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

A. N. Craddock, J. Hannegan, D. P. Ornelas-Huerta, J. D. Siverns, A. J. Hachtel, E. A. Goldschmidt, J. V. Porto, Q. Quraishi, and S. L. Rolston

Phys. Rev. Lett. 123, 213601 – Published 18 November 2019

Abstract

Many remote-entanglement protocols rely on the generation and interference of photons produced by nodes within a quantum network. Quantum networks based on heterogeneous nodes provide a versatile platform by utilizing the complementary strengths of the differing systems. Implementation of such networks is challenging, due to the disparate spectral and temporal characteristics of the photons generated by the different quantum systems. Here, we report on the observation of quantum interference between photons generated from a single ion and an atomic ensemble. The photons are produced on demand by each source located in separate buildings, in a manner suitable for quantum networking. Given these results, we analyze the feasibility of hybrid ion-ensemble remote entanglement generation.

Received 30 July 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.213601

Physics Subject Headings (PhySH)

Hybrid quantum systems Nonlinear optics Photon statistics Quantum optics

Atomic ensemble Rydberg atoms & molecules Trapped ions

Quantum Information, Science & TechnologyAtomic, Molecular & Optical

Authors & Affiliations

A. N. Craddock ^1,†, J. Hannegan ^1,†, D. P. Ornelas-Huerta ¹, J. D. Siverns ¹, A. J. Hachtel¹, E. A. Goldschmidt ², J. V. Porto¹, Q. Quraishi^2,1, and S. L. Rolston^1,*

¹Joint Quantum Institute, National Institute of Standards and Technology and the University of Maryland, College Park, Maryland 20742, USA
²Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, USA

^*Corresponding author. rolston@umd.edu
^†J. H. and A. N. C. contributed equally to this work.

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Vol. 123, Iss. 21 — 22 November 2019

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Figure 1
Experimental layout and energy level diagrams for the two sources. (a) Building $A$ contains a ${}^{138}{Ba}^{+}$ ion which emits photons at 493 nm, and building $B$ contains a ${}^{87}{Rb}$ atomic ensemble producing 780-nm photons. Ion-emitted photons are converted to 780 nm using DFG-1 and sent to building $B$ via PMF. DFG-2 produces 780-nm light used to frequency stabilize the output of DFG-1 by optical beat note locking with reference light sent from building $B$ . Light from the ion and ensemble source is sent to the HOM interferometer for two-photon interference measurements. A half-wave plate (HWP) in one input path allows for control of the relative polarization of the photons. The photons interfere on a nearly $50 ∶ 50$ beam splitter before being coupled into two SMFs which are connected to SPADs linked to a time-tagging device. Here VBG stands for a volume Bragg grating and ULEC for ultralow expansion cavity. (b) Level scheme for ${}^{138}{Ba}^{+}$ . (c) Level scheme for ${}^{87}{Rb}$ .
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Figure 2
Data for stochastic photon production and interference. Second-order intensity autocorrelation functions for the (a) ion source and (b) atomic-ensemble source. The oscillation exhibited in the ion autocorrelation data is attributed to Rabi flopping of the ion. (c) Normalized coincidences for the cases where the relative polarization of the two sources at the interferometer are parallel, $n_{∥} (τ)$ , and perpendicular, $n_{⊥} (τ)$ , using 1-ns bins. Lower (blue) [upper (red)] band corresponds to the expected normalized coincidences when photons from the sources are completely indistinguishable [distinguishable]. For the lower (blue) band expected coincidences are entirely due to atomic-ensemble source multiphoton events, while for the upper (red) band there is an additional contribution from photon distinguishability. Bands indicate the $\pm 1 σ$ confidence interval in this value due to the uncertainty in $g_{atom}^{(2)} (0)$ . Data shown accumulated in $\approx 30$ h. In all cases the error bars denote statistical uncertainties. All curves shown include background subtraction. Raw two-photon interference data and calculation of expected coincidence bands can be found in Ref. [33].
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Figure 3
On-demand pulse sequence and interference. (a) Schematic of pulse sequence for one period. The atomic-ensemble-produced photon profile, and ion-produced photon profile at $t \approx 4.25 μ s$ , are measured directly. The ion-produced photon profile at $t \approx 1.75 μ s$ is a time shifted copy of that at $t \approx 4.25 μ s$ to allow for easy comparison of the photon temporal shapes from the two sources. To lessen the effects of small drifts in the relative arrival time of the photons, we offset the ion- and atomic-ensemble-produced photon average arrival times. (b),(c) Normalized coincidences when the photons from the two sources are temporally overlapped (nonoverlapped) shown in blue (red). Both curves represent the data after software gating, background subtraction, and using 5-ns bins. Dashed lines in (b) indicate the range shown in (c). Theory curve obtained taking into account the nontransform limited nature, probabilistic spectrum of the ion-produced photon, and plausible estimates of the relative drift ( $2 π \times 10 MHz$ ) and offset ( $2 π \times 20 MHz$ ) between the center frequencies of the photons from the two sources. Data presented accumulated over $\approx 22$ h. In all cases the error bars denote statistical uncertainties. Raw two-photon interference data and calculation of theory curves can be found in Ref. [33].
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