A method of reducing the magnitude of a quasi-static electric dipole field at the null position of an oscillating electric quadrupole field of an ion trap, the method comprising: trapping one or more ions (102, 502) in a trapping electric field, wherein the trapping electric field comprises the oscillating electric quadrupole field and wherein the trapping electric field comprises an electric field amplitude which is a function of an electric field amplitude of the oscillating electric field; and inducing a change in an equilibrium position of one of the one or more trapped ions (102, 502) and measuring said change using an interferometry sequence comprising applying a first laser pulse to the one of the one or more trapped ions (102, 502) when the trapping electric field amplitude comprises a first trapping electric field amplitude; characterized in that the interferometry sequence comprises: applying a second laser pulse to the one of the one or more trapped ions when the trapping electric field amplitude comprises a second trapping electric field amplitude different from the first electric field amplitude; and measuring a state of the one of the one or more trapped ions after the application of the first and second laser pulses; and the method comprises: repeating the interferometry sequence a plurality of times in order to obtain a plurality of measurements of the state of the one or more trapped ions (102, 502); determining a probability that the one or more trapped ions (102, 502) change state during the interferometry sequence based on the plurality of measurements of the state of the one or more trapped ions (102, 502); and adjusting the trapping electric field based on the determined probability in order to reduce the magnitude of the quasi-static electric dipole field at the null position of the oscillating electric quadrupole field of the ion trap. trapping one or more ions (102, 502) in a trapping electric field, wherein the trapping electric field comprises the oscillating electric quadrupole field and wherein the trapping electric field comprises an electric field amplitude which is a function of an electric field amplitude of the oscillating electric field; and inducing a change in an equilibrium position of one of the one or more trapped ions (102, 502) and measuring said change using an interferometry sequence comprising applying a first laser pulse to the one of the one or more trapped ions (102, 502) when the trapping electric field amplitude comprises a first trapping electric field amplitude; characterized in that the interferometry sequence comprises: applying a second laser pulse to the one of the one or more trapped ions when the trapping electric field amplitude comprises a second trapping electric field amplitude different from the first electric field amplitude; and measuring a state of the one of the one or more trapped ions after the application of the first and second laser pulses; and applying a second laser pulse to the one of the one or more trapped ions when the trapping electric field amplitude comprises a second trapping electric field amplitude different from the first electric field amplitude; and measuring a state of the one of the one or more trapped ions after the application of the first and second laser pulses; and the method comprises: repeating the interferometry sequence a plurality of times in order to obtain a plurality of measurements of the state of the one or more trapped ions (102, 502); determining a probability that the one or more trapped ions (102, 502) change state during the interferometry sequence based on the plurality of measurements of the state of the one or more trapped ions (102, 502); and adjusting the trapping electric field based on the determined probability in order to reduce the magnitude of the quasi-static electric dipole field at the null position of the oscillating electric quadrupole field of the ion trap. repeating the interferometry sequence a plurality of times in order to obtain a plurality of measurements of the state of the one or more trapped ions (102, 502); determining a probability that the one or more trapped ions (102, 502) change state during the interferometry sequence based on the plurality of measurements of the state of the one or more trapped ions (102, 502); and adjusting the trapping electric field based on the determined probability in order to reduce the magnitude of the quasi-static electric dipole field at the null position of the oscillating electric quadrupole field of the ion trap.