OPD - Operator Digital¶

The OPD is a separate box that enhances the OPX input with 10 more digital signals. These 10 inputs have programmable thresholds and are sampled at 1ns resolution. They are \(50 \Omega\) matched and accept up to 3.3V (LVTTL).

Usage¶

The OPD is directly connected to the OPX. When it is present, the input channels can be defined in the configuration file similarly to other channels. For example:

'controllers': {
        'con1'': {
            'type': 'opx1',
            "analog_outputs": {
                ...
            },
            'digital_inputs': {
                1: {'polarity': 'RISING', 'deadtime': 4, "threshold": 0.1},
                2: {'polarity': 'RISING', 'deadtime': 15, "threshold": 3.2},
            },
            'analog_inputs': {
                ...
            },
        },
    },

When defining a digital input, several parameters needs to be defined:

polarity - Whether it is triggered when RISING (rising edge) or FALLING (falling edge)
deadtime - Minimal time between pulses in ns. e.g., If it is set to 16ns, then only the 1st pulse out of two pulses 10ns apart will be detected. 'deadtime' should be between 4 and 16ns.
threshold - Voltage threshold

Once defined in the controller, it can be used in a quantum element as follows:

"elements": {
    "qubit": {
        "mixInputs": {
            ...
        },
        'digitalOutputs': {
            'out1': ('con1', 2)
        },
        ...
    },
},

Time Tagging¶

Time Tagging with the OPD is done similarly to a normal time tagging measurement:

times = declare(int, size=10)
counts = declare(int)
measure([pulse], [element], [stream], time_tagging.digital([times], [max_time], [counts], [element_output])

times is a vector of integers that is populated by the measurement.
max_time gives the maximum time window, in ns, during which the statement waits for tag arrival.
counts is a variable that is populated with the number of tags which arrived during the measurement.
element_output must be defined if the element has more than one digital output. In this case, it is a string indicating the output to be measured. It can be left empty when the element has only one input (default is empty string).

The time-tagging operation ends either at the set duration or when the times is fully populated (first one of the two).

Note that the information in times is only valid up to counts.

In addition, with the OPD you can directly count pulses:

counts = declare(int)
measure([pulse], [element], [stream], counting.digital([counts], [max_time], [element_outputs]))

counts is a variable that is populated with the number of pulses which arrived during the measurement.
max_time gives the maximum time window, in ns, during which the statement waits for incoming pulses.
element_outputs must be defined if the element has more than one digital input. In this case, it is either a string indicating the output to be measured or a tuple with list of strings corresponding to the outputs to be measured. It can be left empty when the element has only one input (default is empty string).

Wait for trigger¶

It is possible for any element to wait until a pulse arrives at one of the OPD inputs. This is done with the wait_for_trigger() function.

wait_for_trigger(element_waiting, pulse_to_play=None, trigger_element=None)

element_waiting is the element waiting.
pulse_to_play is the name of the pulse to play on the element while waiting for the external trigger. Must be a constant pulse. Can be None to play nothing while waiting.
trigger_element is an element with digital inputs, when one of his digital inputs is triggered, the element_waiting will continue. If trigger_element has multiple digital inputs, a specific one must be chosen: trigger_element can be a tuple, with the 1st item being the element name and the 2nd item being the trigger input. If trigger_element is left empty, then it waits for the trigger input in the back panel of the OPX.

Tip

See also the following guide on external triggering