Description
The spectral separation between resonant transitions of different ion species offers mutual isolation that can be advantageous for scaling trapped ion quantum computing systems, and numerous groups are now performing experiments with mixed-species crystals of trapped ions. In our group at NIST, trapped $^9$Be$^+$ and $^{24/25}$Mg$^+$ ions in a linear RF trap have been utilized to demonstrate sympathetic cooling, logic gates, gate teleportation, quantum-logic spectroscopy, and high-fidelity indirect readout. Here we discuss two further experiments on quantum control of mixed-species trapped-ion chains.
We modify electromagnetically-induced-transparency (EIT) cooling techniques to accommodate the hyperfine structure of $^{25}$Mg$^+$. EIT cooling, as originally conceived, is a broad-bandwidth, rapid, sub-Doppler cooling method suitable for three-level $\Lambda$ systems. EIT cooling also requires less laser power than far-detuned Raman sideband cooling. While the hyperfine levels of $^{25}$Mg$^+$ are suitable for the implementation of qubits, the presence of more than two ground levels (in contrast to $^{24}$Mg$^+$) requires modification of the original EIT cooling approach. We present experimental progress in applying EIT cooling to a single $^{25}$Mg$^+$ ion, theoretical understanding of limitations of the technique, and extensions to sympathetic cooling of mixed-species crystals. In particular, we show EIT cooling of a 1.2 MHz axial mode of a single $^{25}$Mg$^+$ ion to an average motional occupation number $\bar{n}$ < 0.4 in under 1 ms.
We have also coupled pairs of motional modes of various mixed- and same-species ion crystals using an electric potential that oscillates at the difference of two motional frequencies and has suitable spatial dependence. Mode coupling can be used for increased motional control and can aid in sympathetic cooling and quantum-logic spectroscopy. This coupling can enable certain ions within a mixed-species chain to cool and read out states of motional modes in which the addressed ions' participation is small or zero. We demonstrate this by using $^{25}$Mg$^+$ in a $^9$Be$^+$-$^{25}$Mg$^+$-$^9$Be$^+$ crystal to perform sympathetic cooling and repeated readout of the axial “Stretch” mode in which the center $^{25}$Mg$^+$ ion does not participate.
| Presenter name | Jenny Wu |
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| How will you attend ICAP-27? | I am planning on in-person attendance |
