Quantum mechanics is a cornerstone of modern physics. Just as the 19th century was called the Machine Age and the 20th century the Information Age, the 21st century promises to go down in history as the Quantum Age. Yet a fundamental question remains: can we claim to fully understand quantum mechanical principles, and how much do we really believe of what we know?
Answers to such questions require us to revisit the foundational postulates of quantum mechanics and perform precise theoretical and experimental investigations, in order to come up with the right bounds. In our group, a part of our focus is to attempt such investigations using single light particles — single photons — as our primary tool.
Such tests carry great importance in the state-of-the-art theoretical physics scenario, where considerable effort is being devoted to the unification of quantum mechanics and general relativity. These unification attempts would also benefit from a more precise understanding of the principles involved in at least one of the theories — quantum mechanics. Our investigations in this direction include studies of entanglement dynamics, the superposition principle, higher-order interference effects, weak interactions, and quantum measurements.
Quantum mechanics is usually described in terms of the state of a system at a particular time and the outcomes obtained when we measure it. A complementary viewpoint is to describe a quantum system through its possible histories, meaning the different ways it can evolve between preparation and detection. Quantum Measure Theory assigns a generalized measure to sets of such histories, incorporating the interference between them. Unlike an ordinary probability, this quantum measure can exceed one. The difficulty is that a general set of histories does not necessarily correspond to a sequence of ordinary measurements, so its quantum measure is not straightforward to access experimentally. Here we overcome this problem using an ancilla-assisted optical event-filter. In our photonic implementation, polarization is used to tag, select and coherently recombine a chosen set of path histories. The output of the filter is an ordinary detector probability, which always remains between zero and one, but is related by a known factor to the quantum measure of the selected history-event. For the event studied here, the ideal quantum measure is 1.25. Experimentally, we obtain a median value of approximately 1.17, in close agreement with a realistic theoretical model of the apparatus, which predicts about 1.18, and clearly above the classical probability bound of one. The significance goes beyond observing a number greater than one. The experiment gives operational access to a quantity associated with an entire quantum history-event, rather than simply with a final measurement outcome. Histories-based formulations are also natural in attempts to formulate quantum physics for spacetime and quantum gravity, although the present work is a quantum-optics experiment and does not test quantum gravity.
If an unconventional theoretical concept is to play a role in meeting the foundational challenges of quantum theory, it seems important to bring it into contact with experiment as much as possible. Our experiment does this for the quantum measure.
We perform precision tests of the foundational postulates of quantum mechanics including Born's rule, the superposition principle, and the limits of quantum measurement using carefully engineered single-photon states. These experiments directly probe questions at the boundary of quantum theory and probe limits that are relevant to quantum gravity and beyond-standard-model physics.
We study how entanglement the quintessential quantum resource evolves in time under various conditions, including decoherence, environmental noise, and active control. Understanding and controlling entanglement dynamics is key to both fundamental tests of quantum mechanics and practical quantum information tasks.
Standard quantum mechanics predicts that interference occurs only up to second order (Born's rule). We test this prediction rigorously with multi-path single-photon experiments, and also leverage multi-path architectures for quantum information processing tasks using higher-order quantum correlations.
