On the day of the poster session (see the schedule tab), everyone should join the workshop Zoom meeting (the link will be sent by email to registered participants). Each poster presenter will be available in the breakout room whose number matches the one of the below list, and will be sharing their screen to interactively present their poster.
- Apadula, Luca
Title: Relativistic quantum reference frames
Abstract: The framework of Quantum Reference Frames (QRFs) is based on the assumption that no substantial difference should be raised between physical systems and frames of reference, implying that any system defines a viable physical perspective. A thorough formulation of the Galilean covariance within QRFs has been proposed, so we have extended the formalism to relativistic systems, taking in account relativistic scalar particles in a 1+1 dimensional spacetime. Specifically, upon adopting a “timeless” formulation of quantum mechanics, we investigated how the Lorentz group of transformations adapts to quantum frames, obtaining an extension of the original group. This allows for peculiar phenomena such as superposition of transformations, e.g. superposition of Lorentz Boosts, which shed light on a possible quantum understanding of spacetime. - Baumann, Veronika
Title: Non-causal Page Wootters circuits
Abstract: We combine the process matrix framework with a generalization of the Page-Wootters formalism in which one considers several observers, each with their own discrete quantum clock. It enables us to describe processes with indefinite casual order via a history state with multiple clocks. This imposes constraints on the implementability of process matrices into this framework and on the perspectives of the observers. We implement processes were the different definite causal orders are coherently controlled and explain why certain non-causal processes might not be implementable within this setting. - Baumeler, Ämin and Gilani, Amin Shiraz
Title: Unlimited non-causal correlations and their relation to non-locality
Abstract: We investigate the notion of causal order and ask how causal order can be derived in theories without causal order. While previous studies in the field of causal order (the process-matrix framework etc.) indicate that causal order is reestablished in the limit of multi-party interactions, we show the contrary: We present genuinely multi-party non-causal correlations for any number of parties. Therefore, causal order cannot be derived from logical consistency, reversibility, and from computational tameness (previous results), nor from multi-body interactions (this work). Here, moreover, we unveil tight connections between violations of causal order and violations of Bell inequalities. - Chen, Si
Title: Infinitesimal reference frames suffice to determine asymmetry properties of a quantum system
Abstract: Symmetry principles are fundamental in physics, and while they are well understood within Lagrangian mechanics, their impact on quantum channels has a range of open questions. The theory of asymmetry grew out of information-theoretic work on entanglement and quantum reference frames, and allows us to quantify the degree to which a quantum system encodes coordinates of a symmetry group. Recently a complete set of entropic conditions was found for asymmetry in terms of correlations relative to infinitely many quantum reference frames. However these conditions are difficult to use in practice and the physical implications unclear. Here we show that this set of conditions has extensive redundancy, and one can restrict to reference frames forming any closed surface in the state space that has the maximally mixed state in its interior. This in turn implies that asymmetry can be reduced to just a single entropic condition evaluated at the maximally mixed state. Contrary to intuition, this shows that we do not need macroscopic, classical reference frames to determine the asymmetry properties of a quantum system, but instead infinitesimally small frames suffice. Building on this analysis, we provide simple, closed conditions to estimate the minimal depolarization needed so as to make a given quantum state accessible under G-covariant quantum channels. - Cruzeiro, Emmanuel
Title: Quantum measurements, energy conservation and quantum clocks
Abstract: A spin chain extending from Alice to Bob with nearest neighbors interactions, initially in its ground state, is considered. Assuming that Bob measures the last spin of the chain, the energy of the spin chain has to increase, at least on average, due to the measurement disturbance. Presumably, the energy is provided by Bob’s measurement apparatus. Assuming that, simultaneously to Bob’s measurement, Alice measures the first spin, it is shown that either energy is not conserved, – implausible – or the projection postulate doesn’t apply, and that there is signalling. An explicit measurement model shows that energy is conserved (as expected), that the spin chain energy increase is not provided by the measurement apparatus(es), but instead by the clock, that the projection postulate is not always valid – illustrating the Wigner–Araki–Yanase (WAY) theorem – and that there is signalling, indeed. - De La Hamette, Anne-Catherine
Title: Quantum reference frames for general symmetry groups
Abstract: Treating reference frames as physical systems, subject to the laws of quantum mechanics, they become quantum reference frames. Located at the interplay of quantum and gravitational physics, their treatment marks an essential step towards the construction of a relational quantum theory. In this work, we introduce a relational formalism which identifies coordinate systems with elements of a symmetry group G. We define a general operator for reversibly changing between quantum reference frames associated to a group G. This generalises the known operator for translations and boosts to arbitrary finite and locally compact groups, including non-Abelian groups. - Del Santo, Flavio
Title: Generalized probability rules from a timeless formulation of Wigner’s friend scenarios
Abstract: The quantum measurement problem can be regarded as the tension between the two alternative dynamics prescribed by quantum mechanics: the unitary evolution of the wave function and the state-update rule (or “collapse”) at the instant a measurement takes place. The Wigner’s friend gedankenexperiment constitutes the paradoxical scenario in which different observers (one of whom is observed by the other) describe one and the same interaction differently, one –the Friend– via state-update and the other –Wigner– unitarily. This can lead Wigner and his Friend to assign different conditional probabilities to the outcome of the same measurement, given their respective observed results of a previous measurement. In this paper, we apply the Page-Wootters mechanism to give an a priori timeless description of Wigner’s friend-like scenarios, which allows Wigner and his Friend to unambiguously assign two-time conditional probabilities for the gedankenexperiment. We propose three rules to assign two-time conditional probabilities, all of which reduce to standard quantum theory for non-Wigner’s friend scenarios. However, when applied to the Wigner’s friend setup each rule assigns different conditional probabilities, potentially resolving the probability-assignment paradox in a different manner. Moreover, one rule imposes strict conditions on when a joint probability distribution for the measurement outcomes of Wigner and his Friend is well-defined. - De Paiva, Ismael Lucas
Title: Measurements of energy with quantum clocks: energy-time uncertainty relations and non-unitary evolution
Abstract: We use the Page and Wootters timeless framework to investigate the relative “flow of time” between internal and external clocks during energy measurements. In our analysis, we consider both the cases where external and internal systems carry out the measurement. As a result, we obtain a time-energy uncertainty relation for the internal clock when the measurement is performed by ex external subsystem. Moreover, we show that the evolution of systems given by the internal clock is non-unitary. - Flores, Philip Caesar
Title: Time and particles
Abstract: Standard quantum mechanics states that the appearance of a particle is the result of a position measurement which causes the wavefunction to collapse into a Dirac delta function whose support coincides with the observed location of the particle. However, progress on the theory of quantum time of arrival (TOA) operators in one dimension has established an unexpected connection with the collapse of the wavefunction on the appearance of a particle. We discuss this connection by virtue of the dynamics of the TOA eigenfunctions which evolve unitarily to localize at the intended arrival point at a time equal to its eigenvalue. This phenomenon implies that the appearance of a particle is a combination of the collapse of the initial wavefunction into one of the TOA eigenfunctions followed by the unitary Schrödinger evolution of the eigenfunction. It is then argued that particle appearance is the result of a time measurement rather than a position measurement. - Kabel, Viktoria
Title: Falling Through a Massive Superposition
Abstract: The current theories of quantum physics and general relativity on their own do not allow us to study situations in which the gravitational source is quantum. In particular, it is still unclear what the gravitational field sourced by a mass configuration in a spatial superposition looks like and how other objects behave in its vicinity. Here we propose a strategy which allows us to answer such questions utilizing quantum reference frame (QRF) transformations. In particular, we show that as long as the different positions of the mass configuration are related via isometries, it is possible to find a QRF in which the mass configuration becomes definite. The transformation to this frame is achieved via an extension of the current framework of QRFs to “quantum isometries” – superpositions of classical coordinate transformations, which leave the metric invariant. Assuming that the laws of physics are invariant under these transformations, we can change into a reference frame in which the mass configuration is definite and use the fact that we have a fixed spacetime background in this frame to determine the behaviour of other objects in its vicinity, before transforming back to the original frame. As a first application, we use this procedure to determine the motion of a test particle near the mass configuration. Furthermore, the same methods can be used to study the behaviour of clocks, allowing us to determine the time dilation caused by a gravitating object in superposition. - Móller, Natália
Title: Quantum switch in gravity: modeling quantum agents and their internal clocks
Abstract: Here we will present the results reported in arXiv:2012.03989 concerning the possibilities of a realization of a quantum switch in the gravity of Earth. The quantum switch is a task in which two non-commuting operations are applied in a target system in an indefinite order, aiming to represent an indefinite order of time. There are experimental realizations of this task, but they could be seen as simulations, since the role of the proper time of agents who are acting on the state of the target system is not taken into consideration. In our proposal, we explore the quantum switch task taking into consideration the role of the proper time of the agents and their relativistic behaviour in the gravity of Earth. We show that it is possible to arrange the protocol such that the parameters to realize this task are close to the current technology. Moreover, we build a model for a clock which measures the proper time of the agents, allowing the operations in the quantum switch to happen. In summary, we will discuss these possibilities of realization of the quantum switch in the gravity of Earth, and show a model for the clock and for the agents. - Plávala, Martin
Title: Operational Theories in Phase Space: Toy Model for the Harmonic Oscillator
Abstract: We construct a toy model for the harmonic oscillator that is neither classical nor quantum. The model features a discrete energy spectrum, a ground state with sharp position and momentum, an eigenstate with non-positive Wigner function as well as a state that has tunneling properties. The underlying formalism exploits that the Wigner-Weyl approach to quantum theory and the Hamilton formalism in classical theory can be formulated in the same operational language, which we then use to construct generalized theories with well-defined phase space. The toy model demonstrates that operational theories are a viable alternative to operator-based approaches for building physical theories. - Pretti Rossi, Vinicius
Title: Wigner’s friend and the quasi-ideal clock
Abstract: Eugine P. Wigner introduced a thought experiment that highlighted the incompatibility in quantum theory between unitary evolution and wave function reduction in a measurement. This work resulted in a class of thought experiments often called Wigner’s Friend Scenarios, and it recently came back to discussion. In this work, we ask what would happen if the isolated friend in a Wigner’s Friend Scenario did not share a time reference frame with the outer observer, and time should be tracked by a quantum clock. For this purpose, we translate the superobserver’s perspective to a Page-Wootters universe with a Woods-Silva-Oppenheim clock. Our results corroborate with the power of this model of clock, recovering the original WFS for proper clock states. Furthermore, the paradox persists even for an imprecise clock on the superobserver’s side despite the change on agents’ predictions, indicating the importance of deepening the discussion. - Quintino, Marco Túlio
Title: Universal quantum circuits for transforming unitary operations: exponential advantages with causality adaptive strategies and the power of indefinite causality.
Abstract: Given a quantum gate implementing a unitary operation U without any specific description but its dimension, we present a universal quantum circuit that implements its inverse by making k uses of the given operation. In a deterministic scenario, we present a circuit in which the fidelity of the output operation with the inverse of U approaches one exponentially. When considering probabilistic scenarios, our circuit provides the exact unitary inversion with a failure probability that decays exponentially. The protocols employ an adaptive strategy, proven necessary for the exponential performance. Then, we consider the general problem of transforming unitary operations when multiple uses are allowed with a special analysis for the case of unitary complex conjugation and unitary transposition. We then identify scenarios where adaptive circuits exponentially outperforms parallel ones and discuss scenarios where parallelization can be made with no loss in performance. Additionally, we contribute to the discussion on the power and limitations of indefinite causality. This is made by analysing the performance of processes where the use of the input-gates does not necessarily respect a definite causal order, a better performance may be obtained. - Sazim, Sk
Title: Classical communication with indefinite causal order for N completely depolarizing channels
Abstract: If two identical copies of a completely depolarizing channel are put into a superposition of their possible causal orders, they can transmit nonzero classical information. Here we study how well we can transmit classical information with N depolarizing channels put in superposition of M causal orders via a quantum switch. We calculate the Holevo quantity if the superposition uses only cyclic permutations of channels and find that it increases with M and it is independent of N. For a qubit it never reaches 1 if we are increasing M. On the other hand, the classical capacity decreases with dimension d of the message system. Further, for N= 3 and 4 we studied the superposition of all causal orders and uniformly superposed causal orders belonging to different cosets created by a cyclic permutation subgroup. - Vieira, Lucas B.
Title: Temporal correlations in the simplest measurement sequences
Abstract: We investigate temporal correlations in the simplest measurement scenario, i.e., that of a physical system on which the same measurement is performed at different times, producing a sequence of dichotomic outcomes. The resource for generating such sequences is the internal dimension, or memory, of the system. We characterize the minimum memory requirements for sequences to be obtained deterministically, and numerically investigate the probabilistic behavior below this memory threshold, in both classical and quantum scenarios. A particular class of sequences is found to offer an upper-bound for all other sequences, which suggests a nontrivial universal upper-bound of 1/e for the classical probability of realization of any sequence below this memory threshold. We further present evidence that no such nontrivial bound exists in the quantum case. - Zhang, Tian
Title: Quantum correlations in time
Abstract: We investigate quantum correlations in time in different approaches. We assume that temporal correlations should be treated in an even-handed manner with spatial correlations. We compare the pseudo-density matrix formalism with several other approaches: indefinite causal structures, consistent histories, generalised quantum games, out-of-time-order correlations(OTOCs), and path integrals. We establish close relationships among these space-time approaches in non-relativistic quantum theory, resulting in a unified picture. With the exception of amplitude-weighted correlations in the path integral formalism, in a given experiment, temporal correlations in the different approaches are operationally equivalent.