A long-lived memory qubit on a low-decoherence quantum bus

TL;DR

This paper demonstrates long-lived coherence in hyperfine and motional states of trapped-ion qubits, highlighting the potential for high-fidelity quantum logic operations with extended coherence times.

Contribution

It reports the first observation of a hyperfine qubit in extsuperscript{43}Ca extsuperscript{+} with coherence times vastly exceeding those of extsuperscript{40}Ca extsuperscript{+} qubits, and shows long coherence in motional states.

Findings

extsuperscript{43}Ca extsuperscript{+} hyperfine qubit has a T2 of 1.2 seconds.

Motional states have a T2' of 0.18 seconds, limited by heating rate.

Effective coherence time with spin-echo exceeds 45 seconds.

Abstract

We demonstrate long-lived coherence in internal hyperfine states of a single \Ca{43} trapped-ion qubit $[T_2=1.2(2)\s]$, and in external motional states of a single \Ca{40} trapped-ion qubit $[T_2'=0.18(4)\s]$, in the same apparatus. The motional decoherence rate is consistent with the heating rate, which was measured to be 3(1) quanta/sec. Long coherence times in the external motional states are essential for performing high-fidelity quantum logic gates between trapped-ion qubits. The internal-state $T_2$ time that we observe in \Ca{43}, which has not previously been used as a trapped-ion qubit, is about one thousand times longer than that of physical qubits based on \Ca{40} ions. Using a single spin-echo pulse to ``re-phase'' the internal state, we can detect no decoherence after 1\s, implying an effective coherence time $T_2^{\mbox{\tiny SE}} \gtish 45\s$. This compares with timescales in this trap for single-qubit operations of \ish 1\us, and for two-qubit operations of \ish 10\us.