Theory of interaction-dependent instability in quantum detection by means of Luttinger liquid tunnel junction: a rigorous theorem

The low-temperature regime of charge-qubit decoherence due to its Coulomb interaction with electrons tunneling through Luttinger liquid quantum-point contact (QPC) is investigated. The study is focused on quantum detector properties of Luttinger liquid QPC. It is shown, that in low-temperature limit the respective perturbative decoherence- and acquisition of information timescales both tend to diverge, thus, shadowing a true picture of low-temperature quantum detection for such quantum systems. Here I prove two general mathematical statements (S-theorem and S-lemma) about exact re-exponentiation of Keldysh-contour ordered T-exponent for arbitrary Luttinger liquid tunnel Hamiltonian. As the result, decoherence- and acquisition of information time-scales as well as QPC quantum detector efficiency rate are calculated exactly and are shown to have a dramatic dependence on repulsive interaction between electrons in 1D leads of QPC. Discovered abrupt decrease of QPC quantum detector efficiency Q with the increase of g in the close vicinity of value g_cr(T) represents a fingerprint of interaction-dependent instability of all the quantum detection procedure for any Luttinger liquid QPC quantum detector at definite low enough temperatures T_cr(g). The reasons behind these effects are discussed. Also, it is shown that such the low-temperature detection instability effect is able to explain a large unclear mismatch between expected and observed decoherence timescales in two well-known experiments (J.Gorman, D.G.Hasko, D.A.Williams, Phys.Rev.Lett., 95, 090502 (2005); K.D.Petersson, J.R.Petta, H.Lu, A.C.Gossard, Phys.Rev.Lett., 105, 246804 (2010);) on charge-qubit quantum dynamics.