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In physics, things exist in phases, such as solid, liquid and gas states. When something crosses from one phase to another, we talk about a phase transition—like water boiling into steam, turning from liquid to gas.

In our kitchens, boils at 100 degrees C, and its density changes dramatically, making a discontinuous jump from liquid to gas. However, if we turn up the pressure, the boiling point of water also increases, until a pressure of 221 atmospheres where it boils at 374 degrees C. Here, something strange happens: the liquid and gas merge into a single phase. Above this "critical point," there is no longer a phase transition at all, and so by controlling its pressure, water can be steered from liquid to gas without ever crossing one.

Is there a quantum version of a water-like phase transition? "The current directions in quantum magnetism and spintronics require highly spin-anisotropic interactions to produce the physics of topological phases and protected qubits, but these interactions also favor discontinuous quantum phase transitions," says Professor Henrik Rønnow at EPFL's School of Basic Sciences.

Previous studies have focused on smooth, continuous phase transitions in quantum . Now, in a joint experimental and theoretical project led by Rønnow and Professor Frédéric Mila, also at the School of Basic Sciences, physicists at EPFL and the Paul Scherrer Institute have studied a discontinuous phase transition to observe the first ever critical point in a quantum magnet, similar to that of water. The work is now published in Nature.

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