# 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

## 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 $N_MN_U$, which are required to reduce this statistical error below a fixed value of ${\displaystyle \epsilon }$

## Related Papers

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