Commonly used features

station.set_measurement()

Instead of repeatedly supplying the measure argument to the loops, especially if you have many parameters, which you want to keep identical over many measurements, you can use the Station to store default measurement parameters. This is done by running:

station.set_measurement(instrument.input1, instrument.input2)

Now you do not need to specify measure in loop1d, loop2d, loop2dUD.

Looping over time

Often, you will want to monitor your parameters’ evolution over time. To do this, make a custom ‘stepper’ Parameter to loop over, as outlined here.

Other kinds of actions

The primary purpose of measure and station.set_measurement() is of course to tell qcodes++ what to measure. However, what these functions are *really* doing is telling the loop to perform certain actions. If the action is a parameter, then the loop performs parameter() and stores the data. However, there are other options:

Tasks

Within loops, you can also perform a qc.Task, which is a method for introducing functions into the loop. For example, you could wait until your setup has reached a certain temperature before measuring the parameters using

def waitfortemp(setpoint):
    while numpy.abs(instrument.measured_temp()-setpoint)>0.01:
        time.sleep(5)

station.set_measurement(qc.Task(waitfortemp,2),param1,param2)

The loop will wait at each point for the temperature to be within 10 mK of 2 K. Note how arguments to the functions within Task are passed: qc.Task(waitfortemp,0.02), not qc.Task(waitfortemp(0.02)). If the function accepts no arguments, i.e. def func_with_no_args():, simply write qc.Task(func_with_no_args). e.g. wait for the temperature to stabilise within 10 mK no matter the setpoint:

def waitfortemp():
    while numpy.abs(instrument.measured_temp()-instrument.setpoint())>0.01:
        time.sleep(5)

station.set_measurement(qc.Task(waitfortemp),param1,param2)

In loop2d and loop2dUD, you can also to perform actions at each step of the step_parameter by using the step_action argument. For example, if you want to wait for the temperature to stabilise at each step of the step_parameter, you can do:

loop=qc.loop2d(sweep_parameter=instrument.output1,
                start=0,stop=10,num=11,delay=0.1,
                step_parameter=instrument.temperate_setpoint,
                step_start=0,step_stop=10,step_num=11,step_delay=0.1,
                device_info='dummy',
                instrument_info='ACdiv=1e5 DCdiv=1e3 freq=123 Hz',
                measure=[instrument.input1, instrument.input2, instrument.measured_temp],
                step_action=qc.Task(waitfortemp))

Note how in this case, the task is not a part of the measure argument, (nor station.set_measurement()), since we only want to run the task immediately after the step_parameter is set, not at every point of the sweep_parameter.

BreakIf

qc.BreakIf allows the loop to be automatically stopped if a certain condition occurs. For example, if you are supplying a gate voltage and monitoring leakage current, and want to ensure leakage current doesn’t exceed a certain value.

station.set_measurement(qc.BreakIf(lambda: np.abs(k1.curr())>2e-9),param1,param2)

lambda is used to turn the statement k1.curr()>2e-9 into a function with a Boolean output. You can also include BreakIf directly in the measure argument. You can also use or, and, etc

station.set_measurement(qc.BreakIf(lambda: np.abs(k1.curr())>2e-9 or np.abs(k2.curr())>1e-9))

If the loop gets broken during loop.run(), loop.was_broken turns True. You can use this fact to automatically do something smart in the case of the loop breaking.

loop.run()
if loop.was_broken:
    k1.volt.move(0)

See the page on Parameters for more information about the move function.