Quantum Computation II Experimental Realization
fengxiaot Lv4

In this article the experimental prerequisites for ion trapping will be explained. The experimental setup, including a ultra-high vacuum chamber, photoionization, detection and Doppler cooling lasers will be discussed. The experimental control of the trap electrode voltages via a FPGA-based DAC board will be introduced.


  1. Place the ion trap in ultra-high vacuum (UHV) chamber

  2. Lower the pressure of UHV chamber down to 1012mbar10^{-12} \, \mathrm{mbar}.

    • bake the vessel for several weeks in an oven at 150C200C150 ^\circ\mathrm{C}-200 ^\circ\mathrm{C} reduce pressure to 1010mbar10^{-10} \, \mathrm{mbar}.

    • connect with a residual gas analyzer (RGA) monitor residual volatile molecules.

    • connect with an ion gauge to monitor pressure.

    • use a titanium sublimation pump coat the inner surfaces of the chamber with a titanium layer.

      /* Explanation: Clean titanium is very reactive, serving as a getter material. Sublimed titanium molecules can chemically react with reactive gases, like O2 and N2 , and disassociate and diffuse H2 . */

    • use an ion pump to emit electrons and ionize residual molecules or atoms by collisions. The ionized molecules are then directed to and bounded by titanium layer, using strong magnetic field.

    Maintenance: vent under argon reflow

  3. Load neutral calcium.

    • Crystalline calcium is placed into an evaporative oven placed below the trap and then evaporated.
    • The neutral calcium beam is directed into the main vacuum chamber and loaded above the trap through that 100μm100 \, \mathrm{\mu m} wide slit.
  4. Ionize calcium

    • First, a blue 423nm423 \,\mathrm{nm} laser excites the atoms from the 1S0^1S_0 ground state to the 1P1^1P_1 state

    • Second, a red 732nm732 \,\mathrm{nm} laser exciting 1P1^1P_1 to the 1D2^1D_2 state.

    • Last, a infrared 832nm832 \,\mathrm{nm} laser is used for exciting the neutral atoms beyond the ionization limit of 6.1130 eV and creating positively charged 40Ca+^{40}\mathrm{Ca}^+ ions.

      /* Note: Spectroscopic notation 2S+1LJ^{2S+1}L_J */

  5. Doppler Cooling


40Ca+^{40}\mathrm{Ca}^+ could serve as an optical qubit. In the figure above, the electronic energy levels of 40Ca+^{40}\mathrm{Ca}^+ ion are depicted (neglecting the Zeeman splitting sublevels). The qubit is formed by the S1/2S_{1/2} and D5/2D_{5/2} levels and provides an excited state lifetime of τ=1.2s\tau = 1.2 \,\mathrm{s}.

Readout of the qubit state is achieved by detection of fluorescence on the S1/2P1/2S_{1/2} \harr P_{1/2} transition: if the qubit is in the ground state, the 397nm397\, \mathrm{nm} laser will transfer the ground state population into the P1/2P_{1/2} state and the fluorescence counts can be detected with a photomultiplier tube. The excited state will not yield fluorescence due to its long lifetime.

In order to deplete the excited state population and de-excite the qubit, the D5/2P3/2D_{5/2} \harr P_{3/2} transition is driven.

In order to drive the S1/2D5/2S_{1/2} \harr D_{5/2} transition, extremely narrow band lasers with line widths below 200mHz200 \,\mathrm{mHz} are used. Achieving
this accuracy requires laser-locking to ultra-high finesse cavities.

Doppler Cooling

To be continued.