Efficient coherent internal state transfer in trapped ions using Stimulated Raman Adiabatic Passage

We demonstrate experimentally how the process of Stimulated Raman Adiabatic Passage (STIRAP) can be utilized for efficient coherent internal state transfer in single trapped and laser-cooled Ca ions. The transfer from the D3/2 to the D5/2 state, is detected by a fluorescence measurement revealing the population not transfered to the D5/2 state. A coherent population transfer efficiency at the level of 95 % in a setup allowing for the internal state detection of individual ions in a string has been obtained. PACS numbers: 42.50.Hz, 32.80.Qk, 03.67.Lx Efficient coherent internal state transfer in trapped ions using Stimulated Raman Adiabatic Passage2 In many fields of physics, coherent transfer of population from one specific internal state to another in atoms or molecules is desirable. Notable examples are atom clocks and interferometers[1, 2] as well as transitions between atomic and molecular Bose Einstein condensates[3, 4]. In quantum information processing, coherent transfer can be used to momentarily shelve an atom in a state different from one of the qubit states in connection with qubit gate operations [5, 6], or may be used as part of a qubit readout procedure[7, 8]. For such applications, the transfer has to be nearly perfect. High fidelity transfer has previously been achieved in transfer of population between internal states of single atomic ions by applying Rabi [9], Raman [10] or composite [11] π-pulses. Additionally rapid adiabatic passage has proved useful for manipulating the population of individual neutral atoms [12] and ions [13], however the timescales, limited by achievable Rabi frequencies, were 150 μs to a few ms in these experiments. Over such timescales extremely good control of external magnetic fields must be demonstrated. To shorten the experiments higher Rabi frequencies must be achieved, which is possible if an optical Raman transition is driven. This process is referred to as Stimulated Raman Adiabatic Passage (STIRAP)[14, 15, 16], and it has previously been demonstrated in experiments spanning from transitions between metastable states being part of atomic or molecular Λ-systems [15, 16] via excitation of high lying electronic states in an atomic ladder system[17, 18, 19] to efficient atomic beam deflection[20, 21, 22]. However, so far all these experiments have involved ensembles of atoms or molecules. Here, we report on an efficient STIRAP process between the 3D3/2 and 3D5/2 metastable states in single laser-cooled Ca ions. This experiment demonstrates efficient STIRAP transfer in single quantum systems. The success of the presented STIRAP process points to a diversity of single trapped ion manipulation experiments, including robust entanglement schemes[23], molecular state preparation [24], coherent control of chemical reactions[25, 26] and efficient quantum bit readout in quantum computation[6, 7, 27]. The relevant states and laser induced transitions in the Ca ion are presented in Fig. 1, and in Fig. 2, the basics of the experimental setup is sketched. The Ca ions are trapped and Doppler laser-cooled in a segmented linear Paul trap. The rf voltage (∼500 Vpp, Ωrf = 2π×16.8 MHz) is delivered to the two light grey electrodes of length and thickness of 10 mm and 0.2 mm, respectively, separated by a distance of 1.4 mm. On the sectioned electrodes with section lengths: 4.5 mm, 1mm, and 4.5 mm (dark grey/blue), dc voltages of a few volts are applied to achieve axial confinement. The radial and axial trapping frequencies are typically ∼1.5 MHz and ∼0.5 MHz, respectively, allowing for confinement of few-ion strings (1-10 ions). The ions are laser-cooled using the 397 nm 4S1/2 →4P1/2 and the 866 nm 3D3/2 →4P1/2 transitions, while the STIRAP population transfer from the 3D3/2 to the 3D5/2 is driven by light at 850 and 854nm tuned close to the resonance frequency of the 3D3/2 →4P3/2 and 3D5/2 →4P3/2 transition, respectively. We can determine whether an ion is shelved in the 3D5/2 state or not by exposing it simultaneously to 397 nm and 866 nm light and collecting fluorescence light at 397 nm originating from the 4S1/2 →4P1/2 transitions. Only when the ion is in the 4S1/2 or Efficient coherent internal state transfer in trapped ions using Stimulated Raman Adiabatic Passage3 the 3D3/2 state fluorescence is detected. The internal state of the individual ions can be determined by imaging the fluorescence onto an image intensified Charge Coupled Device (CCD) camera. Additionally, a photomultiplier tube (PMT), which provide a fast and efficient way of quantifying the averaged internal state of the ensemble of ions, is used. The 397 nm light is produced by frequency doubling the output of a Ti:Sa laser, while the remaining near infrared laser light at 850, 854 and 866 nm originates from grating stabilized diode lasers. All lasers are locked to temperature stabilized external Fabry Perot resonators with a frequency drift less than 1 MHz per hour. The light from the 866 nm laser is split into two beams. One, which is only present during laser cooling, passes an electro-optical phase-modulator operating at a frequency of 10 MHz for periodically scrambling the polarization of the light to avoid optical pumping into a dark state in the 3D3/2 level[28]. The other beam is polarized linearly along the direction of an applied bias magnetic field of 1 Gauss, and exposes the ions for 1 ms before the STIRAP pulses are applied. This results in optical pumping into the 3D3/2(m = ±3/2) Zeeman sub states, which have the identical coupling strength with respect to the STIRAP pulses when polarized linearly along the same direction. From Raman spectroscopy we find that the optical pumping leads to a population of less than 6 % in the 3D3/2(m = ±1/2) sublevels. In the STIRAP experiments, the 850 and 854nm lasers are both detuned roughly ∆one=2π×600 MHz below the 3D3/2 →4P3/2 and 3D5/2 →4P3/2 transition, respectively. The STIRAP pulses are created using the 1. order diffraction beam from acoustooptical modulators (AOM’s) with controllable rf powers. The pulses generated have nearly gaussian intensity distributions, which can very well be described by I850(t) =