Publications

Indefinite order in quantum theory

[1]  Gravitational quantum switch on a superposition of spherical shells, N. S. M., B. Sahdo, N. Yokomizo, Quantum 8, 1248 (2024), arXiv

[2] Quantum switch in the gravity of Earth, N. S. M., B. Sahdo, N. Yokomizo, Physical Review A 104, 042414 (2021) arXiv

[3] Simulating indefinite causal order with Rindler observers, A. Dimić, M. Milivojević, D. Gočanin, N. S. M., Č. Brukner, Frontiers in Physics 8, 470 (2020) arXiv

Bose-Einstein condensation

[4] Bose-Einstein condensation on curved manifolds, N. S. M., F. E. A. dos Santos, V. S. Bagnato, A. Pelster, New Journal of Physics 22, 063059 (2020) arXiv

[5] Bose-Einstein condensates and the thin-shell limit in anisotropic bubble traps, E. J. P. Biral, N. S. M., A. Pelster, F. E. A. dos Santos, New Journal of Physics 26 013035 (2024) arXiv

Quantum many body systems

[6] A Shielding Property for Thermal Equilibrium States on the Quantum Ising Model, N. S. M., A. L. de Paula Jr, R. C. Drumond, Physical Review E 97, 032101 (2018) arXiv

[7] Bounding entanglement spreading after a local quench, R. C. Drumond, N. S. M., Physical Review A 95, 062301 (2017) arXiv

[8] Shielding Property in Higher Dimensions, N. S. M., A. L. de Paula Jr, C. Duarte, R. C. Drumond, arXiv:1901.08076

Quantum information

[9] Quantum bounds on multiplayer linear games and device-independent witness of genuine tripartite entanglement, G. Murta, R. Ramanathan, N. S. M., M. Terra Cunha, Physical Review A 93, 022305 (2016) arXiv

Highlights on indefinite order in quantum theory

Time in general relativity is dynamical and the ticking rates of clocks experiencing different gravitational fields are distinct. Thus, puzzling effects could happen in a theoretical configuration where a massive body is in a quantum superposition state of different positions. The idea of indefinite order comes from the possibility that, in these configurations, the temporal relations among events would be in a superposition state as well. For instance, two events could be in a superposition of being in the past and future of each other. It is suggested that these configurations could be tested operationally using the proper times measured by clocks lying on such quantum spacetime.

An important research goal nowadays is to find means compatible with the current technological capabilities that make these theoretical ideas possible to be tested. In my most recent papers, we show how to simulate an indefinite order in a quantum spacetime with feasible experiments.

In [2] we show how to produce an indefinite order using two clocks in an entangled state of two different accelerations in a Minkowski spacetime. From the equivalence principle, this setup can be understood as a simulation of indefinite temporal order due to a quantum spacetime generated by a delocalized black hole.

Another scenario that can possibly be simulated is that of our own planet in a delocalized state. If we arrange two clocks in an entangled state of being at two different heights on Earth, this simulates the expected time dilation effects as if Earth itself was delocalized for them. Then, they can be used to produce an indefinite temporal order. However, in the simpler situation in which the clocks remain at the same positions during the whole protocol, this simulation would take one year long, and it is not very practical. But in [1] we discovered that the duration of the protocol strongly depends on the movement of the clocks. In this reference, we found trajectories for the clocks in which the duration of the protocol is reduced to the order of seconds. This is the first realistic proposal of indefinite order that directly involves gravity. Even though it is just a simulation of a quantum spacetime, it is a real proposal for a test of the interface between quantum physics and general relativity, a field which still lacks experiments.

Beyond the experimental goals that we expect to be realized in the near future, these protocols raise foundational questions. Namely, we can discuss to what extent can a quantum clock really be used to measure time consistently, and whether it can be used to operationally characterize what is an event on spacetime. These fundamental topics are under investigation for future works.