# Breakdown of QPU Access Time¶

As illustrated in Figure 105, the time to execute a single QMI on a QPU, QPU access time, is broken into two parts: a one-time initialization step to program the QPU (blue) and typically multiple sampling times for the actual execution on the QPU (repeated multicolor).

Fig. 105 Detail of QPU access time.

The QPU access time also includes some overhead:

$T = T_p + \Delta + T_s,$

where $T_P$ is the programming time, $T_s$ is the sampling time, and $\Delta$ (reported as qpu_access_overhead_time by SAPI) is an initialization time spent in low-level operations, roughly 1-2 ms for D-Wave 2000Q systems and 10-20 ms for Advantage systems.

The time for a single sample is further broken into anneal (the anneal proper; green), readout (read the sample from the QPU; red), and thermalization (wait for the QPU to regain its initial temperature; pink). Possible rounding errors mean that the sum of these times may not match the total sampling time reported.

$T_s / R \approx T_a + T_r + T_d,$

where $R$ is the number of reads, $T_a$ the single-sample annealing time, $T_r$ the single-sample readout time, and $T_d$ the single-sample delay time, which consists of the following optional components[1]:

$\begin{split}T_d = &readout\_thermalization \\ &+ reduce\_intersample\_correlation \\ &+ reinitialize\_state.\end{split}$
 [1] See descriptions of these components under Solver Parameters. The reinitialize_state is used only for reverse annealing.