Control Parameters

Key input parameters employed in defining controls

Single-Qubit Rabi Rate

The Rabi rate, denoted $\Omega(t)$, and modified Rabi rate, denoted $\Omega’(t)$ capture the instantaneous rate of rotation (in angular frequency) for resonantly-driven $(\Delta(t) = 0)$ and off-resonantly-driven $(\Delta(t) \ne 0)$ single-qubit driven operations respectively. This maps to the rate at which the Bloch vector rotates about the relevant control axis on the Bloch sphere. To see this, recall the single-qubit control Hamiltonian may be written

where

Or equivalently,

In this form, the instantaneous rotation generator $\hat{\boldsymbol{n}}(t)\boldsymbol{\sigma} \equiv n_x(t)\sigma_x+n_y(t)\sigma_y+n_z(t)\sigma_z$ drives the infinitesimal rotation $d\theta = \Omega’(t) dt$ about the rotation axis $\hat{\boldsymbol{n}}(t)\in\mathbb{R}^3$ at time $t$, with instantaneous rotational frequency

For the the general case of an off-resonant drive ($\Delta(t) \ne 0$) we obtain

In the limit of zero detuning ($\Delta(t) = 0$) the modified Rabi rate reduces to $\Omega’(t) \rightarrow \Omega(t)$, where the on-resonance Rabi rate satisfies

corresponding to the rotation rate about the axis

in the $xy$-plane (or equatorial plane) of the Bloch sphere, with orientation completely determined by the drive phase $\phi(t)$. This is consistent with the physical interpretation that the Rabi rate is proportional the amplitude of the resonant driving field. We note that in this definition the Rabi rate is strictly positive, with rotational direction determined by the orientation of $\hat{\boldsymbol{n}}(t)$.

The maximum possible amplitude of the driving field in a real hardware system establishes a maximum possible value of the on-resonance Rabi rate. We denote this upper bound as $\Omega_\text{max}$, and require

That is, the instantaneous Rabi rate over the duration of the control, $t\in[0,\tau]$, cannot exceed this upper bound. In particular, we require

where

defines the peak Rabi rate over the duration $t\in[0,\tau]$ of the control drive. therefore bounds the speed at which quantum operations may be driven due to the physical limitations of the control system. Control solutions which exceed these bounds cannot be physically implemented.

Single-Qubit Rotation Angle

In the case of a resonantly-driven rotation enacted over time $\tau$, with a constant Rabi rate $\Omega$ and drive phase $\phi$, we write the rotation operator

and refer to $\theta$ as the net rotation angle.

Two-Qubit Parametric Coupling Rate

For parametrically-driven two-qubit controls the parametric coupling rate, denoted $\Lambda(t)$, captures the rate of rotation (in angular frequency) for driving rotations associated with the two-qubit parametric operations . This is analogous to the single-qubit Rabi rate for single-qubit controls. We illustrate this below for iSWAP operations.

iSWAP Subspace

We consider control solutions implementing the control Hamiltonian , $H_{c}(t) = H_{c,\text{iSWAP}}(t) + H_{c,\text{qubit1}}(t)$, incorporating both parametric iSWAP controls, and single-qubit controls on qubit 1, where

The non-Hermitian operators and drive the iSWAP and single-qubit interactions respectively, and $\mathbb{I}$ is the identity associated with the 2-dimensional Hilbert space of qubit 2. The control parameters $\Lambda(t)$ and $\xi(t)$ for the parametric drive relate to the frequency and phase of the flux drive $\Phi(t)$, and are analogous to the single qubit Rabi rate and drive phase. The maximum value of the parametric coupling rate is constrained by limitations imposed by the hardware system. As with the maximum Rabi rate for single-qubit controls, we denote this upper bound as $\Lambda_\text{max}$, and require

That is, the instantaneous parametric coupling rate over the duration of the control, $t\in[0,\tau]$, cannot exceed this upper bound. In particular, we require

where

defines the peak parametric coupling rate over the duration $t\in[0,\tau]$ of the control drive. therefore bounds the speed at which quantum operations may be driven due to the physical limitations of the control system. Control solutions which exceed these bounds cannot be physically implemented.

Mølmer-Sørensen Rabi Rate

The Mølmer-Sørensen control Hamiltonian cast in the standard form used at Q-CTRL takes the form

where $\delta_{k}$ is the detuning of the $k$th mode from the laser; $\eta_{k}^{(\mu)}$ is Lamb-Dicke parameter capturing the mode-laser coupling; and $\Omega(t)$ and $\phi(t)$ are the Rabi rate and drive phase for resonantly-driven qubit transitions. The maximum possible amplitude of the driving field in a real hardware system establishes a maximum possible value of the on-resonance Rabi rate. We denote this upper bound as $\Omega_\text{max}$, and require

That is, the instantaneous Rabi rate over the duration of the control, $t\in[0,\tau]$, cannot exceed this upper bound. In particular, we require

where

defines the peak Rabi rate over the duration $t\in[0,\tau]$ of the control drive. therefore bounds the speed at which quantum operations may be driven due to the physical limitations of the control system. Control solutions which exceed these bounds cannot be physically implemented.