Probing quantum criticality and symmetry breaking at the microscopic level

We report on an experimental study of the Lipkin-Meshkov-Glick model of quantum spins interacting at infinite range in a transverse magnetic field, which exhibits a ferromagnetic phase transition in the thermodynamic limit. We use Dysprosium atoms of electronic spin $J=8$, subjected to a quadratic Zeeman light shift, to simulate $2J=16$ interacting spins $1/2$. We probe the system microscopically using single magnetic sublevel resolution, giving access to the spin projection parity, which is the collective observable characterizing the underlying $\mathbb{Z}_2$ symmetry. We measure the thermodynamic properties and dynamical response of the system, and study the quantum critical behavior around the transition point. In the ferromagnetic phase, we achieve coherent tunneling between symmetry-broken states, and test the link between symmetry breaking and the appearance of a finite order parameter.Comment: 13 pages, 13 figure

Sensing magnetic fields with non-gaussian quantum fluctuations

http://arxiv.org/abs/1901.06282https://pro.college-de-france.fr/jean.dalibard/publications/2019_PRL.122.173601.pdfThe precision of a quantum sensor can overcome its classical counterpart when its constituants are entangled. In gaussian squeezed states, quantum correlations lead to a reduction of the quantum projection noise below the shot noise limit. However, the most sensitive states involve complex non-gaussian quantum fluctuations, making the required measurement protocol challenging. Here we measure the sensitivity of non-classical states of the electronic spin \J=8\ of dysprosium atoms, created using light-induced non-linear spin coupling. Magnetic sublevel resolution enables us to reach the optimal sensitivity of non-gaussian (oversqueezed) states, well above the capability of squeezed states and about half the Heisenberg limit

Sensing magnetic fields with non-gaussian quantum fluctuations

http://arxiv.org/abs/1901.06282https://pro.college-de-france.fr/jean.dalibard/publications/2019_PRL.122.173601.pdfThe precision of a quantum sensor can overcome its classical counterpart when its constituants are entangled. In gaussian squeezed states, quantum correlations lead to a reduction of the quantum projection noise below the shot noise limit. However, the most sensitive states involve complex non-gaussian quantum fluctuations, making the required measurement protocol challenging. Here we measure the sensitivity of non-classical states of the electronic spin \J=8\ of dysprosium atoms, created using light-induced non-linear spin coupling. Magnetic sublevel resolution enables us to reach the optimal sensitivity of non-gaussian (oversqueezed) states, well above the capability of squeezed states and about half the Heisenberg limit

Quantum-enhanced sensing using non-classical spin states of a highly magnetic atom

Moderate-size coherent superpositions of spin states allow quantum enhancements in metrology. Here, the authors exploit the large electronic spin of dysprosium atoms to realize mesoscopic spin superpositions, allowing a 14-fold quantum enhancement in magnetic field sensitivity, close to the Heisenberg limit

Enhanced Magnetic Sensitivity with Non-Gaussian Quantum Fluctuations

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