Phonon-induced dephasing of singlet-triplet superpositions in double quantum dots without spin-orbit coupling

Abstract

We show that singlet-triplet superpositions of two-electron spin states in a double quantum dot undergo a phonon-induced pure dephasing which relies only on the tunnel coupling between the dots and on the Pauli exclusion principle. As such, this dephasing process is independent of spin-orbit coupling or hyperfine interactions. The physical mechanism behind the dephasing is elastic phonon scattering, which persists to much lower temperatures than real phonon-induced transitions. Quantitative calculations performed for a lateral GaAs/AlGaAs gate-defined double quantum dot yield micro-second dephasing times at sub-Kelvin temperatures, which is consistent with experimental observations.

Phonon-Induced Pure Dephasing of Two-Electron Spin States in Vertical Quantum Dot Molecules

“…Although carrier-phonon coupling is generally stronger in self-assembled structures, the singlet-triplet dephasing rates are comparable to those for gated dots [7] at the same temperatures. This is due to much larger Coulomb energies which lead to increased energy separation from the doubly occupied states and thus reduce the rate of the second-order process.…”

“…Material parameters used may be found in Ref. [1,7]. The pure dephasing rates corresponding to the two--phonon virtual transition as a function of temperature are shown in the left part of Fig.…”

“…The explicit forms of these states before and after the diagonalization of the double quantum dot (DQD) Hamiltonian, as well as the Hamiltonian itself may be found in Ref. [7]. The carrier-phonon interaction is described by…”

“…In a recent work [7], we have shown that the carrier-phonon interaction which, apparently, affects only charge states, in fact destroys also two-electron spin coherence in tunnel--coupled QDs. This dephasing results from elastic phonon scattering and spin-dependent Pauli blocking of certain virtual transitions.…”

Hyperfine interaction induced dephasing of coupled spin qubits in semiconductor double quantum dots

“…44 The existence of such "sweet spots" was also predicted 53,54 in gated QDs. Besides charge noise, the different charge distributions for the singlet and triplet states also allow electron-phonon interaction to cause dephasing in the S-T 0 subspace, 52,55,56 although this dephasing channel should be weak in a double dot if J is not too large. 56 In addition to pure dephasing, the S-T 0 qubit can undergo longitudinal relaxation through phonon emission, which is allowed by spin-orbit coupling [57][58][59] and/or hyperfine mixing between the states.…”