We present an advanced method to enhance the detection and protection of quantum entanglement, a key concept in quantum mechanics for computing, communication, and beyond. Entanglement, where particles remain connected over distance, can be indicated by nonlocality, measurable through the Clauser-Horne-Shimony-Holt (CHSH) inequality. While violating the inequality confirms entanglement, entanglement can still exist without such violations. To overcome this limitation, we use the CHSH violation as an entanglement measure and introduce a variational entanglement witness for more complete detection. Moreover, we propose a nonlocal measurement framework to measure the expectation values in both the CHSH inequality and variational entanglement witness. These nonlocal measurements exploit the intrinsic correlations between entangled particles, providing a more reliable approach for detecting and maintaining entanglement. This paper significantly contributes to the practical application of quantum technologies, where detecting and maintaining entanglement are essential.
{"title":"Detecting and protecting entanglement through nonlocality, variational entanglement witness, and nonlocal measurements","authors":"Haruki Matsunaga, Le Bin Ho","doi":"arxiv-2409.10852","DOIUrl":"https://doi.org/arxiv-2409.10852","url":null,"abstract":"We present an advanced method to enhance the detection and protection of\u0000quantum entanglement, a key concept in quantum mechanics for computing,\u0000communication, and beyond. Entanglement, where particles remain connected over\u0000distance, can be indicated by nonlocality, measurable through the\u0000Clauser-Horne-Shimony-Holt (CHSH) inequality. While violating the inequality\u0000confirms entanglement, entanglement can still exist without such violations. To\u0000overcome this limitation, we use the CHSH violation as an entanglement measure\u0000and introduce a variational entanglement witness for more complete detection.\u0000Moreover, we propose a nonlocal measurement framework to measure the\u0000expectation values in both the CHSH inequality and variational entanglement\u0000witness. These nonlocal measurements exploit the intrinsic correlations between\u0000entangled particles, providing a more reliable approach for detecting and\u0000maintaining entanglement. This paper significantly contributes to the practical\u0000application of quantum technologies, where detecting and maintaining\u0000entanglement are essential.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142268301","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Time crystals, a unique non-equilibrium quantum phenomenon with promising applications in current quantum technologies, mark a significant advance in quantum mechanics. Although traditionally studied in atom-cavity and optical lattice systems, pursuing alternative nanoscale platforms for time crystals is crucial. Here we theoretically predict discrete time-crystals in a periodically driven molecular magnet array, modeled by a spin-S Heisenberg Hamiltonian with significant quadratic anisotropy, taken with realistic and experimentally relevant physical parameters. Surprisingly, we find that the time-crystal response frequency correlates with the energy levels of the individual magnets and is essentially independent of the exchange coupling. The latter is unexpectedly manifested through a pulse-like oscillation in the magnetization envelope, signaling a many-body response. These results show that molecular magnets can be a rich platform for studying time-crystalline behavior and possibly other out-of-equilibrium quantum many-body dynamics.
{"title":"Time Crystals from single-molecule magnet arrays","authors":"Subhajit Sarkar, Yonatan Dubi","doi":"arxiv-2409.10816","DOIUrl":"https://doi.org/arxiv-2409.10816","url":null,"abstract":"Time crystals, a unique non-equilibrium quantum phenomenon with promising\u0000applications in current quantum technologies, mark a significant advance in\u0000quantum mechanics. Although traditionally studied in atom-cavity and optical\u0000lattice systems, pursuing alternative nanoscale platforms for time crystals is\u0000crucial. Here we theoretically predict discrete time-crystals in a periodically\u0000driven molecular magnet array, modeled by a spin-S Heisenberg Hamiltonian with\u0000significant quadratic anisotropy, taken with realistic and experimentally\u0000relevant physical parameters. Surprisingly, we find that the time-crystal\u0000response frequency correlates with the energy levels of the individual magnets\u0000and is essentially independent of the exchange coupling. The latter is\u0000unexpectedly manifested through a pulse-like oscillation in the magnetization\u0000envelope, signaling a many-body response. These results show that molecular\u0000magnets can be a rich platform for studying time-crystalline behavior and\u0000possibly other out-of-equilibrium quantum many-body dynamics.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142248163","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Christian M. Pluchar, Aman R. Agrawal, Dalziel J. Wilson
The optical lever is a precision displacement sensor with broad applications. In principle, it can track the motion of a mechanical oscillator with added noise at the Standard Quantum Limit (SQL); however, demonstrating this performance requires an oscillator with an exceptionally high torque sensitivity, or, equivalently, zero-point angular displacement spectral density. Here, we describe optical lever measurements on Si$_3$N$_4$ nanoribbons possessing $Q>3times 10^7$ torsion modes with torque sensitivities of $10^{-20},text{N m}/sqrt{text{Hz}}$ and zero-point displacement spectral densities of $10^{-10},text{rad}/sqrt{text{Hz}}$. Compensating aberrations and leveraging immunity to classical intensity noise, we realize angular displacement measurements with imprecisions 20 dB below the SQL and demonstrate feedback cooling, using a position modulated laser beam as a torque actuator, from room temperature to $sim5000$ phonons. Our study signals the potential for a new class of torsional quantum optomechanics.
{"title":"Quantum-limited optical lever measurement of a torsion oscillator","authors":"Christian M. Pluchar, Aman R. Agrawal, Dalziel J. Wilson","doi":"arxiv-2409.11397","DOIUrl":"https://doi.org/arxiv-2409.11397","url":null,"abstract":"The optical lever is a precision displacement sensor with broad applications.\u0000In principle, it can track the motion of a mechanical oscillator with added\u0000noise at the Standard Quantum Limit (SQL); however, demonstrating this\u0000performance requires an oscillator with an exceptionally high torque\u0000sensitivity, or, equivalently, zero-point angular displacement spectral\u0000density. Here, we describe optical lever measurements on Si$_3$N$_4$\u0000nanoribbons possessing $Q>3times 10^7$ torsion modes with torque sensitivities\u0000of $10^{-20},text{N m}/sqrt{text{Hz}}$ and zero-point displacement spectral\u0000densities of $10^{-10},text{rad}/sqrt{text{Hz}}$. Compensating aberrations\u0000and leveraging immunity to classical intensity noise, we realize angular\u0000displacement measurements with imprecisions 20 dB below the SQL and demonstrate\u0000feedback cooling, using a position modulated laser beam as a torque actuator,\u0000from room temperature to $sim5000$ phonons. Our study signals the potential\u0000for a new class of torsional quantum optomechanics.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142248118","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Samuel Fedida, Anne-Catherine de la Hamette, Viktoria Kabel, Časlav Brukner
We explore indefinite causal order between events in the context of quasiclassical spacetimes in superposition. We introduce several new quantifiers to measure the degree of indefiniteness of the causal order for an arbitrary finite number of events and spacetime configurations in superposition. By constructing diagrammatic and knot-theoretic representations of the causal order between events, we find that the definiteness or maximal indefiniteness of the causal order is topologically invariant. This reveals an intriguing connection between the field of quantum causality and knot theory. Furthermore, we provide an operational encoding of indefinite causal order and discuss how to incorporate a measure of quantum coherence into our classification.
{"title":"Knot invariants and indefinite causal order","authors":"Samuel Fedida, Anne-Catherine de la Hamette, Viktoria Kabel, Časlav Brukner","doi":"arxiv-2409.11448","DOIUrl":"https://doi.org/arxiv-2409.11448","url":null,"abstract":"We explore indefinite causal order between events in the context of\u0000quasiclassical spacetimes in superposition. We introduce several new\u0000quantifiers to measure the degree of indefiniteness of the causal order for an\u0000arbitrary finite number of events and spacetime configurations in\u0000superposition. By constructing diagrammatic and knot-theoretic representations\u0000of the causal order between events, we find that the definiteness or maximal\u0000indefiniteness of the causal order is topologically invariant. This reveals an\u0000intriguing connection between the field of quantum causality and knot theory.\u0000Furthermore, we provide an operational encoding of indefinite causal order and\u0000discuss how to incorporate a measure of quantum coherence into our\u0000classification.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142248117","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We report how the unique temporal and spectral features of pulsed entangled photons from a parametric downconversion source can be utilized for manipulating electronic excitations through the optimization of their spectral phase. A new comprehensive optimization protocol based on Bayesian optimization has been developed in this work to selectively excite electronic states accessible by two-photon absorption. Using our optimization method, the entangled two-photon absorption probability for a thiophene dendrimer can be enhanced by up to a factor of 20 while classical light turns out to be nonoptimizable. Moreover, the optimization involving photon entanglement enables selective excitation that would not be possible otherwise. In addition to optimization, we have explored entangled two-photon absorption in the small entanglement time limit showing that entangled light can excite molecular electronic states that are vanishingly small for classical light. We demonstrate these opportunities with an application to a thiophene dendrimer.
{"title":"Manipulating Two-Photon Absorption of Molecules through Efficient Optimization of Entangled Light","authors":"Sajal Kumar Giri, George C. Schatz","doi":"arxiv-2409.11368","DOIUrl":"https://doi.org/arxiv-2409.11368","url":null,"abstract":"We report how the unique temporal and spectral features of pulsed entangled\u0000photons from a parametric downconversion source can be utilized for\u0000manipulating electronic excitations through the optimization of their spectral\u0000phase. A new comprehensive optimization protocol based on Bayesian optimization\u0000has been developed in this work to selectively excite electronic states\u0000accessible by two-photon absorption. Using our optimization method, the\u0000entangled two-photon absorption probability for a thiophene dendrimer can be\u0000enhanced by up to a factor of 20 while classical light turns out to be\u0000nonoptimizable. Moreover, the optimization involving photon entanglement\u0000enables selective excitation that would not be possible otherwise. In addition\u0000to optimization, we have explored entangled two-photon absorption in the small\u0000entanglement time limit showing that entangled light can excite molecular\u0000electronic states that are vanishingly small for classical light. We\u0000demonstrate these opportunities with an application to a thiophene dendrimer.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142248273","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Antoine Glicenstein, Apoorva Apoorva, Daniel Benedicto Orenes, Hector Letellier, Alvaro Mitchell Galvão de Melo, Raphaël Saint-Jalm, Robin Kaiser
This study introduces a novel method to investigate in-situ light transport within optically thick ensembles of cold atoms, exploiting the internal structure of alkaline-earth metals. A method for creating an optical excitation at the center of a large atomic cloud is demonstrated, and we observe its propagation through multiple scattering events. In conditions where the cloud size is significantly larger than the transport mean free path, a diffusive regime is identified. We measure key parameters including the diffusion coefficient, transport velocity, and transport time, finding a good agreement with diffusion models. We also demonstrate that the frequency of the photons launched inside the system can be controlled. This approach enables direct time- and space-resolved observation of light diffusion in atomic ensembles, offering a promising avenue for exploring new diffusion regimes.
{"title":"In-situ measurements of light diffusion in an optically dense atomic ensemble","authors":"Antoine Glicenstein, Apoorva Apoorva, Daniel Benedicto Orenes, Hector Letellier, Alvaro Mitchell Galvão de Melo, Raphaël Saint-Jalm, Robin Kaiser","doi":"arxiv-2409.11117","DOIUrl":"https://doi.org/arxiv-2409.11117","url":null,"abstract":"This study introduces a novel method to investigate in-situ light transport\u0000within optically thick ensembles of cold atoms, exploiting the internal\u0000structure of alkaline-earth metals. A method for creating an optical excitation\u0000at the center of a large atomic cloud is demonstrated, and we observe its\u0000propagation through multiple scattering events. In conditions where the cloud\u0000size is significantly larger than the transport mean free path, a diffusive\u0000regime is identified. We measure key parameters including the diffusion\u0000coefficient, transport velocity, and transport time, finding a good agreement\u0000with diffusion models. We also demonstrate that the frequency of the photons\u0000launched inside the system can be controlled. This approach enables direct\u0000time- and space-resolved observation of light diffusion in atomic ensembles,\u0000offering a promising avenue for exploring new diffusion regimes.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142248276","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
It is a sort of ultimate question to examine the continuity of a quantum measurement subject theoretically and has not yet been resolved within a scientific framework. In this article, we approach this question and argue that the continuity of a quantum measurement subject follows as a fundamental consequence of the holographic principle after the classicalization of the quantum state of the bulk space.
{"title":"A remark on quantum measuring systems and the holographic principle","authors":"Eiji Konishi","doi":"arxiv-2409.11594","DOIUrl":"https://doi.org/arxiv-2409.11594","url":null,"abstract":"It is a sort of ultimate question to examine the continuity of a quantum\u0000measurement subject theoretically and has not yet been resolved within a\u0000scientific framework. In this article, we approach this question and argue that\u0000the continuity of a quantum measurement subject follows as a fundamental\u0000consequence of the holographic principle after the classicalization of the\u0000quantum state of the bulk space.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142248102","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The phenomenon of induced coherence without induced emission allows to reconstruct the quantum state of a photon that remains undetected. The state information is transferred to its partner photon via optical coherence. Using this phenomenon, a number of established quantum information protocols could be adapted for undetected photons. Despite partial attempts, no general procedure for such adaptation exists. Here we shed light on the matter by showing the close relation between two very dissimilar techniques, namely the quantum state tomography of qubits and the recently developed quantum state tomography of undetected photons. We do so by introducing a set of parameters that quantify the coherence and that mimic the Stokes parameters known from the polarization state tomography. We also perform a thorough analysis of the environment of undetected photons and its role in the reconstruction process.
{"title":"Visibility Stokes parameters as a foundation for quantum information science with undetected photons","authors":"Jaroslav Kysela, Markus Gräfe, Jorge Fuenzalida","doi":"arxiv-2409.10740","DOIUrl":"https://doi.org/arxiv-2409.10740","url":null,"abstract":"The phenomenon of induced coherence without induced emission allows to\u0000reconstruct the quantum state of a photon that remains undetected. The state\u0000information is transferred to its partner photon via optical coherence. Using\u0000this phenomenon, a number of established quantum information protocols could be\u0000adapted for undetected photons. Despite partial attempts, no general procedure\u0000for such adaptation exists. Here we shed light on the matter by showing the\u0000close relation between two very dissimilar techniques, namely the quantum state\u0000tomography of qubits and the recently developed quantum state tomography of\u0000undetected photons. We do so by introducing a set of parameters that quantify\u0000the coherence and that mimic the Stokes parameters known from the polarization\u0000state tomography. We also perform a thorough analysis of the environment of\u0000undetected photons and its role in the reconstruction process.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142248165","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
What is fundamentally quantum? We argue that most of the features, problems, and paradoxes -- such as the measurement problem, the Wigner's friend paradox and its proposed solutions, single particle nonlocality, and no-cloning -- allegedly attributed to quantum physics have a clear classical analogue if one is to interpret classical physics as fundamentally indeterministic. What really characterizes quantum physics boils down only to phenomena that involve $hbar$, i.e., incompatible observables.
{"title":"Which features of quantum physics are not fundamentally quantum but are due to indeterminism?","authors":"Flavio Del Santo, Nicolas Gisin","doi":"arxiv-2409.10601","DOIUrl":"https://doi.org/arxiv-2409.10601","url":null,"abstract":"What is fundamentally quantum? We argue that most of the features, problems,\u0000and paradoxes -- such as the measurement problem, the Wigner's friend paradox\u0000and its proposed solutions, single particle nonlocality, and no-cloning --\u0000allegedly attributed to quantum physics have a clear classical analogue if one\u0000is to interpret classical physics as fundamentally indeterministic. What really\u0000characterizes quantum physics boils down only to phenomena that involve\u0000$hbar$, i.e., incompatible observables.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142248171","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Modern quantum physics is very modular: we first understand basic building blocks (``XXZ Hamiltonian'' ``Jaynes-Cummings'' etc.) and then combine them to explore novel effects. A typical example is placing known systems inside an optical cavity. The Schrieffer-Wolff perturbation method is particularly suited for dealing with these problems, since it casts the perturbation expansion in terms of operator corrections to a Hamiltonian, which is more intuitive than energy level corrections, as in traditional time-independent perturbation theory. However, the method lacks a systematic approach.% and has largely remained a niche topic. In these notes we discuss how emph{eigenoperator decompositions}, a concept largely used in open quantum systems, can be employed to construct an intuitive and systematic formulation of Schrieffer-Wolff perturbation theory. To illustrate this we revisit various papers in the literature, old and new, and show how they can instead be solved using eigenoperators. Particular emphasis is given to perturbations that couple two systems with very different transition frequencies (highly off-resonance), leading to the so-called dispersive interactions.
{"title":"Eigenoperator approach to Schrieffer-Wolff perturbation theory and dispersive interactions","authors":"Gabriel T. Landi","doi":"arxiv-2409.10656","DOIUrl":"https://doi.org/arxiv-2409.10656","url":null,"abstract":"Modern quantum physics is very modular: we first understand basic building\u0000blocks (``XXZ Hamiltonian'' ``Jaynes-Cummings'' etc.) and then combine them to\u0000explore novel effects. A typical example is placing known systems inside an\u0000optical cavity. The Schrieffer-Wolff perturbation method is particularly suited\u0000for dealing with these problems, since it casts the perturbation expansion in\u0000terms of operator corrections to a Hamiltonian, which is more intuitive than\u0000energy level corrections, as in traditional time-independent perturbation\u0000theory. However, the method lacks a systematic approach.% and has largely\u0000remained a niche topic. In these notes we discuss how emph{eigenoperator\u0000decompositions}, a concept largely used in open quantum systems, can be\u0000employed to construct an intuitive and systematic formulation of\u0000Schrieffer-Wolff perturbation theory. To illustrate this we revisit various\u0000papers in the literature, old and new, and show how they can instead be solved\u0000using eigenoperators. Particular emphasis is given to perturbations that couple\u0000two systems with very different transition frequencies (highly off-resonance),\u0000leading to the so-called dispersive interactions.","PeriodicalId":501226,"journal":{"name":"arXiv - PHYS - Quantum Physics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142248168","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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