We introduce a theoretical framework to study experimental physics using quantum complexity theory. This allows us to address: what is the computational complexity of an experiment? For several ‘model’ experiments, we prove that there is an exponential savings in resources if the experimentalist can entangle apparatuses with experimental samples. A novel example is the experimental task of determining the symmetry class of a time evolution operator for a quantum many-body system. Some of our complexity advantages have been realized on Google’s Sycamore processor, demonstrating a real-world advantage for learning algorithms with a quantum memory.
References: ArXiv:2111.05881 ArXiv:2111.05874 ArXiv:2112.00778
The question of how heat is persistently transported from the Sun’s photosphere (at about 6,000 K) to the much hotter corona (at about 10^6 K) is one of the great open puzzles in astrophysics. Using the quantum Markovian master equation, we show that convection in the stellar photosphere generates plasma waves by an irreversible process akin to Zeldovich superradiance and sonic booms. In the Sun, this mechanism is most efficient in quiet regions with small magnetic fields. Energy is mostly carried by megahertz Alfven waves that scatter elastically until they reach a height at which they can dissipate via mode conversion. This model gives the right power flux for coronal heating and may account for “chromospheric evaporation” leading to impulsive heat transport into the corona.
On the characterisation of quantum correlations: quantum steering and entanglement
Date: Monday, April 4, 2022
Host: Quantum Chaos and Quantum Information (Jagiellonian University)
Speaker: Erik Aurell (KTH Royal Institute of Technology, Stockholm)
A well-studied model in open quantum system theory is a system interacting with a thermal bath of harmonic oscillators at finite temperature. This provides a quantum mechanical model of a classical resistive element in a circuit, and includes as famous examples the Caldeira-Leggett theory of quantum Brownian motion, and the “spin-boson model”. Such environments however also include baths of thermal photons and phonons, and putative baths of gravitons. As long as the environment consists of harmonic oscillators interacting linearly with the system, and starting in a thermal state, the environmental degrees of freedom can be integrated out using the Feynman-Vernon method.
I will first present the open system dynamics of a test particle interacting linearly with a thermal bath of photons, following  and . I will then discuss the resulting energy change of the bath (quantum heat) using the Feynman-Vernon approach. I will discuss what one can say if the bath temperature is very low or zero, i.e. if the test particle interacts with the electromagnetic vacuum.
I will then consider a test particle interacting with a gravitational field quantized in the weak-field (linear) approximation. I will review the recent theory of Parikh, Wilczek and Zahariade  describing an arm of a gravitational wave detector interacting with this kind of quantized gravitational field. Following Parikh et al I will show that an effective nonlinear friction force follows analogously to the way ordinary friction appears in the Caldeira-Leggett theory. I will discuss the random force from the vacuum on the test particle, and the heating of such a gravitational vacuum by the interaction with the test particle.
I will end by discussing what this says or does not say about the entropy production in the electro-magnetic vacuum and gravitational vacuum.
 Heinz-Peter Breuer and Francesco Petruccione, “Destruction of quantum coherence through emission of bremsstrahlung”, Phys. Rev. A 63, 032102 (2001)
 Heinz-Peter Breuer and Francesco Petruccione, Theory of Open Quantum Systems (2002), Chapter 12
 Maulik Parikh, Frank Wilczek and George Zahariade, “Signatures of the quantization of gravity at gravitational wave detectors”, Phys. Rev. D 104, 046021 (2021)
Towards reconciliation of completely positive open system dynamics with equilibration postulate
Date: Wednesday, March 30, 2022
Host: Quantum Information and Quantum Computing Working Group (CTP PAS)
We introduce operational distance measures between quantum states, measurements, and channels based on their average-case distinguishability. To this end, we analyze the average Total Variation Distance (TVD) between statistics of quantum protocols in which quantum objects are intertwined with random circuits and subsequently measured in a computational basis. We show that for circuits forming approximate 4-designs, the average TVDs can be approximated by simple explicit functions of the underlying objects, which we call average-case distances. The so-defined distances capture average-case distinguishability via moderate-depth random quantum circuits and satisfy many natural properties. We apply them to analyze the effects of noise in quantum advantage experiments and in the context of efficient discrimination of high-dimensional quantum states and channels without quantum memory. Furthermore, based on analytical and numerical examples, we argue that average-case distances are better suited for assessing the quality of NISQ devices than conventional distance measures such as trace distance and the diamond norm.
The talk is based on recent preprints: arXiv:2112.14283 and arXiv:2112.14284.
Recently there appeared many works on modified Wigner’s Friend paradoxes, which suggest that quantum theory cannot consistently describe the scenario with many observers. In this presentation I will show an alternative approach to this problem, which indicates that the paradoxes are in fact apparent, and the source of confusion is the undefined status of the measurement process. The talk will be based on recently published work “Physics and Metaphysics of Wigner’s Friends: Even Performed Premeasurements Have No Results” by Marek Żukowski and Marcin Markiewicz, Phys. Rev. Lett. 126, 130402 (2021).
Examples of standing gravitational waves in general relativity
Speaker: Sebastian Szybka (Jagiellonian University)
Standing waves are a quite common phenomenon in physics.They are well understood in linear theories. In Einstein’s gravity, which is a nonlinear theory, the lack of superposition principle complicates studies. I will present exact solutions to Einstein equations that correspond to standing gravitational waves. They provide useful toy-models that allow to investigate the phenomenon.
Wave and particle realism in quantum delayed-choice experiments
Wheeler’s delayed-choice experiment, a scenario wherein a classical apparatus, typically an interferometer, is settled only after the quantum system has entered it, has corroborated the complementarity principle. However, the quantum version of Wheeler’s delayed-choice experiment has challenged the robustness of this principle. Based on the visibility at the output of a quantum-controlled interferometer, a conceptual framework has been put forward which detaches the notions of wave and particle from the quantum state.
In this talk, I will present our results concerning a quantum-controlled reality experiment, a slightly modified setup that is based on exchanging the causal order between the two main operations of the quantum Wheeler’s delayed-choice arrangement. We employed an operational criterion of physical realism to reveal a different state of affairs concerning the wave-and-particle behavior in this new setup.
An experimental proof-of-principle will be presented for a two-spin-1/2 system in an interferometric setup implemented in a nuclear magnetic resonance platform. Finally, it will be discussed how our results validate the complementarity principle.
A resource-theoretic approach to the thermodynamic arrow of time
Date: Monday, March 21, 2022
Host: Quantum Chaos and Quantum Information (Jagiellonian University)