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Output Idle Values

When a Quantum Machine (QM) is open, the hardware holds defined idle values on its output channels between jobs. Understanding what these idle values are, when they apply, and how they interact with runtime changes is important for controlling DC biases (e.g. flux lines) and digital markers reliably across a multi-job workflow.

What Are Idle Values?

Idle values are the hardware state the QM enforces on output channels between jobs:

  • Analog outputs — the DC offset (volts) defined in the controller config under analog_outputs[n]['offset'].
  • Digital outputs — the polarity state set by the 'inverted' flag in digital_outputs[n]. When inverted: True, the digital output holds HIGH between jobs (see Inverted Digital Marker).

QOP 3.4 or earlier

The idle-value lifecycle described in this tab applies to the OPX1000 with QOP or later. For earlier QOP versions, refer to the OPX+ tab.

Lifecycle

Event Hardware output Idle value
QM opened Set to the loaded idle value Loaded from controller's config
Between jobs Holds idle value Unchanged
Job starts Holds idle value unless overwritten Unchanged
Job/QUA API while Job is running Updated immediately Unchanged
qm.update_config() called during job execution Unchanged Updated
Job ends Reverts to idle value Unchanged
qm.update_config() called not during job execution Updates immediately Updated
QM closed Back to default values No longer applied

Runtime Changes Are Temporary

Any of the following changes the current hardware value during a job, but does not update the idle value:

  • set_dc_offset(element, input, offset) — called from inside a QUA program.
  • job.set_output_dc_offset_by_element(element, input, offset) — called via the Job API.
  • A controller config passed alongside a job at submission time (e.g. qm.execute(prog, controller_config=...)): the offset in that config applies only for that job.

When the job ends the hardware reverts to the idle value that was in effect before the job started.

Updating the Idle Value Permanently

To change the idle value without closing and re-opening the QM, use qm.update_config():

qm.update_config({
    "controllers": {
        "con1": {
            "fems": {
                1: {
                    "type": "LF",
                    "analog_outputs": {
                        1: {"offset": 0.25},  # new idle value
                    },
                }
            }
        }
    },
})

The new idle value takes effect between the current job and the next one. Already-compiled or already-running jobs are not affected.

Digital Channels

Digital output polarity follows the same pattern. The inverted flag in the controller config sets the idle state when the QM opens and this state is restored between jobs. Changing digital polarity at runtime does not persist beyond the job.

QOP 3.4 or earlier

Before QOP 3.5, OPX1000 behaved in the same way as the OPX+.

On the OPX+ the offset field in the config is used to initialise the DC offset when the QM is opened, but the persistence model is different from the OPX1000:

  • qm.set_output_dc_offset_by_element() is a QM-level call (not job-level). Changes persist between jobs for as long as the QM remains open.
  • There is no qm.update_config() API on the OPX+.
  • Closing the QM resets all DC offsets to zero, regardless of what value was active. To prevent this, pass keep_dc_offsets_when_closing=True to open_qm() (available from qm-qua 1.2.1 / QOP 2.4.2):
qm = qmm.open_qm(config, keep_dc_offsets_when_closing=True)

Note

keep_dc_offsets_when_closing is an OPX+ only feature and is not applicable to the OPX1000.

FAQ

When a job finishes, is the DC offset on the channel maintained?

Yes — on the OPX1000 (QOP) the channel reverts to its idle value, which is the offset defined in the controller config (or last set via qm.update_config()). On the OPX+ the offset set via qm.set_output_dc_offset_by_element() persists between jobs.

What happens to the DC offset when the QM is closed?

By default, closing the QM resets hardware DC offsets to zero (both OPX1000 and OPX+). On OPX+, you can keep the current offsets by opening the QM with keep_dc_offsets_when_closing=True.

I used job.set_output_dc_offset_by_element() during a job. Which offset will be present after the job ends?

The original idle value (from the controller config, or from the last qm.update_config() call) — not the value you set during the job. This API is OPX1000/QOP 3.x only.

How do I permanently change the DC offset without re-opening the QM?

Use qm.update_config() (OPX1000/QOP 3.x). On the OPX+ use qm.set_output_dc_offset_by_element(), which already persists across jobs.