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Barium isotope 137 has nuclear spin 3/2, which gives rise to hyperfine interaction and ground state splitting of about 8 GHz. Hyperfine states with there effectively infinite lifetime and long coherence are excellent candidates for quantum information storage and processing. We are working on developing a robust scheme for 137Ba+ qubit detection.

The first step in a quantum computation is the initialization of the qubit register. In trapped Ba ions we achieve this by "optical pumping". Laser of appropriately tuned frequency and polariation causes the electron in the ion to go to a particular state (to be "pimped") and remain there. The polarization of the laser is chosen such that when in that staate the electron does not "see" the laser light. We optically pump to the F= 2, mF= 0 Zeeman sublevel of the ground state using a p-polarized 493 nm laser light. The p-polarization means that the electric field of the laser is aligned with the quantuzation axis of the system, defined by the direction of the applied magnetic field. Instead of tuning the polarization itself, we tune the magnetic field direction. Figure on the right shows the mapping of the optical pumping in 137Ba+. Pumping is achieved when the ion "diappears", which corresponds to the "deep-sea" area on the map.

 

To measure the state of the hyperfine qubit, we begin by "shelving" one of the hyperfine states to the metastable 5D5/2 state. That state lifetime is about 30 seconds, much longer than any subsequent operations. The shelving light is coming from a 1762 nm fiber laser stabilized to a high-finesse Zerodur optical cavity. To achieve high efficiency and robustness of the shelfing process, adiabatic passage technique is used. Following the shelving pulse w e apply the 495 nm and 650 nm laser light and detect ion fluorescence. Ion in the "shelved" state does not scatter any light and remains dark, while the non-shelved ion scatters many photons. We can easily sicriminate between the "dark" and the "bright" ion and thus detect the qubit state.

The 6D512 to 5D5/2 transition is a dipole-forbidden, slow transition. However, with sufficient laser power it can be driven fast. We map the transition by changing the frequency of the 1762 nm laser in fine steps and measuring the probablilty of "shelving". Figure on the right shows such a frequency scan for different conditions: no optical pumping, with optcal pumping, both with 2 ms laser exposure time, and with optical pumping and a 5 ms laser exposure time. Note how the additional lines which correspond to different Zeeman sublevels of the ground state diappear when the optical pumping is present. At the longer exposure time, the shelving probablility reaches 50%, which corresponds to a saturated transition. The large width of the resonance line is due in part to power-broadening, but is also a combination of the laser linewidth and the fact that we transition to multiple Zeeman level of the 5D5/2 state.

Finally, putting the initialization and the detection steps together with the 8 GHz transition in between we can observe coherent qubit evolution - the Rabi flops. Graph on the left shows these flops driven by about 1 watt of microwaves. The Rabi frequency is about 10 kHz. The oscillations only reach about 70% at the maxima due to imperfect shelving.

Recently, by better stabilizing the 1762 nm laser, we dramatically reduced the line width of the shelving transition, as seen on the right. We observe more than 2 orders of magnitude line width reduction, and now the spectrum of motion sideband is clearly resolved. We can now apply the adiabatic passage technique to increase the shelving efficiency and thus to improve the qubit detection fidelity.