# Difference between revisions of "Cross-Platform verification of Intermediate Scale Quantum Devices"

This protocol is used to perform cross-platform verification of quantum simulators and quantum computers. This is used to directly measure the overlap and purities of two quantum states prepared in two different physical platforms and thus used to measure the fidelity of two possibly mixed states. This protocol infers the cross-platform fidelity of two quantum states from statistical correlations between the randomized measurements performed on the two different devices.

## Assumptions

• There are no prior assumptions on the quantum states.
• The spin values for the two quantum devices are known.

## Outline

The aim here to perform cross-platform verification by measuring the overlap of quantum states produced with two different experimental setups, potentially realized on very different physical platforms, without any prior assumptions on the quantum states themselves. This can be used to whether two quantum devices have prepared the same quantum state. Here, the cross-platform fidelity is inferred from the statistical correlations between randomized measurements performed on the first and second device.

This protocol to measure the cross-platform fidelity of two quantum states requires only classical communication of random unitaries and measurement outcomes between the two platforms, with the experiments possibly taking place at very different points in time and space.

This protocol consists of the following steps:

• We start with two quantum devices which are based on different physical platforms, each consisting of two different spins. Two quantum operations are prepared in these quantum devices, which are each described by a density matrix.
• We find the reduced density matrices for the sub-systems of identical size for each device using partial trace operator over that sub-system.
• We apply a same random unitary is applied to the two quantum states. This random unitary is defined as the product of local random unitaries acting on all spins of the subsystem. Here, the local random unitaries are sampled independently from a unitary 2-design defined on the local Hilbert space and sent via classical communication to both devices.
• Now projective measurements in a computational basis are performed for both the systems.
• Repeating these measurements for the fixed random unitary provides us with the estimates of probability of measurement outcomes for the both the states.
• This entire procedure is then repeated for many different random unitaries.
• Finally we estimate the density matrix from the second order cross-correlations between the two platforms using the ensemble average of probabilities over random unitaries from the above procedure.
• The purities for the two sub systems are obtained as second-order auto-correlations of the probabilities.

## Notation

• ${\displaystyle N_{M}}$: Finite number of projective measurements performed per random unitary
• ${\displaystyle N_{U}}$: Finite number of random unitaries used to infer overlap
• ${\displaystyle \epsilon }$: Fixed value of statistical error
• ${\displaystyle S_{1},S_{2}}$: Two devices realised on different physical platforms
• ${\displaystyle N_{1},N_{2}}$: Spins consisted in ${\displaystyle S_{1}}$ and ${\displaystyle S_{2}}$ respectively
• ${\displaystyle U_{1},U_{2}}$: Quantum operation prepared in ${\displaystyle S_{1}}$ and ${\displaystyle S_{2}}$ respectively
• ${\displaystyle \rho _{1},\rho _{2}}$: Density matrices of ${\displaystyle U_{1}}$ and ${\displaystyle U_{2}}$ respectively
• ${\displaystyle \rho _{i,A_{i}}}$: Reduced density matrices
• ${\displaystyle A_{i}}$: Sub system of identical size ${\displaystyle N_{A}}$ where ${\displaystyle A_{i}\subseteq S_{i}}$
• ${\displaystyle N_{A}}$: Size of subset ${\displaystyle A_{i}}$. Subsets ${\displaystyle A_{1}}$ and ${\displaystyle A_{2}}$ have the equal size ${\displaystyle N_{A}}$
• ${\displaystyle D_{A}}$: Associated Hilbert space dimension, ${\displaystyle D_{A}=d^{N_{A}}}$
• ${\displaystyle U_{A}}$: Random unitary
• ${\displaystyle U_{k}}$: Local random unitaries acting on spins ${\displaystyle k=1,..,N_{A}}$. Here, the ${\displaystyle U_{k}}$ are sampled independently from a unitary 2-design defined on the local Hilbert space ${\displaystyle C^{d}}$
• ${\displaystyle k}$: Spin
• ${\displaystyle |s_{A}\rangle }$: This denotes a string of possible measurement outcomes for spins. ${\displaystyle |s_{A}\rangle =|s_{1},...,S_{N_{A}}\rangle }$
• ${\displaystyle P_{U}^{(i)}}$: Estimate of probabilities of measurement outcome for different spins.
• ${\displaystyle {\overline {P_{U}^{(i)}(s_{A})}}}$: The ensemble average over random unitaries of the form ${\displaystyle U_{A}}$
• ${\displaystyle D[s_{A},s_{A}']}$: Hamming distance between two strings ${\displaystyle s_{A},s_{A}'}$ is defined as the number of spins where ${\displaystyle s_{k}\neq s_{k}'}$ i.e. ${\displaystyle D[s_{A},s_{A}']=|\{k\in \{1,..,N_{A}\}|s_{k}\neq s_{k}'\}|}$
• ${\displaystyle F_{max}(\rho _{1},\rho _{2})}$: Cross platform fidelity between two quantum states

## Hardware Requirements

• Two quantum devices on two different physical platforms
• Trusted Measurement device.
• Classical communication channel

## Properties

• Figure of merit: Cross-platform fidelity of two quantum states
• We can estimate the density matrix overlap of two quantum states here as well as their purities.
• The present protocol scales, although exponentially, much more favorably with the (sub)system size, allowing practical cross-platform verification for (sub)systems involving tens of qubits on state-of-the-art quantum devices.
• This protocol can be used to perform fidelity estimation towards known target theoretical states, as an experiment-theory comparison.
• In practice, from a finite number of projective measurements performed per random unitary (${\displaystyle N_{M}}$), a statistical error of the estimated fidelity arises. With that, a finite number (${\displaystyle N_{U}}$) of random unitaries used to infer overlap and purities can also cause a statistical error while estimating fidelity. Therefore, the scaling of the total number of experimental runs ${\displaystyle N_{M}N_{U}}$, which are required to reduce this statistical error below a fixed value of ${\displaystyle \epsilon }$
• In the regime ${\displaystyle N_{M}\leq D_{A}}$ and ${\displaystyle N_{U}\gg 1}$, the average statistical error ${\displaystyle |[F_{max}(\rho _{A},\rho _{A})]_{e}-1|~1/(N_{M}{\sqrt {N_{U}}})}$. For unit target fidelity, the optimal allocation of the total measurement budget ${\displaystyle N_{U}N_{M}}$ is thus to keep ${\displaystyle N_{U}}$ small and fixed.
• The fidelity estimation of PR (entangled) states is thus less prone to statistical errors which we attribute to the fact that fluctuations across random unitaries are reduced due to the mixedness of the subsystems.
• The optimal allocation of ${\displaystyle N_{U}}$ vs. ${\displaystyle N_{M}}$ for given ${\displaystyle N_{U}N_{M}}$ depends on the quantum states, in particular their fidelity and the allowed statistical error ${\displaystyle \epsilon }$,and is thus a priori not known
• In larger quantum systems, it gives access to the fidelities of all possible subsystems up to a given size – determined by the accepted statistical error and the measurement budget – and thus enables a fine-grained comparison of large quantum systems.

## Protocol Description

Input: ${\displaystyle S_{1},S_{2}}$ with spins ${\displaystyle N_{1},N_{2}}$

Output: Cross platform fidelity ${\displaystyle F_{max}}$

• Prepare ${\displaystyle U_{1},U_{2}}$ with density matrices ${\displaystyle \rho _{1},\rho _{2}}$ in ${\displaystyle S_{1}}$ and ${\displaystyle S_{2}}$ respectively.
• Denote ${\displaystyle \rho _{i,A_{i}}}$ in ${\displaystyle D_{A}}$ as ${\displaystyle tr_{S_{i}/A_{i}}(\rho _{i})}$ for ${\displaystyle A_{i}\subseteq S_{i}(i=1,2)}$
• For ${\displaystyle 1,....,N_{u}}$
• For spin ${\displaystyle k=1,...,N_{A}}$:
• Sample ${\displaystyle U_{k}}$ independently from a unitary 2-design
• Send ${\displaystyle U_{k}}$ classically to ${\displaystyle S_{1}}$ and ${\displaystyle S_{2}}$
• For ${\displaystyle i=1,2}$:
• Apply ${\displaystyle U_{A}=\bigotimes _{k=1}^{N_{A}}U_{k}}$ to ${\displaystyle \rho _{i,A_{i}}}$ in ${\displaystyle S_{i}}$
• For ${\displaystyle 1,...,N_{M}}$:
• Perform projective measurements in a computational basis in ${\displaystyle S_{i}}$
• Obtain ${\displaystyle |s_{A}\rangle }$
• Get estimates of probabilities ${\displaystyle P_{U}^{(i)}(s_{A})=Tr_{A_{i}}[U_{A}\rho _{i,A_{i}}U_{A}^{\dagger }|s_{A}\rangle \langle s_{A}|]}$
• Obtain ${\displaystyle {\overline {P_{U}^{(i)}(s_{A})}}}$ and ${\displaystyle {\overline {P_{U}^{(j)}(s_{A})}}}$
• Define Tr${\displaystyle [\rho _{i,A_{i}},\rho _{j,A_{j}}]=d^{N_{A}}\sum _{s_{A},s_{A}'}(-d)^{-D[s_{A},s_{A}']}{\overline {P_{U}^{(i)}(s_{A})P_{U}^{(j)}(s_{A}')}}}$
• For density matrix overlap:
• Substitute ${\displaystyle i=1,j=2}$, to get Tr${\displaystyle [\rho _{1,A_{1}},\rho _{2,A_{2}}]}$
• For the purities of the quantum states:
• Substitute ${\displaystyle i=j=1}$ for Tr${\displaystyle \rho _{1,A_{1}}^{2}}$
• Substitute ${\displaystyle i=j=2}$ for Tr${\displaystyle \rho _{2,A_{2}}^{2}}$
• Calculate ${\displaystyle F_{max}}$ using: ${\displaystyle F_{max}={\frac {Tr[\rho _{1,A_{1}},\rho _{2,A_{2}}]}{max\{Tr\rho _{1,A_{1}}^{2},Tr\rho _{2,A_{2}}^{2}\}}}}$

## Further Information

• In principle, the cross-platform fidelity can be determined from full quantum state tomography of the two quantum devices. However due to the exponential scaling with the (sub)system size, this approach is limited to only a few degrees of freedom, In contrast, as demonstrated below, the present protocol scales, although exponentially, much more favorably with the (sub)system size, allowing practical cross-platform verification for (sub)systems involving tens of qubits on state-of-the-art quantum devices.
• Significantly fewer measurements are required here than full quantum state tomography.

## Related Papers

• A.Elben et al (2020) arXiv:1909.01282: Cross-Platform Verification of Intermediate Scale Quantum Devices
*contributed by Rhea Parekh