Entanglement between a trapped-ion qubit and a 780-nm photon via quantum frequency conversion

Future quantum networks will require the ability to produce matter-photon entanglement at photon frequencies not naturally emitted from the matter qubit. This allows for a hybrid network architecture, where these photons can couple to other tools and quantum technologies useful for tasks such as multiplexing, routing, and storage, but which operate at wavelengths different from that of the matter qubit source, while also reducing network losses. Here, we demonstrate entanglement between a trapped ion and a 780-nm photon, a wavelength that can interact with neutral-Rb-based quantum networking devices. A single barium ion is used to produce 493-nm photons, entangled with the ion, which are then frequency converted to 780 nm while preserving the entanglement. We generate ion-photon entanglement with fidelities $\ge 0.93$(2) and $\ge 0.84$(2) for 493-nm and 780-nm photons respectively with the fidelity drop arising predominantly from a reduction in the signal-noise of our detectors at 780 nm compared with at 493 nm.

Figure 1

Photon production and ion qubit control. (a) The ion is prepared into the $|5D_{3/2},m_j=+1/2\rangle$ state. (b) A clean out pulse is used to remove any remaining population in the $|5D_{3/2},m_j=\pm 1/2\rangle$ states. (c) The ion is excited to the $|6P_{1/2},m_j=+1/2\rangle$ state, from which a photon entangled with the ion is emitted. (d) If a photon is detected, coherent ion-qubit operations are driven by an RF antenna tuned to 14.67 MHz. (e) Ion-qubit state detection is performed via optical shelving of the $|0\rangle$ state via the $\mathrm{\Delta }m=0$ quadrupole transition at 1762 nm.

Figure 2

Experimental layout. (a) A ${}^{138}{\mathrm{Ba}}^{+}$ ion produces 493-nm photons that are collected and coupled into a single mode fiber (SMF) by a 0.6 numerical aperture (NA) objective. The photon collection axis is orthogonal to the magnetic field at the ion. A radio-frequency (RF) antenna addresses the ion qubit based on photon measurements on avalanche photodiodes (APD-1 and APD-2). (b) Photon polarization measurements at 493 nm are carried out via a polarization analyzer consisting of a quarter waveplate (QWP), half waveplate (HWP), and polarizing beamsplitter (PBS). A flipper mirror (FM) allows for the analyzer to be bypassed with the photons instead sent to the frequency conversion setup. (c) Frequency conversion is performed using a waveguide written into Zn-doped periodically poled lithium niobate crystal (Zn:PPLN) at the center of two optical loops. Dichroic mirrors (DM) allow for combination and splitting of the multiple colors used. A custom-coated PBS (CPBS) and achromatic half waveplate (AHWP) operating at 493 and 780 nm enable these colors to share the same optical paths. A band-pass filter (BPF) is used to remove the majority of the noise photons produced by the 1343-nm pump light. (d) A second polarization analyzer is used for 780-nm photonic qubit polarization measurements.

Figure 3

Single-photon quantum frequency conversion efficiencies for each polarization, measured from the output of the 493-nm SMF fiber in Fig. 2 to the output of the 780-nm fiber in Fig. 2. The horizontal dashed line represents the conversion efficiency used in this experiment ($34.5\%$). The vertical red and blue lines represent the pump power needed to reach this conversion efficiency for vertical and horizontal input photon polarizations, respectively.

Figure 4

Ion-photon correlations at 493 nm. (a) Calibration scans for the unrotated basis ($z$ basis) where the blue(red) data represents the probability of detecting the ion in state $|1\rangle$ given that the photon is detected in state $|H\rangle (|V\rangle )$ by APD-1(APD-2) for a given position of the HWP in Fig. 2. (b) Calibration scans for rotated basis ($x$ basis), obtained by setting the HWP in Fig. 2 to ${40}^{\circ }$, and for a given phase of the $\pi /2$ pulse applied to the ion. (c) Conditional measurement probabilities at the point of maximum correlations identified by the dashed line in (a) at ${105}^{\circ }$ for the phase in the unrotated basis data. (d) Conditional measurement probabilities at the point of maximum correlations identified by the dashed line in (b) at $7\pi /10$ radians for the phase in the rotated basis data. All error bars are statistical, with 500 photon events for each data point.

Figure 5

Ion-photon correlations at 780 nm. (a) Calibration scans for the unrotated basis ($z$ basis) where the blue(red) data represents the probability of detecting the ion in state $|1\rangle$ given that the photon is detected in state $|H\rangle (|V\rangle )$ by APD-1(APD-2) for a given position of the HWP in Fig. 2. (b) Calibration scans for rotated basis ($x$ basis), obtained by setting the HWP in Fig. 2 to ${100}^{\circ }$, and for a given phase of the $\pi /2$ pulse applied to the ion. (c) Conditional measurement probabilities at the point of maximum correlation identified by the dashed line in (a) at ${75}^{\circ }$ for the phase in the unrotated basis data. (d) Conditional measurement probabilities at the point of maximum correlation identified by the dashed line in (b) at $\pi /2$ radians for the phase in the rotated basis data. All error bars are statistical, with 500 photon events for each data point.

Figure 6

Histograms of photon production attempts between successful detection at 493 nm, using 28 500 total detection events. The exponential fit (red dashed line) suggests a mean (vertical line) of 350 attempts before a successful photon detection. This corresponds to a entangled photon production rate of 137 ${\mathrm{s}}^{-1}$ during the fast loop of the experiment.

Figure 7

Histograms of photon production attempts between successful detection at 780 nm, using 20 700 total detection events. The exponential fit (red dashed line) suggests a mean (vertical line) of 1068 attempts before a successful photon detection. This corresponds to a entangled photon production rate of 45 ${\mathrm{s}}^{-1}$ during the fast loop of the experiment.