Elizabeth M. Bridge

Efficiently scaling quantum networks to long ranges requires local processing nodes to perform basic computation and communication tasks. Trapped ions have demonstrated all the properties required for the construction of such a node, storing quantum information for up to 12 minutes, implementing deterministic high fidelity logic operations on one and two qubits, and ion-photon coupling. While most ions suitable for quantum computing emit photons in visible to near ultraviolet (UV) frequency ranges poorly suited to long-distance fibre optical based networking, recent experiments in frequency conversion provide a technological solution by shifting the photons to frequencies in the telecom band with lower attenuation for fused silica fibres. Encoding qubits in frequency rather than polarization makes them more robust against decoherence from thermal or mechanical noise due to the conservation of energy. To date, ion-photonic frequency qubit entanglement has not been directly shown. Here we demonstrate a frequency encoding ion-photon entanglement protocol in $^{171}$Yb$^+$ with correlations equivalent to 92.4(8)% fidelity using a purpose-built UV hyperfine spectrometer. The same robustness against decoherence precludes our passive optical setup from rotating photonic qubits to unconditionally demonstrate entanglement, however it is sufficient to allow us to benchmark the quality of ion-UV photon correlations prior to frequency conversion to the telecom band.

Isotope selective optical excitation of atoms is important for experiments with neutral atoms, metrology, and work with trapped ions, including quantum information processing. Polarization-enhanced absorption spectroscopy is used to frequency stabilise a tunable external cavity laser diode system at 398.9 nm for isotope selective photoionization of neutral Yb atoms. This spectroscopy technique is used to measure isotope resolved dispersive features from transitions within a see through configuration Ytterbium hollow-cathode discharge lamp. This Doppler-free dichroic polarization spectroscopy is realised by retro-reflecting a laser beam through the discharge and analysing the polarization dependent absorption with balanced detection. The spectroscopy signal is recovered using lock-in detection of frequency modulation induced by current modulation of the external cavity laser diode. Here, we show the use of polarization-enhanced absorption spectroscopy for isotope selectively loading of $^{171}$Yb$^+$, $^{172}$Yb$^+$, or $^{174}$Yb$^+$ ions into an RF Paul trap.

Here we present a cost effective multi-channel opto-mechanical switch and software PID system for locking multiple lasers to a single channel commercial wavemeter. The switch is based on a rotating cylinder that selectively transmits one laser beam at a time to the wavemeter, the wavelength is read by the computer and an error signal is output to the lasers to correct wavelength drifts every millisecond. We use this system to stabilise 740 nm (subsequently frequency doubled to 370 nm), 399 nm and 935 nm lasers for trapping and cooling different isotopes of Yb+ ion. We characterize the frequency stability of the three lasers by using a second, more precise, commercial wavemeter. We also characterise the absolute frequency stability of the 740 nm laser using the fluorescence drift rate of a trapped 174Yb+ ion. For the 740 nm laser we demonstrate an Allan deviation, df/f, of 3 x 10^-10 (at 20 s integration time), equivalent to sub-200 KHz stability.