1.3. Pulse Instantiation

As already briefly mentioned in Pulse Templates, instantiation of pulses is the process of obtaining a hardware interpretable representation of a concrete pulse ready for execution from the quite high-level PulseTemplate object tree structure that defines parameterizable pulses in qupulse.

The entry point is the PulseTemplate.create_program() method of the PulseTemplate hierarchy. It accepts the pulse parameters, and allows to rename and/or omit channels or measurements. It checks that the provided parameters and mappings are consistent and meet the optionally defined parameter constraints of the pulse template. The translation target is defined by the ProgramBuilder argument.

Each pulse template knows what program builder methods to call to translate itself. For example, the ConstantPulseTemplate calls ProgramBuilder.hold_voltage() to hold a constant voltage for a defined amount of time while the SequncePulseTemplate forwards the program builder to the sub-templates in order. The resulting program is completely backend dependent.

Historically, there was only a single program type Loop which is still the default output type. As the time of this writing there is the additional LinSpaceProgram which allows for the efficient representation of linearly spaced voltage changes in arbitrary control structures. There is no established way to handle the latter yet. The following describes handling of Loop object only via the qupulse.hardware.HardwareSetup.

The Loop class was designed as a hardware-independent pulse program tree for waveform table based sequencers. Therefore, the translation into a hardware specific format is a two-step process which consists of the loop object creation as a first step and the transformation of that tree according to the needs of the hardware as a second step. However, the AWGs became more flexibly programmable over the years as discussed in How qupulse models AWGs.

The first step of this pulse instantiation is showcased in Instantiating Pulses: Obtaining Pulse Instances From Pulse Templates where PulseTemplate.create_program() is used to create a Loop program.

The second step of the instantiation is performed by the hardware backend and transparent to the user. Upon registering the pulse with the hardware backend via qupulse.hardware.HardwareSetup.register_program(), the backend will determine which hardware device is responsible for the channels defined in the pulse and delegate the Loop object to the corresponding device driver. The driver will then sample the pulse waveforms with its configured sample rate, flatten the program tree if required by the device and, finally, program the device and upload the sampled waveforms.

The flattening is device dependent because different devices allow for different levels of nested sequences and loops.

For example the Tabor Electronics WX2184C AWG supports two-fold nesting: waveforms into level-1 sequences, level-1 sequences into level-2 sequences. In consequence, the program tree is flattened to depth two, i.e., for all tree paths of larger depth, loops are unrolled and sequences of waveforms are merged into a single waveform until the target depth is reached. Additionally, the AWG requires waveforms to have a minimal length. Any waveform that is shorter is merged by the driver with its neighbors in the execution sequence until the minimum waveform length is reached. Further optimizations and merges (or splits) of waveforms for performance are also possible.

In contrast, the Zurich Instruments HDAWG allows arbitrary nesting levels and is only limited by the instruction cache. However, this device supports increment commands which allow the efficient representation of linear voltage sweeps which is not possible with the Loop class.

The section Instantiated Pulse: Program touches the ideas behind the current program implementations i.e. Loop and LinSpaceProgram.