Absolute_phase#
Each microwave output channel in the QCS can track phase
in one of two ways: consistent phase or coherent phase.
This is configured when adding virtual channels in a channel
mapper by addressing the argument of absolute_phase=True
or absolute_phase=False.
Consistent phase (Absolute_phase=True)#
If the channel is configured with absolute_phase=True then the
IF parameters (phase, frequency and amplitude) are encoded into the
waveform memory. Consequently, if the signal is downconverted and acquired
on a digitizer it will have the same absolute phase (phase of the demodulated
signal in the I/Q plane) in every shot. Because of this, absolute_phase=True
is typically used for readout channels where consistent phase is critical.
Note that subsequent pulses played on a channel with absolute_phase=True
will have the same absolute phase and therefore are not guaranteed to be
phase coherent with one another.
Multiple signals with different frequencies can be multiplexed on channels with
absolute_phase=True. There is no explicit limit on the number of tones
that can be multiplexed, but keep in mind that the combined amplitude of the
output signal must still fit in the output range of the AWG. As more tones are
combined the maximum amplitude of each tone is divided down.
Because all parameters of the pulse are encoded in the waveform memory for this channel type, hardware-accelerated scans are not possible for any parameter other than delay.
Coherent Phase (Absolute_phase=False)#
If the channel is configured with absolute_phase=False then the IF parameters
(phase, frequency and amplitude) are explicitly stored along with each waveform
and only the envelope is encoded in waveform memory. In this configuration, the
waveform is built in real time and the phase is tracked by a digital upconverter
(DUC) on the module’s FPGA.
In this configuration, subsequent pulses will be phase coherent with each other, making this the ideal configuration for qubit control channels. Because the DUC is free-running, acquiring the downconverted waveform on a digitizer will give a different phase each shot. This means that if the time trace is averaged over many shots it will generally tend toward zero.
A physical channel with absolute_phase=False can support up to 16 different
frequencies. Sequences can switch between different frequencies on this channel
without sacrificing phase coherence. In this mode, multiple frequencies cannot
be multiplexed to play back at the same time on a single physical channel.
Hardware sweeps of amplitude, phase, and frequency are only available for
absolute_phase=False channels.
Finally, note that for both absolute_phase=True and absolute_phase=False,
the IF waveform is upconverted by a microwave Numerically Controlled Oscillator
(NCO). This NCO has its phase reset at the start of an experiment but is
free-running from that point on. Because of this, if the microwave signal is
viewed on a high-frequency scope, it will show a different absolute phase
shot-to-shot. Consistent absolute phase is only observed when the microwave
signal passes through a QCS downconverter at the same frequency as the microwave
NCO. For this reason we recommend using the same Local Oscillator (LO)
frequency for downconverters and AWGs used for readout.