In trapped ion quantum computing, while optical qubits have an energy difference of hundreds of THz, hyperfine qubits operate in the microwave regime ($1\sim 10$ THz). Due to this reason, hyperfine qubits have longer lifetimes (thinking in terms of the time-energy Heisenberg uncertainty principle). According to DiVincenzo's criteria, long coherence times are essential for reliable quantum computing. So is there any reason to pursue optical qubits instead of hyperfine qubits?


1 Answer 1


I just had this question myself and it lead me to a 2010 paper (which I just realized was written by a current professor of mine!) Hyperfine and optical barium ion qubits.

In that paper, they show that the same barium ion can be used as an optical qubit or a hyperfine qubit. The advantage for the optical regime being

...the laser wavelengths for the $6S_{1/2}$ to $5D_{5/2}$ infrared “shelving” transition as well as barium’s visible-wavelength cooling transition are the longest of any ionic qubit candidate, which makes it favorable for remote photonic coupling through optical fiber. This property is essential for long distance entanglement between ionic qubits [9,10], since the postselection entanglement process is mediated by emitted photons.


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