Fig. 1: Experimental setup for production, trapping and detection of hydrogen-like ¹¹⁸Sn⁴⁹⁺.

a, The highly charged ions are produced in the Heidelberg EBIT. By means of a room-temperature beamline, the ions are transported into the ALPHATRAP magnet. b, The cryovalve allows to maintain an ultrahigh vacuum within the trap chamber. c, The ‘trap stack’ of the experimental setup. The ions are captured in the capture section by pulsing the applied voltage at the moment the ions are in the trap. Below is the precision trap, a seven-electrode trap in which the frequency ratio Γ₀ = ν_L/ν_c is measured. An image-current detector is used to detect the particle motion in the trap. The voltage applied to the centre electrode is around −59 V. On the bottom of the trap stack, the analysis trap is located, which has a strong magnetic bottle, allowing the detection of the spin state of the bound electron. d, Fourier spectrum of the image-current detector with a ¹¹⁸Sn⁴⁹⁺ particle in resonance. Fitting this ‘dip’ gives the axial frequency of the particle. e, Axial frequency change (about 300 mHz) after flipping the electron spin by microwave irradiation at the Larmor frequency.