The aim of this work package is to devise and test strong interactions, either among atoms and cavity photons for neutrals atoms, or among trapped ions. The goal is to generate spin-squeezed and entangled states and metrological advantage in the OLC context. This includes the possibility of creating a quantum enhanced optical oscillator by exploiting non-linear Hamiltonians and many-body systems, as well as brand-new measurements protocols.

Presently (see [pez10] for a recent review), the state-of-the-art spin-squeezing of collective atomic spin and frequency measurement of Rb clock transitions beyond the QPN was achieved inside a high finesse optical cavity in the strong coupling regime [boh14,cox16], while record spin-squeezing in an optical cavity was achieved in the homogeneous atom-cavity coupling regime [hos16]. These proof-of-principle results are still confined to the RF and MW domain. In the case of ions, the application of quantum coherence and entanglement on multi-particle quantum states engineering and light-matter interaction is basically at the proof-of-principle stage [pez18]. The most prominent example is the use of quantum-logic, where two different qubits are entangled in order to read-out the state of the clock transition (one qubit) with an easily accessible logic state of the other qubit [cho10]. Further examples include a potential sensor application with selective sensitivity enhancement of a measurement of the electric field quadrupole shift effect while removing sensitivity to a noisy magnetic field environment [roo06] with a designed 2-qubit entangled state. The potential for stability enhancement has been demonstrated in a microwave transition with a 6-ion Schrödinger-cat state [lei05] and lately, on a maximally entangled optical 2-qubit state ( ⁸⁸Sr⁺ clock transition), with Rabi spectroscopy at the Heisenberg limit [sha17]. Scalability to a few ion entangled state has been lately demonstrated with stepwise entanglement of 4 ions including shuttling and splitting operations [kau17]. In an earlier experiment GHZ states, commonly suggested for metrological applications, with up to 14 quibts were used to highlight the competition between the gain of phase supersensitivity and the detrimental effect of superdecoherence that can make unprotected entangled states supersensitive to noise [mon11]. The use of protected states with decoherence-free subspaces [lid98, roo06] might enable overcoming this problem. Recently, spin-squeezed states of about 200 ions in a Penning trap have been demonstrated [boh16].

The first task is dedicated to the study of metrological entanglement in strongly-coupled atom-cavity systems, focusing on devising techniques and measurement strategies to overcome the QPN in the full atom-photon quantum regime. The second task is devoted to engineering quantum states of multi-ion strings for entanglement-enhanced spectroscopy. The third task complements and interacts with the previous ones by theoretical studies and engineering of atomic non-Gaussian states and non-linear Hamiltonians.

 

Lead participant

INRIM (Dr Marco Tarallo)

Co-participants

NPL, LUH, CNR, ICFO