# Taking tomographic measurements for photonic qubits 88 ns before they are created

## TL;DR

This paper demonstrates that quantum state tomography can be performed on photonic qubits before their creation using a time-reversed quantum teleportation protocol, achieving high fidelity measurements.

## Contribution

It introduces a novel method for performing tomographic measurements prior to qubit creation via a time-reversal protocol based on quantum teleportation.

## Key findings

- Achieved an average fidelity of 0.90 in reconstructed states.
- Obtained an average process fidelity of 0.84 for the time-reverse channel.
- Protocol is deterministic and does not require post-selection in principle.

## Abstract

We experimentally demonstrate that tomographic measurements can be performed for states of qubits before they are prepared. A variant of the quantum teleportation protocol is used as a channel between two instants in time, allowing measurements for polarisation states of photons to be implemented 88 ns before they are created. Measurement data taken at the early time and later unscrambled according to the results of the protocol's Bell measurements, produces density matrices with an average fidelity of $0.90 \pm 0.01$ against the ideal states of photons created at the later time. Process tomography of the time-reverse quantum channel finds an average process fidelity of $0.84 \pm 0.02$. While our proof-of-principle implementation necessitates some post-selection, the general protocol is deterministic and requires no post-selection to sift desired states and reject a larger ensemble.