diff --git a/docs/src/theory.md b/docs/src/theory.md index be3923d..fbaa427 100644 --- a/docs/src/theory.md +++ b/docs/src/theory.md @@ -57,40 +57,40 @@ The orthogonal polynomial-based chain mapping and the subsequent representation ### Finite Temperature with T-TEDOPA -Assuming a unitary evolution for both the system and environment, the system's dynamics can be isolated by tracing out the environmental degrees of freedom. The density operator for the system at time $t$ is described as: +Assuming a unitary evolution for both the system and environment, the system's dynamics can be isolated by tracing out the environmental degrees of freedom. The density operator for the system at time $t$ is described as ```math \hat{\rho}_S(t) = \text{T}r_E\left\{\hat{U}(t) \hat{\rho}_S(0) \otimes \hat{\rho}_E(0) \hat{U}^\dagger(t)\right\}. ``` -Initially, the system state, $\hat{\rho}_S(0)$, can be pure or mixed, and the environment is in a thermal state defined by the inverse temperature $\beta = (k_B T)^{-1}$. This state is represented by a product of Gaussian states: +Initially, the system state, $\hat{\rho}_S(0)$, can be pure or mixed, and the environment is in a thermal state defined by the inverse temperature $\beta = (k_B T)^{-1}$. This state is represented by a product of Gaussian states ```math -\hat{\rho}_E(0) = \bigotimes_\omega \frac{e^{-\beta \omega \hat{b}_\omega^\dagger \hat{b}_\omega}}{Z_\omega(\beta)}, +\hat{\rho}_E(0) = \bigotimes_\omega \frac{e^{-\beta \omega \hat{a}_\omega^\dagger \hat{a}_\omega}}{Z_\omega(\beta)} . ``` -The system's evolution is dictated by the environment's two-time correlation function, which in turn is determined by the spectral density function $J$ and the temperature $\beta$: +The system's evolution is dictated by the environment's two-time correlation function $S(t)$, which in turn is determined by the spectral density function $J$ and the temperature $\beta$ via the Bose-Einstein distribution $n_\omega(\beta)$ ```math -\hat{S}(t) = \int_0^\infty d\omega J(\omega)\left[e^{-i\omega t}(1 + \hat{n}_\omega(\beta)) + e^{i\omega t} \hat{n}_\omega(\beta)\right], +S(t) = \int_0^\infty d\omega J(\omega)\left[e^{-i\omega t}(1 + n_\omega(\beta)) + e^{i\omega t} n_\omega(\beta)\right] . ``` -To simulate finite temperature effects using a zero-temperature model with the T-TEDOPA method [^tamascelli_efficient_2019], we extend the spectral density function to cover both positive and negative frequencies, allowing us to use a pure state description for the environment. This is achieved by defining a new spectral density function $J(\omega, \beta)$ that incorporates the Boltzmann factors, supporting the entire real axis: +To simulate finite temperature effects using a zero-temperature model with the T-TEDOPA method [^tamascelli_efficient_2019], we extend the spectral density function to cover both positive and negative frequencies, allowing us to use a pure state description for the environment. This is achieved by defining a new spectral density function $J(\omega, \beta)$ that incorporates the Boltzmann factors, supporting the entire real axis ```math J(\omega, \beta) = \frac{\text{sign}(\omega)J(\left|\omega\right|)}{2} \Big(1 + \coth\Big(\frac{\beta \omega}{2}\Big)\Big). ``` -This modified bath allows us to maintain a pure state description of the environment, represented as a vacuum state, and avoid the computational complexities of density matrices: +This modified bath has the same correlation function $S(t)$ and thus allows us to maintain a pure state description of the environment, represented as a vacuum state, and avoid the computational complexities of density matrices ```math -\ket{\text{vac}} = \bigotimes_\omega \ket{0}_\omega, +\left|\text{vac}\right\rangle = \bigotimes_\omega \left|0_\omega\right\rangle . ``` The Hamiltonian of the system interacting with this extended bath now includes temperature-dependent interactions: ```math -\hat{H} = \hat{H}_S + \int_{-\infty}^{+\infty} d\omega \omega \hat{b}_\omega^\dagger \hat{b}_\omega + \frac{\hat{\sigma}_x}{2} \otimes \int_{-\infty}^{+\infty} d\omega \sqrt{J(\omega,\beta)}\left(\hat{b}_\omega^\dagger+\hat{b}_\omega\right), +\hat{H} = \hat{H}_S + \int_{-\infty}^{+\infty} d\omega \omega \hat{a}_\omega^\dagger \hat{a}_\omega + \frac{\hat{\sigma}_x}{2} \otimes \int_{-\infty}^{+\infty} d\omega \sqrt{J(\omega,\beta)}\left(\hat{a}_\omega^\dagger+\hat{a}_\omega\right), ``` This method simplifies the simulation of finite temperature effects by treating them within an effective zero-temperature framework, thereby keeping the computational advantages of using pure states.