Bidirectional shot noise in a singly occupied channel

We calculate the power spectrum of the noisy current in a narrow channel that admits one charge carrier at a time from the left L or right R reservoirs. The random positive and negative currents are produced by the random arrival and departure of discrete charges ata measuring device on one side. The unidirectional currents are: not independent because the presence of one charge in the channel blocks the entrance of another. Four classes of trajectories are possible: the trans trajectories LR and RL and cis trajectories LL and RR. The power spectrum of the total current is described by explicit formulas, depending only on the statistics of the interarrival times and times spent in the channel. These formulas generalize those of shot (e.g., Schottky) noise and predict more complex behavior than the sum of the noises of the four types of trajectories, if they were independent: (i) the mean current and the intensity of fluctuations saturate as the arrival time on one side decreases and (ii) the noise intensity depends nonlinearly on the mean net current. Explicit formulas are given and special cases are analyzed.