Quantum Interference between Photons from an Atomic Ensemble and a Remote Atomic Ion

Abstract

Advances in the distribution of quantum information will likely require entanglement shared across a hybrid quantum network [1][2][3]. Many entanglement protocols require the generation of indistinguishable photons between the various nodes of the network [4,5]. This is challenging in a hybrid environment due to typically large differences in the spectral and temporal characteristics of single photons generated in different systems [1]. Here we show, for the first time, quantum interference between photons generated from a single atomic ion and an atomic ensemble, located in different buildings and linked via optical fibre. Trapped ions are leading candidates for quantum computation and simulation with good matter-to-photon conversion [6][7][8][9][10][11][12][13]. Rydberg excitations in neutral-atom ensembles show great promise as interfaces for the storage and manipulation of photonic qubits with excellent efficiencies [14][15][16][17]. Our measurement of high-visibility interference between photons generated by these two, disparate systems is an important building block for the establishment of a hybrid quantum network.Recently, Rydberg atoms have proven to be a useful tool in the field of quantum information. The strong optical nonlinearity exhibited by neutral-atom Rydberg ensembles enables the construction of single-photon sources [15], gates [16], and transistors [17]. Strong light-matter interactions make them well suited as quantum memories [14], and for implementing quantum repeaters [18,19]. Furthermore, arrays of Rydberg atoms are a powerful new platform for quantum simulation [20, 21]. The continued success of trapped-ion systems in quantum computation [6,7], simulation [8, 9], and communication [12] owes to their long coherence and trapping lifetimes [10], high fidelity operations [11], and ease of generating ion-photon entanglement [12, 13].Given the wide-ranging applications of both platforms, future efforts in quantum information will benefit from the construction of remote hybrid atomic-ensemble-ion networks. Flying photonic qubits provide an excellent means arXiv:1907.04387v1 [quant-ph]

Indistinguishable photons from a trapped-ion quantum network node

“…The achieved visibilities and coincidence rates in our experiments would already allow for remote ion entanglement over tens of kilometers. Consider the entanglement swapping protocol of [8], which leads to maximal entanglement of two remote (ion) qubits with state fidelity F (T )=(1 + V (T ))/2 [32] at a heralded rate R swap (T ) ∝ R gen × C ⊥ (T ), where R gen is the photon-generation attempt rate at each ion-trap network node. Using R gen = 30 kHz, the achieved values without photon conversion ( Figure 2) would allow for 3 km ion-ion entanglement distribution with F (T = 9µs) = 0.736 ± 0.004 at a rate of R swap (9µs) = 30 Hz.…”

“…A twofold coincidence projects the two ions into an entangled state. In such an entanglement swapping experiment, the fidelity F (with respect to a maximally entangled two qubit state) of the two ion state can be shown to be proportional to the HOM visibility V [32]. Furthermore, the rate at which the entangled states are created is related to the success probability P succ defined in Eq.…”

C-band single photons from a trapped ion via two-stage frequency conversion

“…This reduces the range of ion quantum networks and prevents the integration of these networks into existing telecommunications infrastructure. Of particular interest are photons produced via S-P dipole transitions, enabling direct entanglement between the photons and commonly used ground state qubits of ions such as Yb + [1,3,5,7,8,12], Ba + [3,11,12,[16][17][18], and Ca + [6,9,10]. Ground-state qubits currently demonstrate the longest coherence times in trapped ions [8], as well as the highest two-qubit gate fidelities [10].…”

“…Quantum frequency conversion (QFC) has been used by trapped ion groups to down-convert ion-emitted photons in the near-IR (≈ 850 nm) to telecom wavelengths [13][14][15], greatly improving potential networking range, though not using photons directly entangled with ground state qubits. QFC of visible (493 nm) ion-emitted photons, which may be directly entangled with ground state ion qubits, to the near IR (≈ 780 nm) [16,17] has been demonstrated, serving to both increase networking range and providing a pathway for the implementation of hybrid networks using both trapped ion and neutral atomic systems [17][18][19].…”

“…This two-step approach circumvents noise produced via spontaneous parametric down conversion (SPDC) that would be present in any single-stage QFC scheme taking visible light to the C-band [15,20]. Additionally, this scheme offers the advantage of having the intermediate first-stage conversion resonant with neutral-atom based systems for potential integration into a hybrid quantum network [17][18][19]21]. Using low levels of laser light, we demonstrate quantum efficiencies of 0.37 and 0.56 for the first-and second-stage respectively.…”