Enhanced Quantum Interface with Collective Ion-Cavity Coupling

Abstract

We prepare a maximally entangled state of two ions and couple both ions to the mode of an optical cavity. The phase of the entangled state determines the collective interaction of the ions with the cavity mode, that is, whether the emission of a single photon into the cavity is suppressed or enhanced. By adjusting this phase, we tune the ion-cavity system from sub-to superradiance. We then encode a single qubit in the two-ion superradiant state and show that this encoding enhances the transfer of quantum information onto a photon.Sub-and superradiance are fundamental effects in quantum optics arising in systems that are symmetric under the interchange of any pair of particles [1][2][3]. Superradiance has been widely studied in many-atom systems, in which effects such as a phase transition [4,5] and narrow-linewidth lasing [6] have recently been observed. For few-atom systems, each atom's state and position can be precisely controlled, and thus collective emission effects such as Rydberg blockade [7] and the Lamb shift [8] can be tailored. In a pioneering experiment using two trapped ions, variation of the ions' separation allowed both sub-and superradiance to be observed, with the excited-state lifetime extended or reduced by up to 1.5% [9]. The contrast was limited because spontaneous emission from the ions was not indistinguishable, as the ions' separation was on the order of the wavelength of the emitted radiation. This limitation can be overcome by observing preferential emission into a single mode, such as the mode defined by incident radiation [1] or by an optical cavity. In a cavity setting, indistinguishability is guaranteed when the emitters are equally coupled to the mode, even if they are spatially separated. Subradiance corresponds to a suppressed interaction of the joint state of the emitters with the cavity mode, while for the superradiant state, the interaction is enhanced.In the context of quantum networks [10,11], superradiance can improve a quantum interface when one logical qubit is encoded across N physical qubits. In the DLCZ protocol for heralded remote entanglement, efficient retrieval of stored photons is based on superradiance [12,13]. Superradiance can also improve the performance of a deterministic, cavity-based interface, which enables the direct transmission of quantum information between network nodes [14]. If a qubit is encoded in the state.. ↓ N , the coupling rate to the cavity is enhanced from the single-qubit rate g to the effective rate g √ N , relaxing the technical requirements for strong coupling between light and matter [15]. This state corresponds to the first step in the superradiant cascade described by Dicke [1]. In contrast, subradiant states are antisymmetrized, resulting in suppressed emission. From a quantum-information perspective, subradiant states are interesting because they span a decoherence-free subspace [16][17][18]. A subradiant state of two superconducting qubits coupled to a cavity has recently been prepared [19].Here, we generate collective states of tw...

Unambiguous atomic Bell measurement assisted by multiphoton states

“…Current experimental realizations report coherent transport and controlled positioning of neutral atoms in optical cavities [29,30,42,43], where a dipole trap is used as a conveyor belt to displace them. Two trapped ions have also been reported to be coupled in a controlled way to an optical resonator [27,28]. The cavity can be shifted with respect to ions, allowing to tune the coupling strength between ions and optical cavity.…”

“…Nevertheless, it is an unambiguous protocol as there are four successful events that lead to postselection of four different Bell states. The scheme is based on basic properties of the two-atom TavisCummings model [26] and on resonant matter-field interactions which are already under experimental investigation [27,28,29,30]. These considerations make our scheme compatible with a quantum repeater or a quantum relay based on coherent multiphoton states, atomic qubits and resonant matter-field interaction.…”

Multiphoton-state-assisted entanglement purification of material qubits

“…The proposed scheme can be already implemented in a one-atom maser setup. However, other implementations may include condensed-matter qubits which are coupled to single-mode radiation fields [41], trapped ions inside a cavity [42], and neutral atoms coherently transported into an optical resonator [43].…”

Intracavity Rydberg-atom electromagnetically induced transparency using a high-finesse optical cavity

“…The matter can be neutral atoms [23,24], ions [25,26], molecules [27], quantum dots [28], nitrogen-vacancy centers [29], etc., and the resonator includes millimeter-wave cavities [30], optical cavities [31], microtoroid cavities [32], photonic crystal defect cavities [33], fiber cavities [34], superconducting stripline resonators [35], surfaces [36], etc. Principally, the cavity restricts the field modes with which the matter inside the cavity can interact and allows the emitted light corresponding to those modes to be detected as it leaks out of the cavity.…”