Figure 2

The color circles indicate the occupied energy levels. Each green arrow represents a classical pulse, which is resonant with the transition between the two neighbor levels close to each green arrow. In (e) and (g), the sequence for applying the pulses is from top to bottom, and the lower pulses are turned on after the upper pulses are switched off. In (i), the sequence for applying the pulses is from bottom to top, and the upper pulses are turned on after the lower pulses are switched off. For the details on the applied pulses, see the descriptions given in the text. Note that in (a–j), the left levels are for qudit 1 while the right levels are for qudit 2. For simplicity, we here consider the case that the spacings between adjacent levels become narrow as the levels move up, which is actually unnecessary.