Abstract
Quantum spins of mesoscopic size are a well-studied playground for engineering non-classical states. If the spin represents the collective state of an ensemble of qubits, its non-classical behavior is linked to entanglement between the qubits. In this work, we report on an experimental study of entanglement in dysprosium's electronic spin. Its ground state, of angular momentum $J=8$, can formally be viewed as a set of $2J$ qubits symmetric upon exchange. To access entanglement properties, we partition the spin by optically coupling it to an excited state $J'=J-1$, which removes a pair of qubits in a state defined by the light polarization. Starting with the well-known W and squeezed states, we extract the concurrence of qubit pairs, which quantifies their non-classical character. We also directly demonstrate entanglement between the 14- and 2-qubit subsystems via an increase in entropy upon partition. In a complementary set of experiments, we probe decoherence of a state prepared in the excited level $J'=J+1$ and interpret spontaneous emission as a loss of a qubit pair in a random state. This allows us to contrast the robustness of pairwise entanglement of the W state with the fragility of the coherence involved in a Schr\"odinger cat state. Our findings open up the possibility to engineer novel types of entangled atomic ensembles, in which entanglement occurs within each atom's electronic spin as well as between different atoms.
Highlights
Entanglement is a hallmark of nonclassical behavior in compound quantum systems
The virtual absorption of a photon is interpreted as the annihilation of a qubit pair in a state defined by the light polarization, leaving a set of 14 qubits in the excited electronic level [see Fig. 1(a)]
As shown in Ref. [48], the existence of a strictly negative value Z (n) constitutes a necessary and sufficient criterion of nonclassicality. To apply this criterion to our system, we use the connection between the mean values of spin projection and the Husimi function of qubit pairs extracted from the electronic spin J, Ln = Qpair(n) − Qpair(−n), Ln2 = Qpair(n) + Qpair(−n), leading to the expression Z (n) = α Cn, where we introduce the coefficient α = ( Qpair(−n) − Qpair(n))2 − 1 and the distribution
Summary
Entanglement is a hallmark of nonclassical behavior in compound quantum systems. Minimal entangled systems of qubit pairs, as realized with correlated photon pairs, play a central role in testing the foundations of quantum mechanics [1,2]. We study quantum entanglement between subsystems of the electronic spin of dysprosium atoms, of angular momentum J = 8 in its ground state and prepared in nonclassical spin states. The virtual absorption of a photon is interpreted as the annihilation of a qubit pair in a state defined by the light polarization, leaving a set of 14 qubits in the excited electronic level [see Fig. 1(a)]. This process realizes a partition of the electronic spin J in two subsystems—the excited electronic spin J = J − 1 and the photon angular momentum L = 1. Such systems would combine entanglement between atoms and within each electronic spin, allowing one to scale up entanglement depth and its application to quantumenhanced sensing
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
