration of atomic spins. They did this by employing a quantum mechanical ruse.

There are phases in some materials that cannot transition because they are shielded by a specific type of symmetry. These are known as topological phases in physics. One illustration of this is the Haldane phase, which happens in antiferromagnetic spin-1 chains and is named after Duncan Haldane, the physics laureate of the 2016 Nobel Prize. A group of scientists at MPQ have now achieved this unusual state of matter in a straightforward system of ultracold atoms. They achieved the appropriate atomic spin form using a quantum gas microscope, assessed the system's characteristics, and discovered the internal hidden order characteristic of the Haldane phase.




the cavity is being aligned for the second harmonic generation. The UV light generated in this area is used by the researchers to cool the lithium atoms in their experiment.

Each phase of any issue has the potential to meld into the next. Water serves as an illustration of this because it can exist in liquid form, as ice, or as steam, depending on the weather. The chemical makeup of the various physical phases is the same, but their levels of internal organisation vary. At some point, the phase of the water changes depending on factors like temperature or pressure. However, in some materials, there are phases between which a transition is not feasible because they are guarded by a particular sort of symmetry-a characteristic of the system that, as an example, does not change during a reflection or rotation. A phase transition can only occur if the symmetry is broken. The study of these topological phases, as they are known in physics, has recently expanded our understanding of the structure of quantum systems.


Topological phases of quantum systems: detection of the Haldane phase

As of now, these characteristics have largely only been discovered through theoretical calculations and models, or by doing indirect observations on materials. However, a group of scientists from the Max Planck Institute of Quantum Optics (MPQ) in Garching have now achieved the creation and experimental analysis of a unique, exemplary type of topological phase. A so-called Haldane phase was produced by the researchers in the MPQ Department of Quantum Many-Body Systems together with other researchers in the field of quantum optics. It is named for the British physicist Duncan Haldane, who with with two other researchers shared the 2016 Nobel Prize in Physics for his groundbreaking work in describing topological phases of quantum systems.


Haldane concentrated on a number of things, including the potential for a topological phase to arise in a chain of antiferromagnetic spin-1 particles. The angular momentum of a particle when it rotates about its own axis can be simply understood as the spin, a quantum mechanical property of particles like electrons or atoms. In an antiferromagnetic substance, the spins prefer that other spins in their immediate proximity rotate in a different direction.

In spin-1 systems, this can result in a periodic ordering of the spins, which is imperceptible in traditional measurements. Theoretical predictions stated that although there is an order, it is "hidden." It wouldn't be able to monitor every spin concurrently and separately to detect it in solids. However, the MPQ researchers employed synthetic materials with substantially greater spin separation. Therein, they generated a spin-1 chain having the traits Haldane listed.

The integer-spin cell technique

This was challenging to understand up until today, claim researchers at QunatumDDX. Because of this, researchers turned to a ruse: In an indirect manner, according to the researchers, a spin-1 chain was built up from spins of the value 1/2, to which two were added each. Cells with integer spin were produced in this fashion, and they were arranged in a chain.

The scientists made use of a device known as a quantum gas microscope to realise this unique shape. The magnetic characteristics of individual atoms that have previously been arranged in a specific way can be studied, for instance, using a device like this. Therefore, scientists also mention a quantum simulator, which creates matter artificially from its fundamental constituents. According to the researchers - To do this, standing waves of laser light are used that build a kind of lattice for atoms. Then, using more lasers and numerous small, adjustable mirrors, this lattice is sculpted into the required shape.

Atoms were arranged in a two-dimensional optical lattice for the tests on the topological Haldane phase, according to the physicist. The researchers chose a lattice structure that gave the atoms, together with their spins, the shape of a ladder - with two "legs" and "rungs" in between. "In a vacuum and at a temperature near to absolute zero, the atoms subsequently arranged themselves exactly in the way required by light." According to the researchers in the field of quantum optics. The rungs of these so-called Fermi-Hubbard ladders each coupled two atomic spins to form unit cells with spin 1. The AKLT model, as it is known in theoretical physics, was used in this configuration.

One atom separated the two legs of the quantum scales from one another.

An example of the lattice potential utilised is shown on the left, and a screenshot of a single ladder with 14 distinct atoms visible in green is shown on the right, illustrating the major ideas of the research. A schematic illustration of how the ladder shape is translated onto a spin-1 chain is provided below. Grey is used to depict the spinning dangling edge.

The two legs of the quantum ladder were separated from one another by one atom, which researchers in this field describe as the "highlight of the experiment." The atoms' half-integer spins could be combined in a diagonal offset in this manner to create unit cells. Individual spins without a direct partner "dangled" at both ends of the system as a result of this structure; these states are known as edge states in technical parlance. According to Dominik Bourgund, "Such spins and their magnetic moments can acquire new orientations without any further energy input." In doing so, they endow the system with distinctive qualities based on the unique symmetry, which are the distinguishing features of the Haldane phase. The Max Planck scientists also produced a "trivial" topological phase devoid of edge states as a point of reference.

The scientists used a quantum gas microscope to monitor the magnetization of both the individual spins and the system of all atoms along a mental string in order to analyse the properties of the two phases. The "secret" internal order could only be discovered in this manner. According to the researchers - these results corroborate the expected topological features of both the overall system and the edge states. This demonstrates how a straightforward system was able to be used to access the complicated structure for measurements.

The basis for experimentally confirming topological phase predictions made by theory

The Max Planck team' findings not only set the stage for physically confirming topological phase predictions made in theory. Their fresh discoveries might eventually be put to use in quantum computers. Qubits, which are basic computing units in the form of quantum states, are the foundation of their operation. The technical realization's current weakness is its lack of stability; if the qubits lose their value, the data is likewise gone. It would be much easier to compute using a quantum computer if they could be represented by topological phases, which are extremely resistant to outside influence because of their tight link to a fundamental symmetry.


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