Fast nuclear-spin gates and electrons-nuclei entanglement of neutral atoms in weak magnetic fields

We present fast Rydberg-mediated entanglement involving nuclear spins of divalent atoms with $^{171}$Yb as an example. First, we show a nuclear-spin controlled phase gate of an arbitrary phase realizable either with two laser pulses when assisted by Stark shifts, or with three pulses. Second, we propose to create a state $(\lvert\text{cc}\rangle_{\text{e}} \otimes \lvert\Phi\rangle_{\text{n}} + \lvert\Phi\rangle_{\text{e}} \otimes \lvert\Psi\rangle_{\text{n}} )/\sqrt{2}$ entangled between the electrons~(e) and nuclear spins~(n) of two atoms, where $\lvert\Phi\rangle$ and $\lvert\Psi\rangle$ are two orthogonal Bell states and $\lvert \text{c}\rangle_{\text{e}}$ denotes the clock state. For want of a better term, it is called a Super Bell State for it mimics a ``large'' Bell state incorporating three ``smaller'' Bell states. Third, we show a protocol to create a three-atom state $(\sqrt{3}\lvert\text{ccc}\rangle_{\text{e}} \otimes \lvert\Lambda\rangle_{\text{n}} + \lvert \text{W}\rangle_{\text{e}} \otimes \lvert \text{GHZ}\rangle_{\text{n}} )/2$, where $\lvert\Lambda\rangle_{\text{n}}$ is a nuclear-spin state, $\lvert \text{W}\rangle_{\text{e}}$ is a W state in the ground-clock state space, and $\lvert \text{GHZ}\rangle_{\text{n}}$ is the Greenberger-Horne-Zeilinger~(GHZ) state in the nuclear-spin state space. The four protocols possess high intrinsic fidelities, do not require single-site Rydberg addressing, and can be executed with large $\Omega_{\text{m}}$ in a weak, Gauss-scale magnetic field for they involve Rydberg excitation of both nuclear-spin qubit states in each atom. The latter two protocols can enable measurement-based preparation of Bell, hyperentangled, and GHZ states.Comment: 19 pages, 11 figure

Quantum logic and entanglement by neutral Rydberg atoms: methods and fidelity

Quantum gates and entanglement based on dipole-dipole interactions of neutral Rydberg atoms are relevant to both fundamental physics and quantum information science. The precision and robustness of the Rydberg-mediated entanglement protocols are the key factors limiting their applicability in experiments and near-future industry. There are various methods for generating entangling gates by exploring the Rydberg interactions of neutral atoms, each equipped with its own strengths and weaknesses. The basics and tricks in these protocols are reviewed, with specific attention paid to the achievable fidelity and the robustness to the technical issues and detrimental innate factors.Comment: 57 pages, 10 figure

Hyperentanglement of divalent neutral atoms by Rydberg blockade

Hyperentanglement~(HE), the simultaneous entanglement between two particles in more than one degrees of freedom, is relevant to both fundamental physics and quantum technology. Previous study on HE has been focusing on photons. Here, we study HE in individual neutral atoms. In most alkaline-earth-like atoms with two valence electrons and a nonzero nuclear spin, there are two stable electronic states, the ground state and the long-lived clock state, which can define an electronic qubit. Meanwhile, their nuclear spin states can define a nuclear qubit. By the Rydberg blockade effect, we show that the controlled-Z~(C$_{\text{Z}}$) operation can be generated in the electronic qubits of two nearby atoms, and simultaneously in their nuclear qubits as well, leading to a C$_{\text{Z}}\otimes$C$_{\text{Z}}$ operation which is capable to induce HE. The possibility to induce HE in individual neutral atoms offers new opportunities to study quantum science and technology based on neutral atoms.Comment: 19 pages, 10 figure

Fast atom-photon entangling gates with a superconducting coplanar waveguide

Entanglement between atoms and microwave photons in a superconducting coplanar waveguide~(SCW) can enable hybrid quantum devices and interface static and flying qubits. We study a one-step controlled-Z~(C$_{\text{Z}}$) gate between a neutral atom trapped near a SCW and a microwave mode in the SCW, which is an extension of the gate proposed in [J. D. Pritchard, et.al., Phys. Rev. A 89, 010301(R) (2014)]. The gate protocol is simple and requires one laser pulse for exciting a transition between the ground and Rydberg states of the neutral atom.Comment: 7 pages, 6 figure