Talk by Prof. Urbasi Sinha


Revealing new facets in experimental quantum information processing with photons



When: July 20th 2022, 10:30 h
Location: Ernst-Mach Lecture Hall, 2nd floor, Boltzmangasse 5, 1090 Vienna
Hosted by: Markus Arndt



We cover two different aspects of experimental photonic quantum information processing that have been recently explored at the Quantum Information and Computing lab at RRI, Bangalore, India.

In the first part, we present the first loophole-free experiment wherein both the Leggett Garg inequality (LGI) as well as the WLGI inequalities have been decisively violated using single photons [1], thus providing a comprehensive refutation of the classical realist worldview along with measurements ensured to be non-invasive. This is the first experiment that closes/circumvents all known loopholes plaguing such experiments till date. Our results also demonstrate perfect matching of these observed violations with quantum-mechanical predictions incorporating experimental nonidealities, again not analysed in earlier such experiments. Our carefully designed strategies make this setup a powerful platform for harnessing this most general unambiguous signature of nonclassicality of single photon states towards various information theoretic applications wherein the single photon is a ubiquitous workhorse.

In the second part of the talk, we talk about our new quantum state estimation method, which we call Quantum State Interferography [2]. Quantum state tomography (QST) has been the traditional method for characterization of an unknown state. Recently, many direct measurement methods have been implemented to reconstruct the state in a resource efficient way. We present an interferometric method, in which any qubit state, whether mixed or pure, can be inferred from the visibility, phase shift, and average intensity of an interference pattern using a single-shot measurement—hence, we call it quantum state interferography. This provides us with a “black box” approach to quantum state estimation, wherein, between the incidence of the photon and extraction of state information, we are not changing any conditions within the setup, thus giving us a true single shot estimation of the quantum state. In contrast, standard QST requires at least two measurements for pure state qubit and at least three measurements for mixed state qubit reconstruction. We then go on to show that QSI is more resource efficient than QST for quantification of entanglement in pure bipartite qubits. We experimentally implement our method with high fidelity using the polarization degree of freedom of light. An extension of the scheme to pure states involving d−1 interferograms for d-dimensional systems (qudits) is also presented. Thus, the scaling gain is even more dramatic in the qudit scenario for our method, where, in contrast, standard QST, without any assumptions, scales roughly as d2.


[1] Loophole free interferometric test of macrorealism using heralded single photons,

K.Joarder, D.Saha, D.Home, U.Sinha, PRX Quantum, 3, 010307, 2022.

[2] Quantum State Interferography, S.Sahoo, S. Chakraborti, A.K.Pati, U.Sinha,

Phys. Rev. Lett. 125 123601, 2020.