Atomic Source of Single Photons in the Telecom Band

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Atomic Source of Single Photons in the Telecom Band

A. M. Dibos, M. Raha, C. M. Phenicie, and J. D. Thompson

Phys. Rev. Lett. 120, 243601 – Published 11 June 2018

Abstract

Single atoms and atomlike defects in solids are ideal quantum light sources and memories for quantum networks. However, most atomic transitions are in the ultraviolet-visible portion of the electromagnetic spectrum, where propagation losses in optical fibers are prohibitively large. Here, we observe for the first time the emission of single photons from a single ${Er}^{3 +}$ ion in a solid-state host, whose optical transition at $1.5 μ m$ is in the telecom band, allowing for low-loss propagation in optical fiber. This is enabled by integrating ${Er}^{3 +}$ ions with silicon nanophotonic structures, which results in an enhancement of the photon emission rate by a factor of more than 650. Dozens of distinct ions can be addressed in a single device, and the splitting of the lines in a magnetic field confirms that the optical transitions are coupled to the electronic spin of the ${Er}^{3 +}$ ions. These results are a significant step towards long-distance quantum networks and deterministic quantum logic for photons based on a scalable silicon nanophotonics architecture.

Received 31 December 2017

DOI:https://doi.org/10.1103/PhysRevLett.120.243601

Physics Subject Headings (PhySH)

Atoms, ions, & molecules in cavities Nanophotonics Quantum memories Quantum repeaters

Rare-earth doped crystals

Quantum Information, Science & TechnologyAtomic, Molecular & Optical

Authors & Affiliations

A. M. Dibos, M. Raha, C. M. Phenicie, and J. D. Thompson^*

Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA

^*jdthompson@princeton.edu

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Vol. 120, Iss. 24 — 15 June 2018

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Images

Figure 1
Experimental configuration for enhancing ${Er}^{3 +}$ emission with a silicon photonic crystal. (a) Schematic illustration of the fabricated devices. Silicon waveguides patterned with photonic crystal cavities evanescently couple to ${Er}^{3 +}$ ion impurities in a YSO crystal. The suspended, tapered ends of the Si waveguides protrude off the edge of the YSO crystal, allowing coupling to a lensed fiber. The axes ( $D_{1}$ , $D_{2}$ , $b$ ) denote the orientation of the YSO crystal [32]. (b) Scanning electron microscope image of a photonic crystal cavity and tapered waveguide prior to transfer onto the YSO substrate. (c) Schematic layout of the experiment. The Si-YSO device is situated in a cryostat ( $T \approx 4 K$ ). A stabilized laser in combination with a double-pass acousto-optic modulator (AOM) and electro-optic intensity modulator (IM) produces short pulses of light with an on/off ratio of 90 dB. A superconducting nanowire single-photon detector (SNSPD) inside a second cryostat detects the return light from the cavity. (d) The PLE measurement sequence consists of a $10 μ s$ excitation pulse, followed by a fluorescence collection window. (e) Reflection spectrum of the cavity used in these experiments, with quality factor $Q = 7.3 \times 10^{4}$ , limited by internal losses.
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Figure 2
Photoluminescence excitation spectrum of single ions in a dilute ${Er}^{3 +}$ ensemble. (a) PLE spectrum, measured by scanning the laser frequency and cavity resonance (with linewidth $κ = 2 π \times 3.85 GHz$ ) together through a spectral region near the bulk Er:YSO absorption resonance at 1536.46 nm (determined from a second crystal, and indicated schematically by the green dashed line). The vertical axis indicates the average number of photons detected in an $82 μ s$ integration window following each excitation pulse [as in Fig. 1]. The spectrum shows individually resolvable peaks, which we interpret as the optical transitions of single ${Er}^{3 +}$ ions. The interruptions in the scan result from spectral regions that are inaccessible with our laser stabilization technique. Inset: Energy levels of ${Er}^{3 +}$ in YSO. The crystal field splits the ${{}^{2 S + 1}L}_{J}$ states of the free ${Er}^{3 +}$ ion (magenta lines) into $J + 1 / 2$ Kramers doublets (black lines), which further split into single states in a magnetic field (orange lines). The cavity is resonant with the $Z_{1} - Y_{1}$ transition at 1536.46 nm (YSO site 1). (b) Expanded view of the red portion of (a), showing an isolated line with a width of 5 MHz (FWHM) on a nearly dark-count-limited background (gray dashed line), characteristic of the tails of the inhomogeneous distribution. Inset: The background-subtracted height of this isolated peak saturates with increasing excitation power. The solid line is a model based on independently measured parameters [33].
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Figure 3
Quantifying the ${Er}^{3 +}$ -cavity coupling, and measurement of $g^{(2)}$ . (a) Time-resolved fluorescence from a single cavity-coupled ${Er}^{3 +}$ ion (blue) following an excitation pulse, which decays to the detector dark count rate (gray dashed line). The fluorescence from a bulk ensemble without cavity enhancement is shown for comparison (orange; measured in a second crystal, and on an arbitrary vertical scale). (b) Fixing the laser frequency to the atomic transition and sweeping the cavity resonance reveals that the decay rate enhancement $Γ / Γ_{0}$ varies with the atom-cavity detuning. A Lorentzian fit to the data (red dashed curve) yields a width of $2.62 \pm 0.04 GHz$ (in agreement with the cavity linewidth), and a maximum decay rate enhancement of $P = 658 \pm 5$ . (c) Second-order autocorrelation function ( $g^{(2)}$ ) of the fluorescence from a single ion. We bin all photons detected after a single excitation pulse into a single time bin, so the horizontal axis shows the autocorrelation offset in units of the pulse repetition period ( $100 μ s$ ). The data are symmetric around zero offset since a single detector is used to record the fluorescence and compute a true autocorrelation; however, both positive and negative offsets are plotted for clarity.
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Figure 4
Zeeman splitting of a single ${Er}^{3 +}$ ion spectral line. A magnetic field splits a single spectral line into two transitions. The field is oriented approximately 135 deg between the $D_{1}$ and $D_{2}$ axes in the $D_{1} − D_{2}$ plane. The predicted transition frequencies are shown with red lines, including an offset field of around 1 G. Since the $g$ factors are highly anisotropic, the slope of the predicted splitting is not linear when the applied field is smaller in magnitude than the offset field, which has a different orientation.
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