@online{Daniel,

title = {Inflation versus Finner in the triangle network},

author = {Daniel Centeno Díaz},

year  = {2022},

date = {2022-01-02},

urldate = {2022-01-01},

abstract = {Standard Bell nonlocality studies correlations in a simple experiment where two space-like separated parties measure a physical system distributed by a source. Bell nonlocality in networks deals with experiments where several parties measure systems distributed by statistically independent sources organised in a network.  The parties choose freely and independently a measurement (called input) to perform on their respective share of the systems from which they obtain a result (called outcome). After many repetitions of the experiment, we can calculate the correlations between the measurement results of the different parties. It has been shown that the strength of the correlations achievable through the distribution of classical systems is bounded, and furthermore, this bound can be surpassed if we distribute quantum systems instead. Moreover, one can consider sources distributing systems beyond just classical or quantum, e.g., systems described by any general probabilistic theory (GPT) the goal in studying these more general scenarios is to find constraints on the correlations which can be generated depending on the nature of the distributed systems. In this work, we are interested in the relationship between two ways of obtaining constraints: the Finner inequality, a generalisation of Hölder inequality, and the inflation technique, a generic method to find constraints in networks. We focus on the well-known triangle network, in which we have three parties connected via a source pairwise, with no inputs, for a family of correlations. This group of correlations is the noisy GHZ distribution, P(abc)=p[000]+q[111]+(1-p-q)/6[other], where p and q are two parameters which fulfill p+q≤1. We know that when comparing an inflation of the triangle, which is a six-partite ring using two copies of the parties and sources of the triangle, with the Finner inequality, we find for almost every possible values of p and q that inflation is more restrictive. However, there is a region of values of p where the Finner inequality is more restrictive than the six-partite inflation. Moreover, it has been proven that when you take into account bigger inflations the restrictions you will find should be tighter to the set of distributions feasible in the original network. We are trying to understand how the inflation technique converges to Finner inequality in this specific case.

Supervisors: Marc-Olivier Renou, Gaël Massé.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{AdrianSolymos,

title = {Shareability of OO states},

author = {Adrian Solymos},

year  = {2022},

date = {2022-01-01},

abstract = {Unlike classical states, quantum states cannot necessarily be shared (extended) in such a way that the two-particle reduced states are all identical. More precisely, only the separable states are those that can be extended in such a way. The so-called shareability or extendibility number describes to how many parties a given state can be extended to. This is a good entanglement measure (i.e., a LOCC-monotone function), however, it has been calculated only for a few types of states. The poster presents the set of shareable OO-states for low shareability-number.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{AkshataShenoy,

title = {Satellite Based Quantum Communication},

author = {Akshata Shenoy},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {The goals and progress of a project aimed at advancement of quantum technologies to enable fundamental tests of quantum mechanics in space is presented. Specifically, we report the generation, manipulation, and control of high-quality entangled photon pairs for satellite quantum communication. The quality of the generated photon pairs onboard is characterised by the measurement of Bell parameter and state fidelity. This information acts as feedback to a microcontroller unit and enables active self-calibration of the entangled source with very less human intervention. The designed set-up will be used to implement the BBM92 protocol for secure key generation.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{ArindamMitra,

title = {Compatibility of quantum instruments},

author = {Arindam Mitra},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Incompatibility of quantum devices is a useful resource in various quantum information theoretical tasks, and it is at the heart of some fundamental features of quantum theory. While the incompatibility of measurements and quantum channels is well studied, the incompatibility of quantum instruments has not been explored in much detail. In this work, we revise a notion of instrument compatibility introduced in the literature that we call traditional compatibility. Then, we introduce the notion of parallel compatibility and show that these two notions are inequivalent. We then argue that the notion of traditional compatibility is conceptually incomplete and prove that, while parallel compatibility captures measurement and channel compatibility, traditional compatibility does not capture channel compatibility. Hence, we propose parallel compatibility as the conceptually complete definition of the compatibility of quantum instruments. 

 

Authors- Arindam Mitra and Máté Farkas 

 

J. ref. - Arindam Mitra and Máté Farkas, Phys. Rev. A 105, 052202 (2022) 

 Doi-https://doi.org/10.1103/PhysRevA.105.052202},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{BeataZjawin,

title = {Quantifying EPR: the resource theory of nonclassicality of common-cause assemblages},

author = {Beata Zjawin},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Einstein-Podolsky-Rosen (EPR) steering is often (implicitly or explicitly) taken to be evidence for spooky action-at-a-distance. An alternative perspective on steering - endorsed by EPR themselves - is that Alice has no causal influence on the physical state of Bob's system; rather, Alice merely updates her knowledge of the state of Bob's system by performing a measurement on a system correlated with his. In this work, we elaborate on this perspective (from which the very term `steering' is seen to be inappropriate), and we are led to a resource-theoretic treatment of correlations in EPR scenarios. For both bipartite and multipartite scenarios, we develop the resulting resource theory, wherein the free operations are local operations and shared randomness (LOSR). We show that resource conversion under free operations in this paradigm can be evaluated with a single instance of a semidefinite program, making the problem numerically tractable. Moreover, we find that the structure of the pre-order of resources features interesting properties, such as infinite families of incomparable resources. In showing this, we derive new EPR resource monotones. We also discuss advantages of our approach over a pre-existing proposal for a resource theory of `steering', and discuss how our approach sheds light on basic questions, such as which multipartite assemblages are classically explainable.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{BrunaGabriellydeMoraesAraújo,

title = {The universe as a quantum computation over continuous variables using a conformal field theory in 2 dimensions.},

author = {Bruna Gabrielly Moraes Araújo},

year  = {2022},

date = {2022-01-01},

abstract = {In this work, I will describe an ongoing research project which has as goal connect foundations of quantum mechanics with advanced tools of theoretical physics such as CFT and AdS/CFT correspondence. 

 

 

The investigation of some fundamental questions from both perspectives through a quantum computation based on quantum gates over continuous-variable like trapped ions systems, for example. 

 

The smaller blocks to build a computation are the gates, in quantum computation: the quantum gates. The universe can be looked at through continuous variables using the coherent states - the formalism also used in quantum optics. Thus, we propose to do a generalization of quantum gates over continuous variables using Conformal Field Theory (CFT) -2d, specifically, vertex operators on a coherent basis and using effective field theory to do some correspondences between observables that are interested in the quantum gravity theory to systems based in a quantum laboratory.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{CameronForeman,

title = {Practical randomness amplification and privatisation with implementations on quantum computers},

author = {Cameron Foreman},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Authors: Cameron Foreman, Sherilyn Wright, Alec Edgington, Mario Berta, Florian J. Curchod 

 

We present an end-to-end and practical randomness amplification and privatisation protocol based on Bell tests. This allows the building of device-independent random number generators which output (near-)perfectly unbiased and private numbers, even if using an uncharacterised quantum device potentially built by an adversary. Our generation rates are linear in the repetition rate of the quantum device and the classical randomness post-processing has quasi-linear complexity - making it efficient on a standard personal laptop. The statistical analysis is also tailored for real-world quantum devices. 

 

Our protocol is then showcased on several different quantum computers. Although not purposely built for the task, we show that quantum computers can run faithful Bell tests by adding minimal assumptions. In this semi-device-independent manner, our protocol generates (near-)perfectly unbiased and private random numbers on today's quantum computers. 

 https://arxiv.org/abs/2009.06551},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{CarlaAFerradini,

title = {A causal modelling framework for classical and quantum cyclic causal structures},

author = {Carla A. Ferradini},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {In 1937 the theoretical existence of closed time-like curves (CTCs) as solutions in General Relativity was discovered. These are time-like curves that allow a particle to return to its starting point in space-time and, thus, seem to imply that time travel backwards in time is theoretically possible. This gives rise to several paradoxes such as the well-known grandfather paradox. Therefore, it is of physical interest to characterise and give a description of such closed time-like curves from a causal point of view and derive logically consistent solutions. The study of these solutions requires to introduce cyclic causal models and deduce how to evaluate probability distributions. Here, we provide a method that allows us to determine whether an arbitrary cyclic causal graph admits a logically consistent solution and eventually evaluate probabilities. This method can be used both in classical or quantum scenarios. In both cases, we prescribe how to reduce the initial causal graph to an acyclic one and then recover cyclicity through post-selection. Classically, we show that for an acyclic graph this reduces to the joint probability distribution that can be derived using the classical theory of acyclic causality. In the quantum scenario, we also show the equivalence between loop composition of the causal box framework and post-selected CTCs in our formalism. The obtained results do not only describe CTCs that may arise in exotic solutions of General Relativity, but also can be used to model ordinary feedback processes.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{CarolynWood,

title = {Composite particles with minimum uncertainty in spacetime},

author = {Carolyn Wood},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {We often take for granted that the quantum states which minimise the Heisenberg uncertainty principle will be the most suitable for describing well-localised particles. However, until now, all studies of the free propagation of composite particles (like atoms and molecules) have found that they delocalise into separate internal energy components travelling at different velocities. This destroys their spatial coherence, rendering them unsuitable for experimental applications and as idealised clocks which are represented mathematically by composite particles. 

 We solve the delocalisation problem by introducing a class of states with minimum uncertainty in spacetime. The relevant physics comes from minimising the uncertainty between position and velocity, rather than position and momentum, while correctly including relativistic mass—energy equivalence. Here, I will discuss the process of deriving these states, and explore some of their unique properties that make them optimal for describing idealised clocks, such as their covariant transformation under boosts, and cohesive propagation even in gravitationally-induced free-fall.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{ChungYunHsieh,

title = {Quantum Channel Marginal Problem},

author = {Chung Yun Hsieh},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Given a set of local dynamics, are they compatible with a global dynamics? We systematically formulate these questions as quantum channel marginal problems. These problems are strongly connected to the generalization of the no-signaling conditions to quantized inputs and outputs and can be understood as a general toolkit to study notions of quantum incompatibility. In fact, they include as special cases channel broadcasting, channel extendibility, measurement compatibility, and state marginal problems. After defining the notion of compatibility between global and local dynamics, we provide a solution to the channel marginal problem that takes the form of a semidefinite program. Using this formulation, we construct channel incompatibility witnesses, discuss their operational interpretation in terms of an advantage for a state-discrimination task, prove a gap between classical and quantum dynamical marginal problems and show that the latter is irreducible to state marginal problems.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{CrislanedeBrito,

title = {Twin-photon quantum correlations and their relation to the Gouy phase and Sorkin parameter},

author = {Crislane Brito},

year  = {2022},

date = {2022-01-01},

abstract = {Type-I spontaneous parametric down-converted biphotons can be described approximately by a double-Gaussian wavefunction in configuration space. 

Using an effective propagator in the Fresnel approximation, the time evolution and transversal spreading of the two-particle wavefunction, we find that the pair of photons develops a Gouy phase while it propagates. We show that the two-photon entanglement can be connected to the biphoton Gouy phase, as these quantities are Rayleigh-length-related. 

Furthermore, we obtain the Gouy phase of the pair of photons wavepacket being focused through a thin lens, which is in reasonable agreement with the experimental data. Indicating that the Gouy phase obtained from the effective double-Gaussian biphoton wavefunction can be used as good approximation in exploring quantum correlations of 

twin photons. 

Afterwards, we exploit the relation between phase-space correlations of biphotons diffracting through a double-slit as encoded in the logarithmic negativity and the Gouy phase. 

We observe that measurements of Gouy phase differences provide information on the two-photons phase-space entanglement variation, governed by the 

physical parameters of the experiment and expressed by the logarithmic negativity via covariance matrix elements. 

Finally, we evaluate the Sorkin parameter $kappa$ for biphotons and massive particles, which results from nonclassical path contributions of the kink type and loops to double- and triple-slit interferometry. 

This simple unidimensional model for the evaluation of $kappa$ predicts that kinked nonclassical paths may lead to $kappa approx 10^-5$ for biphotons. 

We show that such a model reproduces well the Sorkin parameter for matter waves found in more involved approaches in the literature. Moreover, we establish a hierarchy of approximations based on the shape of the nonclassical paths for matter waves and compare their size with leading relativistic corrections to the propagator.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{CristianBoghiu,

title = {A Python package for quantum causal compatibility},

author = {Cristian Boghiu},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Finding which causes are behind observed correlations in nature is at the core of the scientific discipline. Observed correlations are represented as the joint probability distribution of a set of classical random variables. One can ask whether certain causal relationships between variables are compatible with the observed correlations (e.g., if one variable could have a direct causal influence on another), or if there exist other unobserved variables, also called latent variables, with an unknown joint distribution that could have such influence (e.g., the existence of hidden common-cause variables). Bayesian causal networks, in the form of directed acyclic graphs (DAGs), give the tools to formalise such questions. Each causal hypothesis can be encoded as a DAG, with causal influence between variables being represented as directed edges. Whether some observed correlations are compatible with a particular DAG is known as the causal compatibility problem. 

 

In quantum theory, our most accurate theory at a fundamental level, one cannot always assign a deterministic output to measurement results: the theory is inherently probabilistic. Thus, one needs to represent measurement results as random variables. This naturally leads to causal analysis: which cause-and-effect relationships can explain observed measurement statistics? 

 

One can ask, on the one hand whether observed correlations arising from measuring quantum systems are compatible with a *classical* cause-and-effect explanation (leading to notions such as Bell nonlocality) or whether these correlations are compatible with a *quantum* cause-and-effect explanation, with the appropriate definitions of what it means to have a "quantum cause". 

 

Both these questions can be handled with a technique called quantum inflation. Our Python package can take as input any DAG with any assumption on the "quantumness" of each variable and then automatically implement the appropriate techniques for testing either causal compatibility, or also, for optimizing functions over correlations. 

 There is no open-source tool for systematically applying quantum inflation techniques flexible enough to accept any DAG as input; our package is the first instance of this. Furthermore, we add extra features which are not published yet, which are small improvements to the quantum inflation technique, which make the},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{DávidJakab,

title = {The shareability of Werner and Isotropic states.},

author = {Dávid Jakab},

year  = {2022},

date = {2022-01-01},

abstract = {When can quantum correlations in overlapping subsystems of a larger, composite system be compatible with each other? This fundamental question of quantum physics is the quantum marginal problem. We investigate a permutation symmetric sub scenario of the quantum marginal problem, known as shareability, or symmetric extendability problem. The essence of the shareability problem is the following: Alice and Bob are each given a composite quantum system. Is it possible that each bipartite subsystem in which one part is owned by Alice, and the other by Bob, is simultaneously in the state (rho)? The maximal sizes of the two composite systems in which a given bipartite state (rho) can be shared in such a way serves as a measure of the entanglement of (rho). We restrict the candidates for (rho) to SU(d) symmetric Werner and isotropic states, which allows us to apply the representation theory of Lie groups to the problem, and derive necessary and sufficient conditions for the general, k-l shareability of both classes of bipartite states.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{EwaBorsuk,

title = {On safe post-selection for Bell tests with ideal detectors: Causal diagram approach},

author = {Ewa Borsuk},

year  = {2022},

date = {2022-01-01},

abstract = {The poster focuses on finding a simple criterion for safe post-selection in Bell-type experiments. Reasoning about Bell non-locality from the correlations observed in post-selected data is always a matter of concern. This is because conditioning on the outcomes in the process of post-selection may be a source of non-causal correlations, known as a selection bias, rising doubts whether the conclusion concerns the actual causal process or maybe it is just an effect of processing the data. Thus we propose a simple criterion, called the all-but-one principle which shows when the conclusions about non-locality from breaking Bell inequalities with post-selected data remain in force. 

 

The problem is important since even in the idealized case without detection inefficiencies, post-selection is an integral part of experimental designs, not least because it is a part of the entanglement generation process itself. In this poster we discuss a broad class of scenarios with post-selection on multiple spatially distributed outcomes. Generality of this result, attained by adopting the high-level diagrammatic tools of causal inference, provides safe grounds for systematic reasoning based on the standard form of multipartite Bell inequalities in a wide array of entanglement generation schemes, without worrying about the dangers of selection bias. In particular, it can be applied to post-selection defined by single-particle events in each detection channel when the number of particles in the system is conserved. 

 

Based on a joint work with Pawel Blasiak and Marcin Markiewicz, in Quantum 5, 575 (2021): https://doi.org/10.22331/q-2021-11-11-575 .},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{GabrielPereiraAlves,

title = {Any pair of incompatible rank-one projective measurements is optimal for some non-trivial Bell inequality},

author = {Gabriel Pereira Alves},

year  = {2022},

date = {2022-01-01},

abstract = {Bell non-locality is an important feature of quantum mechanics as the correlations established among distributed quantum systems are stronger than those allowed by classical physics. In this context, entanglement combined with incompatibility of measurements [1] are two necessary prerequisites to generate non-locality. Here, we show that for rank-one and projective measurements acting on the same finite-dimensional space there is a tight connection between non-locality and incompatibility. 

 

In ref. [2] a Bell functional was constructed in a way that it is tailored to mutually unbiased bases (MUBs), i.e., its maximum quantum violation - or the quantum value of the functional - is achieved if the two possible measurements performed by one of the parts correspond to rank-one MUBs projectors. In this work, we generalize this construction to an arbitrary pair of incompatible rank-one projective measurements. 

 

The following development consists of building a family of Bell functionals whose quantum realization can be obtained from a previously defined Bob’s pair of incompatible measurements and a suitable choice of measurements of Alice, as well as a proper entangled state. Once the family of functionals is set, a couple of results can be extracted, as follows. 

 

Result 1. If a pair of rank-one projective measurements are incompatible, then they can be used to generate nonlocal correlations. Moreover, there exists a non-trivial Bell inequality for which these measurements are optimal. 

 

In this context, the non-triviality of a Bell inequality means that the quantum value is strictly larger than the local one, forming a gap. Our proof to result in 1 is fully constructive; for every pair of measurements of Bob, we can explicitly construct a functional and show that it is non-trivial. Now, it is natural to ask for what functionals corresponding to particular measurements we have the biggest gap, that is, what is the pair of measurements that turn the quantum value the biggest possible when compared to the local one? 

 Result 2. For rank-one projective measurements acting on a d-dimensional complex space, where d is even, the largest gap between the quantum and local values is achieved if and only if the rank-one projective measurements correspond to a direct sum of MUBs in dimension 2.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{GuilhermeFiusa,

title = {Asymmetry resource theory and its applications in holographic quantum codes},

author = {Guilherme Fiusa},

year  = {2022},

date = {2022-01-01},

abstract = {The AdS/CFT correspondence emerged as a duality conjecture between a theory of quantum gravity and a conformal field theory. Recent results suggest that the way information encoding works in this duality precisely resembles a quantum error-correcting code. In particular, arguments using error correction properties and holographic codes point toward quantum gravity being incompatible with global symmetries. In this work, by employing methods from quantum resource theories of asymmetry and reference frames, we shed some light on the tradeoffs between covariant and approximate quantum codes as well as explore some of its consequences for the holographic correspondence.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{IsmaelPaiva,

title = {Non-Hermitian dynamics in the Page and Wootters framework},

author = {Ismael Paiva},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Recently, much effort has been put towards a broader understanding of time in quantum mechanics within the Page and Wootters (PaW) framework. This framework consists of having one or more quantum systems used as references for time to study the dynamics of other systems of interest. In this presentation, we discuss how a non-Hermitian Hamiltonian may govern the effective dynamics from the perspective of some clocks. First, we use the PaW framework to study the total energy measurement of a system that contains an internal clock. We analyze whether quantum mechanics requires a minimum duration for these measurements from the perspective of this clock or of others external to the system and derive a new time-energy uncertainty relation. Most importantly, however, we show that the dynamics from the perspective of the internal clock is non-Hermitian. Because the clock's energy is being measured, there is a sense in which this clock constitutes a non-inertial reference frame. Then, to further investigate this aspect, we consider general accelerating clocks. We prove that they lead to non-Hermitian dynamics. Finally, we discuss some challenges, questions, and research opportunities associated with these results.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{KacperPrech,

title = {Violations of Thermodynamic and Kinetic Uncertainty Relations versus Entanglement - Different Manifestations of Quantum Coherence},

author = {Kacper Prech},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Entanglement, a form of correlation that is stronger than what classical systems are capable of, is a prominent manifestation of quantum coherence. A different manifestation of coherence is provided by overcoming classical bounds on the signal-to-noise ratio, so-called thermodynamic uncertainty relations (TURs) and kinetic uncertainty relations (KURs). We systematically analyze the relation between these different manifestations of coherence in a serial double quantum dot. In this system, entanglement can be generated by driving a charge current through the double dot. That same current may exhibit a suppression of fluctuations due to quantum coherence, allowing for TUR and KUR violations. We find that TUR and KUR violations are present for the same range of tunnel-couplings as entanglement. However, while TUR violations require small bias voltages (keeping dissipation small), entanglement and KUR violations are maximized in the large bias limit. Increasing the bias voltage, we observe a cross-over from TUR violations to entanglement and KUR violations with a window of co-existence for intermediate voltages. Coulomb interactions between the dots enhance coherence resulting in larger TUR and KUR violations, stronger entanglement, and, in the case of strong interactions, non-local quantum states.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{KarthikHS,

title = {Noise adapted quantum random access codes},

author = {Karthik H S},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Quantum random access codes (QRACs) are the quantum counterparts of RACs where the dataset held by Alice is encoded onto a quantum state sent over a transmitting channel for the other party Bob to partly decode using appropriate quantum measurements. Here, the acts of state preparation and measurement correspond as encoding and decoding functions at Alice’s and Bob’s ends respectively. Analogous to a RAC, dimension witnesses are shown to be useful in certifying the security aspect of semi-device -independent quantum key distribution. Dimension witnesses offer the distinction between classical and quantum systems by lower bounding the dimension d required in establishing a random-access code. These witnesses form a simple method in recognizing the characteristic feature of quantum systems as a potentially more useful resource in comparison to its classical counterpart of the same dimension, d. Restricting to the case of d= 2, we investigate the effect of stochastic operations on a noisy quantum channel employed for a QRAC. We show that it is possible to enable restoration/activation of the quantum advantage in the task of RAC even in the presence of noise in the channel.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{LeonardoOleynik,

title = {Studies of covariance and nonlocality in Bohmian mechanics},

author = {Leonardo Oleynik},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {The principle of general covariance motivates a relational prescription of the world, implying that reference frames must be conceived as physical systems rather than coordinate systems. With this prescription, it has been shown that entanglement is not invariant upon changes of quantum reference frames, meaning that, at least for pure states, nonlocality is a frame-dependent quantity. Something similar happens in classical mechanics. When one applies it to non-inertial frames, fictitious forces induce non-local effects, identified by Newton’s second law. Here, we explore a broader notion of this phenomenon within Bohmian mechanics, a causal interpretation of quantum mechanics given in terms of particles and their trajectories. Our analysis shows that the nonlocality prescribed by classical and quantum mechanics is intimately associated with each theory’s definition of physical state. With that in mind, we propose a way to quantify entanglement by analysing a distant particle’s equation of motion.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{MarceloJanovitch,

title = {Breakdown of wave-particle duality in the classical limit of a quantum heat engine},

author = {Marcelo Janovitch},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Quantum heat engines have spurred a lot of interest in the recent years, and, although their classical counterparts have been thoroughly investigated in the last centuries, the quantum-to-classical transition remains elusive. In this ongoing investigation, we consider a quantum heat engine based on photon-assisted Cooper pair tunnelling. Its quantum description exhibits a dual wave-particle behaviour, but a comparison with viable classical models has been lacking. Here, we derive different classical limits: (i) A particle model, where photons hop between two neighbouring resonators; (ii) a wave model, in which electromagnetic waves interfere. We contrast these with the quantum model at the level of current fluctuations, where non-classical features usually emerge.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{MarcinKarczewski,

title = {Genuine multipartite indistinguishability and its detection via the generalized Hong-Ou-Mandel effect},

author = {Marcin Karczewski},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {The question whether two indistinguishable particles are bosons or fermions can be answered by observing the Hong-Ou-Mandel effect on a beam splitter. However, already for three particles one can consider symmetries that are neither bosonic nor fermionic. In this work, we describe a simple method of identifying them experimentally and propose a measure of a genuine multipartite indistinguishability.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{MaríaCiudadAlañón,

title = {An oracle for the bilocal network},

author = {María Ciudad Alañón},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Bell's theorem states that a physical theory based on local hidden variables alone is not compatible with all quantum predictions. This is certified by the violation of the so-called “Bell inequalities”, expressions that bound the strength of the correlations achievable through local hidden variable theories between two parties that cannot communicate instantly; by using quantum mechanics one can overcome these bounds. 

In recent years, the study of nonlocality has been generalized to more sophisticated scenarios, known as quantum networks, in which there exist several statistically independent sources, and multiple parties. While correlations in the standard Bell scenario form a convex set, in networks the set of correlations is non-convex. This makes characterizing correlations in such case challenging. Our main goal is to characterize correlations in the simplest nontrivial bilocal scenario, i.e., one with 3 parties--Alice, Bob and Charlie-- on a line and two shared sources between Alice-Bob and Bob-Charlie (with binary outputs for everyone and binary inputs for Alice and Charlie). For this purpose, we use a black-box oracle that determines whether a probability distribution is bilocal or not using an implementation of a numerical method for quadratic programming from a bilinear solver. This works well for very small networks, but it does not scale efficiently, and it does not provide an analytical certificate that proves whether a correlation is compatible with a network or not. 

The motivation is two-fold. First, from the foundational point of view, by studying the simplest network scenario we will better understand the transition from standard nonlocality to network nonlocality, which can help us characterise more complex scenarios. Secondly, from the quantum information perspective this understanding might help in designing protocols for the use of quantum phenomena, such us entanglement swapping, cryptography, and other information processing tasks. Supervisors: Paolo Abiuso, Cristian Boghiu and Marc-Olivier Renou},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{MartinRenner,

title = {Simulating entangled qubit pairs with classical communication},

author = {Martin Renner},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Bell's famous theorem shows that quantum correlations cannot be reproduced by local hidden variables. However, it is possible to simulate quantum entanglement when the parties are allowed to transmit some classical communication. For the simplest case of projective measurements on two maximally entangled qubits, Toner and Bacon (Phys. Rev. Lett. 91, 187904, 2003) proved that already a single bit is sufficient. At the same time, somehow counterintuitively, all previously known protocols to simulate non-maximally entangled qubits require more resources. In this work, however, we present a protocol to simulate two weakly entangled qubits with a single bit and another protocol to simulate every entangled qubit pair with a single trit. We also study the simulation cost of other scenarios. More precisely, we present a protocol that uses only two bits to simulate a qubit channel in any prepare and measure scenario even when the measurements are allowed to be positive operator-valued measurements (POVMs). For that case, previously known protocols required an unbounded amount of communication in the worst case. We show that this protocol is optimal by presenting a task in which qubits can outperform trits. Finally, we use this result to generalize the protocol of simulating entangled qubit pairs to the scenario where the parties are allowed to perform POVMs.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{MichalGrňo,

title = {[No title]},

author = {Michal Grňo},

year  = {2022},

date = {2022-01-01},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{NicolasLoizeau,

title = {Locality from random matrices},

author = {Nicolas Loizeau},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Quantum many-body systems are endowed with a tensor product structure. A structure that is essentially inherited from probability theory, where the probability of two independent events is the product of the probabilities. The tensor structure of a Hamiltonian gives a natural decomposition of the system into coupled subsystems. Examples of such tensor structures are 1-local (no interactions), 2-local (at most 2 particles interactions), 1D chains, bipartition etc. Considering a particular Hamiltonian and a particular tensor structure; one can ask : is there a basis in which this Hamiltonian has this tensor structure? that is to say: is there another Hamiltonian with the same spectrum that has this particular tensor structure? We have developed numerical methods to find such a preferred basis and we find that the spectrum of large random matrices can be very well approximated by the spectrum of 2-local Hamiltonians. These results are helpful for thinking about the preferred basis problem and the emergence of locality from quantum theory itself.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{NicolásMedinaSánchez,

title = {Operational reconstruction of quantum statistics},

author = {Nicolás Medina Sánchez},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {A fascinating fact about the collective behavior of indistinguishable quantum particles is the existence of only two types of statistics: bosonic and fermionic, characterized by the exchange symmetry of their associated quantum states. So far, all attempts to explain the origin of these symmetries resort on oblivious assumptions added to the quantum formalism (e.g. dimensionality of space). Hereby we introduce an information-theoretic study of particle statistics in the space of modes. We show that there are infinitely many statistics compatible with the unitary symmetry and the Fock space structure, divided in two big families of fermionic-like and bosonic-like statistics.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{NidhinSudarsananRagini,

title = {Labeling of quantum observables},

author = {Nidhin Sudarsanan Ragini},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {We identify a particular class of discrimination problems for observables (positive operator-valued measures), in which observables with identical range but permuted effects are involved, as the labeling problem for these observables. Consequently, we identify the set of observables those can be “labeled” perfectly and, study the minimum-error scenario as well as the unambiguous scenario for labeling, using the framework of process POVMs which describe test procedures for testing quantum channels with one time step. Studies presented here involve binary observables, those which are described by two effects. These occur equiprobably as well.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{PaoloAbiuso,

title = {Characterizing (non-) Markovianity through Fisher Information},

author = {Paolo Abiuso},

year  = {2022},

date = {2022-01-01},

abstract = {A non-isolated physical system typically loses information to its environment, and when such loss is irreversible the evolution is said to be Markovian. Non-Markovian effects are studied by monitoring how information quantifiers, such as the distance between physical states, evolve in time. Here we show that the Fisher information metric emerges as the natural object to study in this context; we fully characterize the relation between its contractivity properties and Markovianity, both from the mathematical and operational point of view. We prove, for classical dynamics, that Markovianity is equivalent to the monotonous contraction of the Fisher metric at all points of the set of states. At the same time, operational witnesses of non-Markovianity based on the dilation of the Fisher distance cannot, in general, detect all non-Markovian evolutions, unless specific physical postprocessing is applied to the dynamics. Finally, we show for the first time that non-Markovian dilations of Fisher distance between states at any time correspond to backflow of information about the initial state of the dynamics at time 0, via Bayesian retrodiction. All the presented results can be lifted to the case of quantum dynamics by considering the standard CP-divisibility framework.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{PolinaPogrebinskaya,

title = {Duality between classical waves and particles},

author = {Polina Pogrebinskaya},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Interference of single particles lies at the core of quantum mechanics. The 

most prominent demonstration of this effect is the double-slit experiment: a 

single experimental run indicates an experiment with single particles, however 

the statistics of repeated runs reassembles interference fringes. This is the source 

of the celebrated wave-particle duality. In 

this work we show that classical wave mechanics combined with the statistical 

detection model can completely reproduce quantum interference experiments with 

single particles. The recreation of quantum double-slit experiment using classical waves shows that the dual behaviour between waves and particles (at least its part described in this work) is not necessarily proof of a genuine quantum effect.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{RafaelWagner,

title = {Inequalities bounding coherence, contextuality and non-locality},

author = {Rafael Wagner},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Quantum coherence, nonlocality, and contextuality have been identified as key resources for quantum advantage in tasks involving metrology, communication, and computation. We introduce a graph approach to derive classicality inequalities that, depending on how we associate quantum preparations and measurements to vertices and edges, represent bounds on either local, non-contextual, or coherence-free models. We prove that our approach i) recovers all non-contextuality inequalities obtainable using the exclusivity graph approach; ii) generalizes recently proposed basis-independent coherence witnesses using overlaps, and iii) provides witnesses of generalized contextuality. We describe an algorithm to find all such classicality inequalities, and use it to analyze many simple scenarios.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{RamanChoudhary,

title = {Contextuality of n-qubit Pauli operators},

author = {Raman Choudhary},

year  = {2022},

date = {2022-01-01},

abstract = {N-qubit Pauli operators (Paulis) hold key importance in the foundational as well as the application aspects of Quantum Mechanics (QM). Examples like Peres-Mermin square, Mermin star exploit Paulis to reveal non-classicality (via contextuality) within QM. Any Hermitian operator A over an n-qubit Hilbert space can always be decomposed into Paulis since the latter constitute a basis for the former- a fact, heavily utilized by NISQ algorithms like VQEs, QAOA etc. by decomposing the Hamiltonian encoding the problem into Pauli basis. This multifacetedness of Paulis makes them a good candidate to explore the link between potential quantum advantage (of NISQ algorithms) and quantum contextuality. More specifically, Paulis could help pinpoint whether contextuality could supply quantum advantage within the NISQ algorithms. But to explore this link a full characterization of the contextuality of Paulis needs to be determined. An attempt for this characterization was made in [Phys. Rev. Lett. 123, 200501] where the authors restricted their discussion to State-Independent Strong Contextuality (Peres-Mermin square like) proofs for Paulis. But this left open the case of State dependent weak contextuality (i.e. the NC inequality violation) of Paulis. Here, we fully characterize this case. We show that the necessary and sufficient conditions for a set of Paulis to violate NC inequalities is the presence of a 4-cycle subgraph in the overall compatibility graph of the set. In our attempt to characterize the same we also discover peculiar features of allowed compatibility cycles of Paulis.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{RheaAlexander,

title = {Statistical mechanical constraints on magic state distillation for qubit CSS protocols},

author = {Rhea Alexander},

year  = {2022},

date = {2022-01-01},

abstract = {Magic states are fundamental building blocks on the road to fault-tolerant quantum computing. For odd prime dimensional systems in the magic state injection model, Wigner function negativity and contextuality have been shown to be equivalent and necessary for obtaining computational speedups beyond what is classically achievable. However, for even dimensional systems such as qubits, this link is not as clear. A better understanding of how these structures extend to the qubit case has important foundational and practical implications (e.g. many quantum algorithms are formulated in terms of multi-qubit systems). 

 Previous work has cast quantum computing with magic states for odd prime dimensions within a phase space setting, which enables universal quantum computing to be described using the statistical mechanics of Wigner functions. Here we extend this framework to the important $d=2$ qubit case and show that we can exploit common features of CSS protocols that lead to new distillation bounds that out-perform previous monotone bounds in many regimes of interest. In the particular case of CSS code reductions, we arrive at a novel cut-off result on the dimension of the CSS code which implies that for fixed error probability and success probability that one needs only consider CSS codes below a threshold number of qubits.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{RicardoHeras,

title = {Locality vs nonlocality in the Aharonov-Bohm effect},

author = {Ricardo Heras},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {There is a longstanding and unfinished debate on the physical interpretation of the Aharonov-Bohm effect (AB) between those who argue that this effect reflects the physical significance of the vector potential in quantum mechanics (A-explanation) and those who argue that this effect reflects a nonlocal action of the magnetic field (B-explanation). Here we argue in favor of a nonlocal explanation in the context of the AB effect in a closed flux line by introducing a gauge in which the vector potential vanishes in all space except on the surface surrounded by the closed flux line. This means that the potential is zero along the trajectory of the charged particle except on the crossing point where this trajectory intersects the surface bounded by the closed flux line, a result that questions the alleged physical significance of the vector potential and thereby the local interpretation of the AB effect. 

 Reference: R. Heras, The Aharonov-Bohm effect in a closed flux line, Eur. Phys. J. Plus 137, 641 (2022).},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{SebastianStengele,

title = {A Framework for Universality Across Disciplines},

author = {Sebastian Stengele},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {What is the scope of universality across disciplines? And what is its relation to undecidability? To address these questions, we build a (categorical) framework for universality. Its instances include Turing machines, spin models, and others. We introduce a hierarchy of universality and argue that it distinguishes universal Turing machines as a non-trivial form of universality. We also outline the relation to undecidability by drawing a connection to Lawvere’s Fixed Point Theorem. 

 Authors - Sebastian Stengele, Tobias Reinhart, Tomáš Gonda and Gemma De las Cuevas},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{SeungbeomChin,

title = {Graph picture of linear quantum networks and entanglement},

author = {Seungbeom Chin},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {The indistinguishability of quantum particles is widely used as a resource for the generation of entanglement. Linear quantum networks (LQNs), in which identical particles linearly evolve to arrive at multimode detectors, exploit the indistinguishability to generate various multipartite entangled states by the proper control of transformation operators. However, it is challenging to devise a suitable LQN that carries a specific entangled state or compute the possible entangled state in a given LQN as the particle and mode number increase. This research presents a mapping process of arbitrary LQNs to graphs, which provides a powerful tool for analyzing and designing LQNs to generate multipartite entanglement. We also introduce the perfect matching diagram (PM diagram), which is a refined directed graph that includes all the essential information on the entanglement generation by an LQN. The PM diagram furnishes rigorous criteria for the entanglement of an LQN and solid guidelines for designing suitable LQNs for the genuine entanglement. Based on the structure of PM diagrams, we compose LQNs for fundamental $N$-partite genuinely entangled states. In addition, we present a more recent graph-theoretic approach to generate entanglement of bosons by subtraction operators that do not use postselections at the measurements.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{ShashaankKhanna,

title = {Which Causal Scenarios Might Support “Non-Local" Correlations},

author = {Shashaank Khanna},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{SidineyBrunoMontanhano,

title = {Differential Geometry of Contextuality},

author = {Sidiney Bruno Montanhano},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Contextuality has been related for a long time as a topological phenomenon. In this work, such a relationship is exposed in the more general framework of generalized contextuality. The main idea is to identify states, effects, and transformations as vectors living in a tangent space, and the non-contextual conditions as discrete closed paths implying null vertical phases. Two equivalent interpretations hold. The geometrical or realistic view, where flat space is imposed, implies that the contextual behavior becomes equivalent to the curvature (non-trivial holonomy) of the probabilistic functions, in analogy with the electromagnetic tensor; as a modification of the valuation function, it can be used to connect contextuality with interference, non-commutativity, and signed measures. The topological or anti-realistic view, where the valuation functions must be preserved, implies that the contextual behavior can be translated as topological failures (non-trivial monodromy); it can be used to connect contextuality with non-embeddability and a generalized Vorobyév theorem. Both views can be related to contextual fraction, and the disturbance in ontic models can be presented as non-trivial transition maps.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{TamoghnaDas,

title = {Non-locality of quantum optical modes induced by a single photon},

author = {Tamoghna Das},

year  = {2022},

date = {2022-01-01},

abstract = {Violation of local realism by spatially separated quantum optical modes plays an essential role in implementing various quantum information protocols including quantum cryptography, dense coding, one way quantum computing. Here I demonstrate the non-classical feature of a vacuum-single-photon excitation of a pair of spatially separated modes by robustly violating the Clauser-Horne inequality via weak field homodyne measurements. We proved that it is essential to exploit the particle and wave nature of the single-photon excitation as there exists a local hidden variable model for the phase-sensitive constant field homodyne measurement.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{TathagataKarmakar,

title = {Tomography of a continuously monitored qubit},

author = {Tathagata Karmakar},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {We are now able to track and perform feedback control on a quantum system in real-time by performing time-continuous measurements. This work theoretically investigates a continuously monitored qubit coupled to a cavity in the strong dispersive regime. A microwave probe drive excites the cavity, and homodyne measurements on the transmitted signal allow us to monitor the system's state. An additional spectroscopy drive is introduced to ensure full state tomography of the qubit. This analysis goes beyond the Markovian qubit evolution and accounts for the qubit-resonator interaction at a quantum level. We characterize the stochastic trajectories of the qubit-cavity system under measurements. Finally, we implement the Bayesian mean estimation (BME) procedure to perform tomography of the initial state of the qubit. Our results provide insights into the real-time state tracking of a qubit-cavity system and bear relevance to circuit QED experiments at the forefront of quantum computing research endeavors.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{VincenzoFiorentino,

title = {Uncertainty relations for the support of pure quantum states},

author = {Vincenzo Fiorentino},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {A signal and its Fourier transform cannot be localised arbitrarily well simultaneously. Inequalities for the support of pure states capture this property in finite-dimensional quantum systems. In spaces of prime dimensions, sharp additive uncertainty relations have been derived for the support of a state in two orthonormal bases related by a discrete Fourier transform. We extend these state-independent uncertainty relations for pairs of bases to complete sets of mutually unbiased bases. We establish a lower bound which is shown to be sharp for dimension three. Analytic and numerical results for dimensions up to d ≤ 19 suggest, however, that it cannot be saturated for any larger prime dimension.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{ViniciusRossi,

title = {On characterising assemblages in Einstein-Podolsky-Rosen scenarios},

author = {Vinicius Rossi},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

abstract = {Characterising non-classical quantum phenomena is crucial not only from a fundamental perspective, but also to better understand its capabilities for information processing and communication tasks. In this work, we focus on exploring the characterisation of Einstein-Podolsky-Rosen inference (a.k.a. steering): a signature of non-classicality manifested when one or more parties in a Bell scenario have their systems and measurements described by quantum theory, rather than being treated as black boxes. We propose a way of characterising common-cause assemblages from the correlations that arise when the trusted party performs tomographically-complete measurements on their share of the experiment, and discuss the advantages and challenges of this approach. Within this framework, we show that so-called almost quantum assemblages satisfy the principle of macroscopic noncontextuality, and demonstrate that a subset of almost quantum correlations recover almost quantum assemblages in this approach.},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{VivekRm,

title = {Drawing Classical Analogues to Quantum Systems: Pair Annihilation and Creation as a phase transition process},

author = {Vivek Rm},

year  = {2022},

date = {2022-01-01},

keywords = {},

pubstate = {published},

tppubtype = {online}

}

@online{ZekiSeskir,

title = {On the Cardinality of Many-Worlds in Discrete Spacetime Structures},

author = {Zeki Seskir},

year  = {2022},

date = {2022-01-01},

abstract = {We make an analysis over a variation of causal sets where the light cone of an event is represented by finitely branching trees with respect to any given arbitrary dynamics. We argue through basic topological properties of Cantor space that under certain assumptions about the spacetime structure and causation, given any event x, if all worldlines extending the event x are `eventually deterministic', then within the many-worlds interpretation the number of alternate universes with respect to x is exactly ℵ0. We also observe that if there are countably many alternate universes with respect to x, then at least one of these universes must be necessarily `decidable' in the sense that there is an algorithm which determines whether or not any given event belongs to the given worldline. We finally point out the fact that there can be only countably many universes with an ultimate end. 

 Arxiv link: https://arxiv.org/abs/2109.14042},

keywords = {},

pubstate = {published},

tppubtype = {online}

}