for the noise. The noise is simply given by the sum of the quantum partitioning noise (first term) and the noise due to the dynamics of the impurity (second term). Note the crossover of the noise (42) from sub-Poissonian F ≤ 1 for a fast fluctuator with ℏ Γ ≫ qV [the first term in (42) dominates] to super-Poissonian F ≥ 1 when the fluctua- tor is slow ℏ Γ ≪ qV [the second term in (42) dominates], provided that G$_{1}$ = G$_{2}$ , see Fig. 2. In our model these rates are given as the basic input pa- rameters γ$_{ij}$ defining our two-level fluctuator. The con- dition of rare charge switching events corresponds to the requirement that ℏ Γ ≪ qV . In this limit Eq. (39) reduces to the result of the classical point contact, Eq. (17), with the conductances given by We observe that the statistics is dominated by the fluc- tuations of the impurity and the results derived in the previous sections remain valid in the case of a quantum wire when the classical conductance is replaced by the Landauer formula (40). However, our result Eq. (39) is also valid in the op- posite regime ℏ Γ ≫ qV , i.e., when the two-level fluctu- ator noise acts on a timescale which is fast compared to the partitioning noise. Then only the first term in (39) contributes and the cumulant generating function is the average of the expressions (32) for the quantum point contact, to be taken over the positions x$_{1}$$_{,}$$_{2}$ with weights given by the probabilities P eq 1 , $_{2}$. Having access to both regimes, it is possible to study the crossover from clas- sical noise (due to the two-level fluctuator) to quantum- partitioning noise (due to the quantum point contact). To this end, we calculate the long-time asymptotics of the first two moments of (37), for the average charge and for the zero-frequency noise due to the two-level fluc- tuator [second term in Eq. (42)]. At a bias voltage V ≃ 1 mV with a rate Γ ≃ 100 MHz, the noise due to the fluctuator is given by S$_{tl}$ (0) ≃ 10 − $^{28}$A$^{2}$s dominated by the quantum-partitioning noise with S$_{tl}$ (0) ≃ 10 − $^{27}$A$^{2}$s. Note that in experiments rates of the order of 10 − 100 kHz have been observed. 33,34 We expect that rates in the Tunneling of an electron between the two quantum dots with a rate Γ leads to a change in the conduc- tance. In Ref. 33, this change was of the order of 0 . 1 q $^{2}$/ ℏ . However, in order to being able to observe the crossover the capacitive coupling of the double dot to the quan- tum point contact should be reduced to a level such that G$_{1}$ − G$_{2}$ ≈ 0 . 001 q $^{2}$/ ℏ . This provides us with the estimate (using P eq 1 = P eq 2 = 1 / 2) when the system is tuned in the middle of a conductance step with T ≈ 1 / 2. Note that for bias voltages V ≥ 0 . 1 mV the quantum-partitioning noise is larger than the thermal noise floor. at 50 mK. The quantum-partitioning noise [first term in Eq. (42)] can be estimated as To observe the crossover, both the noise S$_{tl}$ (0) from the classical two-level fluctuator and the quantum- partitioning noise S$_{qp}$ (0) (at zero frequency) have to dominate over the thermal Nyquist-Johnson noise which is given by It is difficult to experimentally confirm the crossover from sub- to super-Poissonian noise as described in the previous section as the quantum-partitioning noise is typ- ically small and thus the noise of the classical two-level fluctuator will dominate. As a promising setup, we envi- sion coupling a quantum point contact in GaAs/AlGaAs to a double dot, e.g., in an InAs nanowire, which serves as tunable two-level fluctuator. Such a system was studied recently, see Ref. 33. 7 FIG. 2: Plot of the Fano factor F as a function of the bias voltage V for a single mode wire with P eq 1 = P eq 2 = 1 / 2, T 1 = 1, and T 2 = 0 , 0 . 1 , 0 . 3, and 0.5 . The Fano factor starts of at a value T 2 (1 − T 2 ) / (1+ T 2 ) < 1 for small voltages. In the opposite regime, it is given by [(1 − T 2 ) $^{2}$/ (1 + T 2 )] qV/ 2 π ℏ Γ which becomes superpoissonian for V large enough. S$_{tl}$ (0) = V $^{2}$( G$_{1}$ − G$_{2}$ ) 2 Γ ≃ V $^{2}$[mV$^{2}$] Γ[MHz] 10 − $^{26}$A$^{2}$s (45) S$_{qp}$ (0) ≈ q $^{3}$V 8 π ℏ ≃ V [mV]10 − $^{27}$A$^{2}$s (44) S$_{NJ}$ (0) ≈ 2 q $^{2}$k$_{B}$ T π ℏ ≃ 10 − $^{28}$A$^{2}$s (43) G$_{1}$$_{,}$$_{2}$ = q 2 2 π ℏ ∑ γ T γ 1 , $_{2}$. (40) 〈 Q 〉 qtl ≫ = V ( P eq 1 G$_{1}$ + P eq 2 G$_{2}$ ) t (41) 〈〈 Q $^{2}$〉〉 qtl ≫ = qV t q 2 2 π ℏ ∑ n,γ P eq n T γ $_{n}$(1 − T γ $_{n}$) + 2 P eq 1 P eq 2 V $^{2}$( G$_{1}$ − G$_{2}$ ) $^{2}$t Γ (42) VI. EXPERIMENTAL TEST F T$_{2}$ = 0 . 1 T$_{2}$ = 0 . 25 T$_{2}$ = 0 . 5 qV/ ℏ Γ T$_{2}$ = 0 0.1 10 0.1 1 10 1