Tunable ion–photon entanglement in an optical cavity

Abstract

Proposed quantum networks require both a quantum interface between light and matter and the coherent control of quantum states1,2. A quantum interface can be realized by entangling the state of a single photon with the state of an atomic or solid-state quantum memory, as demonstrated in recent experiments with trapped ions3,4, neutral atoms5,6, atomic ensembles7,8, and nitrogen-vacancy spins9. The entangling interaction couples an initial quantum memory state to two possible light–matter states, and the atomic level structure of the memory determines the available coupling paths. In previous work, these paths’ transition parameters determine the phase and amplitude of the final entangled state, unless the memory is initially prepared in a superposition state4, a step that requires coherent control. Here we report the fully tunable entanglement of a single 40Ca+ ion and the polarization state of a single photon within an optical resonator. Our method, based on a bichromatic, cavity-mediated Raman transition, allows us to select two coupling paths and adjust their relative phase and amplitude. The cavity setting enables intrinsically deterministic, high-fidelity generation of any two-qubit entangled state. This approach is applicable to a broad range of candidate systems and thus presents itself as a promising method for distributing information within quantum networks.

Polarisation-preserving photon frequency conversion from a trapped-ion-compatible wavelength to the telecom C-band

“…It is a challenge to efficiently collect 854-nm photons from the ion (or have them absorbed by the ion): in free space, resonant excitation to the excited �P 3∕2 ⟩ leads to the emission of an 854-nm photon in only ∼1/17 of cases [34], in most cases a 393-nm photon is emitted. However, an optical cavity around the ion can enhance emission on this transition [35,36]. In such a CQED system, both near-maximal entanglement between a travelling 854-nm polarisation qubit and the ion qubit [36], and state mapping from ion qubit to photon [37] have been achieved with high fidelity.…”

“…However, an optical cavity around the ion can enhance emission on this transition [35,36]. In such a CQED system, both near-maximal entanglement between a travelling 854-nm polarisation qubit and the ion qubit [36], and state mapping from ion qubit to photon [37] have been achieved with high fidelity. While the bandwidth of an 854-nm photon in free space is 23 MHz (directly related to the excited state lifetime of 6.9 ns) [34], it can be significantly narrower in a CQED setup as the photon leaks slowly out of a high-finesse cavity (e.g.…”

“…While the bandwidth of an 854-nm photon in free space is 23 MHz (directly related to the excited state lifetime of 6.9 ns) [34], it can be significantly narrower in a CQED setup as the photon leaks slowly out of a high-finesse cavity (e.g. ∼ 50 kHz [36]). Such narrowband photons present challenges and opportunities for low-noise QFC.…”

“…triggered by a laser pulse), and are entangled with, the Ca + . In [36], the repetition rate for attempting to generate photons was ∼1 kHz, while photons were actually collected into optical fiber at a rate of ∼100 Hz. In future CQED ion trap systems, that use state-of-the-art mirror coatings with losses of only a few parts per million [44] (to enable higher collection efficiencies), and faster cooling schemes for state reinitialisation [45] (to enable higher repetition rates), it is feasible that triggered 854 nm photons could be collected at a rate of 10 kHz from the ion.…”

Novel Ion Trap Design for Strong Ion-Cavity Coupling

“…On the other hand, employing ionized atoms instead offers clear advantages such as robust trapping and precise quantum state control [4]. After the early demonstrations of localized ions in optical cavities [5,6], several important experimental landmarks have been demonstrated using trapped ions, such as the generation of single photons [7,8], the generation of entanglement between single ions and single photons [9] and the heralded entanglement of two intra-cavity ions [10]. Despite these successful demonstrations, cavity-based ion-photon interfaces are currently limited by the weak interaction between the ions and cavity field.…”