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Physical Review A

Experimental study of the coherence of a terahertz-separated metastable-state qubit in C40a+

Kenji Toyoda, H. Shiibara, Shinsuke Haze, Rekishu Yamazaki, S. Urabe

Published Feb 18, 2009• 11 citations
Publisher

Abstract

Relaxation of the off-diagonal density-matrix elements between the metastable $3\text{ }{^{2}D}_{3/2}$ and $3\text{ }{^{2}D}_{5/2}$ states in a single ${^{40}\text{C}\text{a}}^{+}$, which are separated from each other by 1.82 THz, is studied experimentally by using the Ramsey method of separated oscillating fields. The two metastable states are connected with a stimulated Raman transition by using two infrared lasers, which are phase locked with their frequency difference bridged by an optical frequency comb. By performing Ramsey measurements for two Zeeman components of the stimulated Raman transition with different sensitivities to magnetic fields, the coherence is investigated quantitatively and the effect of magnetic-field fluctuations on that is evaluated. By using a spin-echo $\ensuremath{\pi}$ pulse in a Ramsey sequence which is effective for inhomogeneous broadening processes, a visibility decay time of 5.1 ms is obtained. This is more than twice as long as that for normal sequences. This work is an essential step for realization of a qubit using the two metastable states and demonstrates coherent manipulation of the internal states separated by around a terahertz using phase-locked light sources.

DOI
10.1103/physreva.79.023419

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We describe the results of stimulated Raman spectroscopy between the fine-structure levels, specifically 3d2D3/23{d}^{2}{D}_{3/2}3d2D3/2​ and 3d2D5/23{d}^{2}{D}_{5/2}3d2D5/2​ metastable states, in a single trapped 40Ca+{^{40}\text{C}\text{a}}^{+}40Ca+. Transitions between different Zeeman sublevels, split by an applied magnetic field, are clearly observed and the fine-structure level separation of $\ensuremath{\nu}=1\text{ }819\text{ }599\text{ }021\text{ }504\ifmmode\pm\else\textpm\fi{}37\text{ }\text{Hz}$ is determined. This measurement provides a direct observation of this level separation. From the spectroscopic data, a prospect of using the DDD-fine structure levels as a qubit for the quantum information processing is reviewed.
We report on the first realization of a novel neutral atom qubit encoded in the spin-orbit coupled metastable states ^{3}P_{0} and ^{3}P_{2} of a single ^{88}Sr atom trapped in an optical tweezer. Raman coupling of the qubit states promises rapid single-qubit rotations on par with the fast Rydberg-mediated two-body gates. We demonstrate preparation, readout, and coherent control of the qubit. In addition to driving Rabi oscillations bridging an energy gap of more than 17 THz using a pair of phase-locked clock lasers, we also carry out Ramsey spectroscopy to extract the transverse qubit coherence time T_{2}. When the tweezer is tuned into magic trapping conditions, which is achieved in our setup by tuning the tensor polarizability of the ^{3}P_{2} state via an external control magnetic field, we measure T_{2}=1.2 ms. A microscopic quantum mechanical model is used to simulate our experiments including dominant noise sources. We identify the main constraints limiting the observed coherence time and project improvements to our system in the immediate future. Our Letter opens the door for a so-far-unexplored qubit encoding concept for neutral atom-based quantum computing.
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Abstract We report on interference studies in the internal and external degrees of freedom of metastable triplet helium atoms trapped near quantum degeneracy in a $$ 1.5\kern0.166667em \upmu \mathrm{m} $$ 1.5 μ m optical dipole trap. Applying a single π/2 \pi /2 π/2 π / 2 rf pulse we demonstrate that 50% of the atoms initially in the m=+1 m=+1 m=+1 m = + 1 state can be transferred to the magnetic field insensitive m=0 m=0 m=0 m = 0 state. Two π/2 \pi /2 π/2 π / 2 pulses with varying time delay allow a Ramsey-type measurement of the Zeeman shift for a high precision measurement of the 23S1 {2}^3{S}_1 23S1​ 2 3 S 1 – 21S0 {2}^1{S}_0 21S0​ 2 1 S 0 transition frequency. We show that this method also allows strong suppression of mean-field effects on the measurement of the Zeeman shift, which is necessary to reach the accuracy goal of 0.1 kHz on the absolute transition frequencies. Theoretically the feasibility of using metastable triplet helium atoms in the m=0 m=0 m=0 m = 0 state for atom interferometry is studied demonstrating favorable conditions, compared to the alkali atoms that are used traditionally, for a non-QED determination of the fine structure constant.
Evidence of coherence effects has been observed in the Zeeman sublevels by means of nonlinear level crossing in a metastable Ca beam with resonance width 3 orders of magnitude less than the radiative linewidth of the investigated transitions. By using two separate traveling waves, Ramsey-like fringes have been observed. The final measured widths are determined by the transit time, are weakly dependent on the magnetic-field inhomogeneities, and are independent of the natural width of the optical transitions and the laser bandwidth.