Fig. 1: Atomic qubit control using lasers as the local oscillator (LO).

a For optical qubits, the laser light is resonant with the qubit transition. b For hyperfine qubits, the LO is derived from the beatnote between two closely spaced optical frequencies. For continuous-wave (CW) lasers, this is performed by off-set phase locking two lasers by the qubit frequency. For modelocked (ML) lasers, this is performed by interfering two frequency combs at the atom position, with one comb shifted in frequency such that the qubit is driven by the frequency difference between comb pairs. c Noise away from the laser carrier frequency leads to a time-dependent variation in the LO control field, which can itself be expressed as a power spectral density, as shown in (d). e The time-dependent noise causes perturbations in qubit evolution on the Bloch sphere. f Example servo loops for stabilising laser frequency noise. g The servo loops act to reduce noise within the servo bandwidth, while free-running noise at higher frequencies is unaffected.