Two-color STIRAP in trapped-ion quantum states

Q. A. Turchette, B. E. King, Christopher J. Myatt +6 more

Florian Gebert, Y Wan, Fabian Wolf +2 more

Efficient preparation and detection of the motional state of trapped ions is important in many experiments ranging from quantum computation to precision spectroscopy. We investigate the stimulated Raman adiabatic passage (STIRAP) technique for the manipulation of motional states in a trapped ion system. The presented technique uses a Raman coupling between two hyperfine ground states in 25 Mg + , implemented with delayed pulses, which removes a single phonon independent of the initial motional state. We show that for a thermal probability distribution of motional states the STIRAP population transfer is more efficient than a stimulated Raman Rabi pulse on a motional sideband. In contrast to previous implementations, a large detuning of more than 200 times the natural linewidth of the transition is used. This approach renders STIRAP suitable for atoms in which resonant laser fields would populate nearby fluorescing excited states and thus impede the STIRAP process. We use the technique to measure the wavefunction overlap of excited motional states with the motional ground state. This is an important application for force sensing applications using trapped ions, such as photon recoil spectroscopy, in which the signal is proportional to the depletion of motional ground state population. Furthermore, a determination of the ground state population enables a simple measurement of the ion's temperature.

P. E. Toschek, Sascha Wallentowitz

Jens Lykke Sørensen, Ditte Møller, Theis F. Q. Iversen +5 more

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 40 Ca + ions. The transfer from the D 3/2 to the D 5/2 state, is detected by a fluorescence measurement revealing the population not transferred to the D 5/2 state. A coherent population transfer efficiency at the level of 95% in a set-up allowing for the internal state detection of individual ions in a string has been obtained.

Chitra Rangan

Theories of quantum control have, until recently, made the assumption that the Hilbert space of a quantum system can be truncated to finite dimensions. Such truncations, which can be achieved for most quantum systems via bandwidth restrictions, have enabled the development of a rich variety of quantum control and optimal control schemes. Recent studies in quantum information processing have addressed the control of infinite-dimensional quantum systems such as the quantum states of a trapped-ion. Controllability in an infinite-dimensional quantum system is hard to prove with conventional methods, and infinite-dimensional systems provide unique challenges in designing control fields. In this paper, we will discuss the control of a popular system for quantum computing the trapped-ion qubit. This system, modeled by a spin-half particle coupled to a quantized harmonic oscillator, is an example for a surprisingly rich variety of control problems. We will show how this infinite-dimensional quantum system can be examined via the lens of the Finite Controllability Theorem, two-color STIRAP, the generalized Heisenberg system, etc. These results are important from the viewpoint of developing more efficient quantum control protocols, particularly in quantum computing systems. This work shows how one can expand the scope of quantum control research to beyond that of finite-dimensional quantum systems.

K. N. Zlatanov, M. Mallweger, M. Hennrich +1 more

The strong dipole-dipole interaction of trapped Rydberg ions offers the possibility of sub-microsecond entanglement gates. For example a two-qubit Control-Phase gate in 88 Sr + ions can be realized, by simultaneous excitation to the Rydberg states via stimulated Raman adiabatic passage (STIRAP) with simultaneous microwave induced dipole-dipole interaction. We show that this excitation protocol distorts the dark-state of the STIRAP stage and is prone to decay from the intermediate state. Here, we propose a novel pulse ordering, in which the STIRAP and the microwave dressing of the Rydberg states occurs in separate stages, preventing mutual interference effects that are detrimental to the gate fidelity. We show that, for experimentally feasible parameters, the proposed excitation scheme can achieve a fidelity of 99.93%, surpassing the experimentally demonstrated gate. In addition, we demonstrate a non-adiabatic speed-up to 400 ns by employing asymmetric pulse shapes in the STIRAP stage. The entangling phase is then controlled solely through the interaction strength by nonresonant asymmetric chirping of the microwave field.