Figure 3: Echo-detected Ramsey fringes.

(a) Schematic pulse sequence showing the pump pulse 1 (solid red), pump pulse 2 (open red) and rephasing pulse 3 (open blue), and the echoes associated with each pump (grey). (b) The detected signal in the echo direction k_e for a laser frequency near resonance with the 1s(A₁)→2p₀ transition. The zero of the abscissa (τ₁₂) occurs when the pumps overlap in time, and the delay between pulses 2 and 3 (τ₂₃) was fixed at 50 ps. Black points: all three laser pulses unblocked; orange line: pulse 2 blocked showing the two-pulse echo due to pulses 1 and 3. In the black data, there is increased scatter near τ₁₂=0 (and, less obviously, near τ₁₂=−50), and this is due to interference fringes, not noise, as shown in other panels. (c) The autocorrelation of pulses 1 and 2 with the laser off-resonance (pulse 3 blocked). The inset shows a higher-resolution (step size 0.5 mm) version of the main figure (points) near τ₁₂=0, with a sine wave fit at the laser frequency (red line). The fringe contrast is ~98%, that is, the minimum is ~1% of maximum. (d) The envelope of the fringes in the echo-detected Ramsey fringe data (from b), produced with a Fourier transform filter centred on the laser frequency. Two Gaussian fits are shown corresponding to autocorrelation of scatter and the Ramsey fringes (blue and green lines, respectively). (e) The envelope of the fringes in the autocorrelation data and a Gaussian fit (red line). See Methods for details of filters, fits, discussion of Ricean noise and so on.