Context: Recently, Scientists from Raman Research Insitute experimenting with new ways to manipulate quantum states so that they can be harnessed for computing, communication, and metrology, has found a novel way to characterise and estimate such states.
Key Points

• Scientists from the Raman Research Institute (RRI) have found a novel way to estimate and characterise quantum states.
• RRI is an autonomous institute under the Department of Science and Technology, GOI located in Bengaluru.
• Scientists experiment with new ways to manipulate quantum states so that they can be harnessed for computing, communication, and metrology.
• This new method called Quantum State Interferography, can help make such manipulations simpler so that several crucial operations in quantum technologies become less cumbersome.
• In this new method, the scientists have found a new way of inferring the state of a system (both two-dimensional qubits, the simplest quantum system as well as higher-dimensional “qubits”) from an interference pattern.

Problem with the current method of estimation

• The determination of an unknown quantum state is usually done by a method known as Quantum State Tomography (QST).
• This involves measuring the projection of the quantum state on various directions in state space and reconstructing the quantum state from the information obtained.
• However, in particular, scenarios where the dimensions are large, the operations needed to perform tomography increase quadratically.
• The experimental settings often need to be changed many times, thus making the process very cumbersome.

‘Black Box’ Approach

• The RRI team showed that without changing any settings in the experimental setup, it is possible to infer the unknown quantum state of a higher dimensional system.
• The setup requires only two interferometers from which many interferograms can be obtained to reconstruct the state.
• This provides a ‘black box’ approach to quantum state estimation – between the incidence of the photon and extraction of state information, conditions within the set-up are not changed, thus providing a true single-shot estimation of the quantum state.