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Register a new userObservation of Entangled States of a Fully Controlled 20-Qubit System
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We generate and characterise entangled states of a register of 20 individually controlled qubits, where each qubit is encoded into the electronic state of a trapped atomic ion. Entanglement is generated amongst the qubits during the out-of-equilibrium dynamics of an Ising-type Hamiltonian, engineered via laser fields. Since the qubit-qubit interactions decay with distance, entanglement is generated at early times predominantly between neighbouring groups of qubits. We characterise entanglement between these groups by designing and applying witnesses for genuine multipartite entanglement. Our results show that, during the dynamical evolution, all neighbouring qubit pairs, triplets, most quadruplets, and some quintuplets simultaneously develop genuine multipartite entanglement. Witnessing genuine multipartite entanglement in larger groups of qubits in our system remains an open challenge.
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We study a qubit-oscillator system, with a time-dependent coupling coefficient, and present a scheme for generating entangled Schrodinger-cat states with large mean photon numbers and also a scheme that protects the cat states against dephasing caused by the nonlinearity in the system. We focus on the case where the qubit frequency is small compared to the oscillator frequency. We first present the exact quantum state evolution in the limit of infinitesimal qubit frequency. We then analyze the first-order effect of the nonzero qubit frequency. Our scheme works for a wide range of coupling strength values, including the recently achieved deep-strong-coupling regime.
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Euclidean volume ratios characterizing the typicality of entangled and separable states are investigated for two-qubit and qubit-qutrit quantum states. For this purpose a new numerical approach is developed. It is based on the Peres-Horodecki criterion, on a characterization of the convex set of quantum states by inequalities resulting from Newton identities and Descartes rule of signs and on combining this characterization with standard and Multiphase Monte Carlo algorithms. Our approach confirms not only recent results on two-qubit states but also allows for a numerically reliable numerical treatment of so far unexplored special classes of two-qubit and qubit-qutrit states. However, our results also hint at the limits of efficiency of our numerical Monte Carlo approaches which is already marked by the most general qubit-qutrit states forming a convex set in a linear manifold of thirtyfive dimensions.
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