Friday 16 September 2022
9:00 – 9:25
Geometric Event-Based Relativistic Quantum Mechanics
Universita’ di Pavia
We propose a special relativistic framework for quantum mechanics. It is based on introducing a Hilbert space for events. Events are taken as primitive notions (as customary in relativity), whereas quantum systems (e.g. fields and particles) are emergent in the form of joint probability amplitudes for position and time of events. Textbook relativistic quantum mechanics and quantum field theory can be recovered by dividing the event Hilbert spaces into space and time (a foliation) and then conditioning the event states onto the time part. Our theory satisfies the full Poincare’ symmetry as a ‘geometric’ unitary transformation, and possesses observables for space (location of an event) and time (position in time of an event).
9:25 – 9:50
The p-adic qubits
Within the framework of quantum mechanics over (a quadratic extension of) the non-
Archimedean field of p-adic numbers Qp, we provide a general definition of a quantum state
relying on an algebraic approach and on a suitable p-adic model of probability theory. As in
the standard complex case, a distinguished class of physical states are related to a notion of a trace for a bounded operator, and one can define a suitable class of trace class operators in the non-Archimedean setting. Eventually, we will particularize our discussion to two-dimensional systems, thus obtaining a p-adic model of the qubit.
9:50 – 10:15
Incompatibility of observables, channels and instruments in information theories
Every theory of information, including classical and quantum, can be studied in the framework of operational probabilistic theories—where the notion of test generalizes that of quantum instrument, namely a collection of quantum oper- ations summing to a channel, and simple rules are given for the composition of tests in parallel and in sequence. Here we study the notion of compatibil- ity for tests an operational probabilistic theory. Following the quantum litera- ture, we first introduce the notion of strong compatibility, and then we illustrate its ultimate relaxation, that we deem weak compatibility. It is shown that the two notions coincide in the case of observation tests—which are the counter- part of quantum POVMs—while there exist weakly compatible channels that are not strongly compatible. We prove necessary and sufficient conditions for a the- ory to exhibit incompatible tests. We show that a theory admits of incompatible tests if and only if some information cannot be extracted without disturbance.
10:15 – 10:40
Multi-parameter digital quantum estimation
Global quantum estimation strategies allow to extract information on a
phase or a set of phases encoded on a quantum state without any prior
knowledge about them. As far as we know, unlike the local estimation case,
a general model encompassing all of the global strategies does not exist
yet. We show that Holevo’s estimation theory provides a good candidate,
i.e. a model that allows to retrieve both parallel and sequential
strategies. Moreover, it yields a framework for multiparameter global
estimation, since the model can be straightforwardly generalized for the
estimation of more than one phase. In particular, we focus on the case of
double-parameter estimation and explore the advantages of multiparameter
estimation with respect to multiple single-parameter estimations in terms
of mutual information.
10:40 – 11:10
11:10 – 11:35
Quantized-gravity and “gravitzed”-quantum, testing spin-statistics and quantum collapse in the cosmic silence
The VIP experiment,at the Laboratori Nazionali del Gran Sasso of INFN,is explor- ing Quantum Mechanics (QM) foundations,investigating models of dynamical wave function collapse and performing high sensitivity tests of the Pauli Exclu- sion Principle (PEP) for electrons.Motivated by the awareness that space-time fluctuations would induce decoherence in quantum systems,the idea to ”gravi- tize” QM aroused growing interest in the last decades.We will report the strong experimental constrains on the gravity-related collapse models developed by Diosi and Penrose,obtained by searching for an unavoidable effect of the col- lapse mechanism,namely a faint radiation emission by charged particles.The de- velopment of non-Markovian and dissipative versions of the models,and their impact on the spontaneous radiation will be outlined.Recently emerged that PEP violation may be induced by space-time non-commutativity,a class of universality for several models of Quantum Gravity.X-ray surveys,searching for atomic tran- sitions prohibited by the PEP,represent stunning candidates to test Quantum Gravity,at unexpectedly high energy scales.The results of exploratory studies will be presented.
Gate fidelities in arrays of noisy flip-flop qubits
A Flip-Flop Qubit (FFQ) is defined on a mixture of nuclear and bonded electron spin states of a 31P atom in a spin-free 28Si substrate . High-fidelity one- qubit operations are implemented by exploiting electric dipole spin resonance  and two-qubit ones could be created by using electric dipole interaction. The long-range dipole-dipole interaction between FFQs can relax the stringent fabri- cation requirements for spin-based qubits, particularly on the lateral positioning of gates/donors thus easing the fabrication specs from some tens of nm to few hundreds of nm range. Parallel gating is a central ingredient for quantum com- putation, but gate parallelism is limited by unwanted inter-qubit interactions. Those interactions reduce gate fidelities thus parallel one-qubit and two-qubit gates performances are simulated in FFQ-arrays embedded in a realistic noisy environment . Such results are presented, also providing additional insights for system scaling-up towards a silicon-based quantum processor.  G. Tosi et al., Nat Commun. 6 8(1):450, 2017,  R. Savytskyy et al., arXiv:2202.04438v1,  D. Rei, E. Ferraro and M. De Michielis, Adv. Quantum Technol., 5: 2100133, 2022
11:35 – 12:00
Certification and quantification of non-classicality through causal inference
Quantum technologies have promised to bring advances in several fields. How- ever, to correctly exploit them, we need to certify the quantum operation of a given device. Device-independent protocols tackle this problem, requiring minimal assumptions and relying on quantum causal inequality violations (e.g. Bell tests). Unfortunately, on one hand, such testable constraints are not always available, and, on the other, for practical purposes, we also need to quantify non-classicality, e.g. to evaluate the randomness that is extractable from quan- tum random numbers generators. Here, we present two methods to address these issues and their implementation on a photonic platform. First, we show how the amount of causal influence among variables (measured through direct interventions on the apparatus) allows to distinguish classical and non-classical features within a given process, even if no standard quantum inequality violations are possible. Then, we design an optimizer over the set of behaviors allowed by quantum mechanics, based on an artificial neural network. This tool extends cur- rently used methods, being able to deal with non-linear objective functions and optimization constraints.
Energy-efficient entanglement generation and readout in a spin-photon interface
Spin photon interfaces (SPI) are devices where a spin is strongly coupled to a one dimensional atom. They are essential building blocks for quantum technologies. In particular, they can be used to generate cluster states of photons needed to implement measurement based quantum computing. So far, SPIs have been probed by using coherent pulses or single photons as input light. Here we study the potential of a purely quantum resource, i.e. quantum superpositions of zero and single photon states, which have recently been shown to be within experi- mental reach. Based on an exact resolution of the light-matter dynamical equa- tions, we show that such quantum superpositions are more energy efficient than coherent pulses, generating more spin-light entanglement with the same input energy. We show that this energetic quantum advantage is transferred at the classical level, quantum superpositions giving rise to more precise spin readouts than coherent pulses of the same energy. Estimations with realistic numbers taken from semiconducting devices (quantum dots) show that these effects can be observed with state-of-the-art SPIs.
12:00 – 12:25
Collapse dynamics are diffusive
Testing the limits of validity of the superposition principle is of crucial impor- tance in the foundations of quantum mechanics and the development of quan- tum technologies. A way to quantify possible breakdowns of the superposition principle is given by collapse models. These models modify quantum mechanics by introducing a nonlinear interaction with a classical noise that induces collapse in space. The natural way of testing collapse models is through interferometric experiments of systems with large masses, which is challenging. For this reason, non-interferometric experiments were considered. These exploit the fact that the noise responsible for the collapse induces a diffusion in momentum, in prin- ciple detectable even in localized systems by performing high precision position measurements. We first give a summary of the bounds on collapse models from non-interferometric experiments. Then we show how the diffusion in momen- tum is not just a property of collapse models but it is a universal feature of any dynamics inducing collapse in space. This implies that non-interferometric ex- periments test the quantum superposition principle in a stronger sense than one might suppose.
Managing light-matter interaction on timescales faster than the loss of electronic coherence is key for achieving the full quantum control of final products in solid- solid transformations. In this work, we demonstrate coherent electronic con- trol of the photoinduced insulator-to-metal transition in the prototypical Mott insulator V2O3. Selective excitation of a specific interband transition with two phase-locked light pulses manipulates the orbital occupation of the correlated bands in a way that depends on the coherent evolution of the photoinduced superposition of states. Comparison between experimental results and numer- ical solutions of the optical Bloch equations provides an electronic coherence time on the order of 5 fs. Temperature dependent experiments suggest that the electronic coherence time is enhanced in the vicinity of the insulator-to-metal transition critical temperature, thus highlighting the role of fluctuations in deter- mining the electronic coherence. These results open new routes to selectively switch functionalities of quantum materials and store quantum information in solid-solid transformations.
12:25 – 12:50
Optimal interferometry and Bell-nonclassicality with a single photon state
Bell nonclassicality of a single photon superposition in two modes, often referred to as ’nonlocality of a single photon’, is one of the most striking nonclassical phe- nomena discussed in the context of foundations of quantum physics. We recon- sider the all-optical weak homodyne-measurement based experimental schemes aimed at revealing Bell nonclassicality (‘nonlocality’) of a single photon. We focus on the schemes put forward by Tan, Walls and Collett (TWC, 1991) and Hardy (1994). There are consequential differences between TWC and Hardy setups: (i) The initial state of Hardy is a superposition of a single photon excitation with vacuum in one of the input modes of a 50-50 beamsplitter. In the TWC case, there is no vacuum component. (ii) In the final measurements of Hardy’s proposal the local settings are specified by the presence or absence of a local oscillator field (on/off). Eventually, we show that the TWC experiment can be described by a local hidden variable model, hence the claimed nonclassicality is apparent. The nonclassicality proof proposed by Hardy remains impeccable. We investi- gate which feature of Hardy’s approach is crucial to disclose the nonclassicality. Nowadays, photon-number resolving weak-field homodyne measurements al- low the realization of emblematic gedankenexperiments revealing correlations of optical fields. Here we show how to robustly violate local realism within the weak-field homodyne measurement scheme for any superposition of one photon with vacuum. Our modification of the previously proposed setups involves tun- able beamsplitters at the measurement stations, and the local oscillator fields significantly varying between the settings, optimally being on or off. We find a condition for optimal measurement settings for the maximal violation of the Clauser-Horne inequality with weak-field homodyne detection, which states that the reflectivity of the local beam-splitter must be equal to the strength of the local oscillator field. We show that this condition holds not only for the vacuum- one-photon qubit input state but also for the superposition of a photon pair with vacuum, which suggests its generality as a property of weak-field homodyne de- tection with photon-number resolution. Our findings suggest a possible path to employ such scenarios in device-independent quantum protocols. T. Das, M. Karczewski, A. Mandarino, M. Markiewicz, B. Woloncewicz and M. Zukowski, Wave-particle complementarity: detecting violation of local realism with photon- number resolving weak-field homodyne measurements, New J. Phys. 24 033017 (2022), T. Das, M. Karczewski, A. Mandarino, M. Markiewicz, B. Woloncewicz and M. Zukowski, Remarks about Bell-nonclassicality of a single photon, Phys. Lett. A 435 128031 (2022), T. Das, M. Karczewski, A. Mandarino, M. Markiewicz, B. Woloncewicz and M. Zukowski, Can single photon excitation of two spa- tially separated modes lead to a violation of Bell inequality via weak-field homo- dyne measurements? New J. Phys. 23 073042 (2021), T. Das, M. Karczewski, A. Mandarino, M. Markiewicz, and M. Zukowski, Optimal interferometry for Bell- nonclassicality by a vacuum-one-photon qubit, arXiv:2109.10170
Cavity protection for intersubband polaritons in strong magnetic field
We analyse the effect of a perpendicular homogeneous magnetic field on an op- tical intersubband transitions coupled to a cavity. It turns out that the magnetic field changes the shape of the intersubband optical density, strongly affecting the resulting polaritonic emission linewidth. In the strong coupling regime and when the magnetic field reaches the Hall regime for the confined electrons the polaritonic linewidth can be reduced by several order of magnidutes. This effect may pave the way for the full implementation of an infrared laser with intersub- band transitions and opens to more fundamental studies regarding the interplay between quantum Hall physics and cavity light-matter interactions.
12:50 – 15:00
15:00 – 15:25
Here we summarize recent theoretical studies on the dynamical Casimir effects (DCEs) in optomechanical systems.
We studied the DCE using a fully quantum-mechanical description and without linearizing the dynamics. We have shown that the resonant generation of pho- tons from the vacuum is determined by a ladder of mirror-field vacuum Rabi split- ting. We find that vacuum emission can originate from the free evolution of an initial pure mechanical excited state, in analogy with the spontaneous emission from excited atoms. We also show that the DCE can also be driven by incoherent mechanical pumping. We then applied this framework to study the interaction of two mechanical oscillators mediated by the exchange of virtual photon pairs. Specifically, we demonstrated that mechanical quantum excitations can be co- herently transferred among spatially separated mechanical oscillators, through a dissipationless quantum bus, due to the exchange of virtual photon pairs. This system can also operate as a mechanical parametric downconverter.
15:25 – 15:50
Cavity optomechanics with levitated nanosphere: Quantum signatures in hybrid light-mechanical states and advances in two-dimensional cooling
Optically levitated nanospheres in high-finesse cavities offer a unique platform for the study of macroscopic quantum mechanics in all three spatial dimensions and the achievement of quantum coherent control of their motion, with appli- cations ranging from quantum foundations and information processing to direc- tional quantum sensing. We report on cavity-optomechanical experiments in which the motion of a nanosphere levitated in high vacuum is strongly coupled to a single cavity mode by coherent scattering of the tweezer photons. The two- dimensional motion on the plane orthogonal to the tweezer axis and the optical cavity mode define an optomechanical system with three degrees of freedom. In the quantum-coherent strong-coupling regime, i.e. when the optomechani- cal coupling exceeds the total decoherence rate, we observe the formation of hybrid light-mechanical states with a peculiar vectorial nature. Such states give rise to polaritonic dispersion relations characterized by two avoided crossings at different frequencies, unambiguous signature of the strong three-body interac- tions. For appropriate frequencies and polarization of the tweezer beam, the motion of the particle is strongly cooled in the plane orthogonal to the tweezer axis. We demonstrate a regime in which the fully 2D dynamics of the nanopar- ticle exhibits strong non-classical properties, and we introduce an indicator that quantifies how close the system is to a minimum uncertainty state. These find- ings pave the way to novel protocols for the transfer of quantum information between photonic and phononic components and represent an important step towards the demonstration of optomechanical entangled states at room tem- perature.
15:50 – 16:15
Two-membrane cavity optomechanics and Quantum Technologies
The linear and non-linear dynamics of an optomechanical system made of a two- membrane ethalon in a high-finesse Fabry-Pérot cavity is presented. This two- membrane setup has the capacity to enhance the single-photon optomechanical coupling, and in the linearized interaction regime to cool simultaneously two me- chanical oscillators. The experimental characterization of the optical, mechan- ical, and especially optomechanical properties of a two-membrane sandwich within an optical cavity will be presented. In the non-linear regime, a truthful de- tection of membrane displacements much above the usual linear sensing limited by the cavity linewidth is presented. The non-linear dynamics of the mechanical oscillator provides a novel procedure for the determination of the single-photon optomechanical coupling rate, that is the optomechanical interaction strength of the system. In the second part of the talk, we will illustrate the QUANTum Exper- imental Platform (QUANTEP) INFN project, which aims at the development and implementation of a complete Silicon Photonics Integrated platform for Quan- tum Computation with linear optics circuits, focusing on its recent achievements.
16:15 – 16:40
Quantum-jump trajectories from non-linear rate operators: continuous measurements and non-Markovianity
Stochastic methods with quantum jumps are routinely used to describe open quantum system dynamics, both for the advantage they provide from a computa- tional point of view, and for their insight into fundamental topics, such as the role of measurements in quantum mechanics and the description of non-Markovian memory effects. In this talk, I will present a recently introduced quantum-jump approach , named rate operator quantum jumps (ROQJ), whose quantum- jump trajectories are defined in terms of state-dependent, non-linear rate op- erators. I will first show that ROQJ is associated with a systematic continuous- measurement scheme for a wide and physically relevant class of dynamics, in- cluding a set of master equations with negative decay rates, where the standard Monte Carlo wave function (MCWF) approach  to quantum jumps does not apply. I will then discuss how ROQJ can be extended to deal with general non- Markovian evolutions, going beyond the current non-Markovian generalizations of MCWF  and helping clarify the different types of memory effects within the context of quantum jumps. Finally, I will show that different quantum-jump pic- tures can be formulated by exploiting the freedom in how to assign the terms of the underlying master equation to the deterministic and jump parts of the trajectories, which can result in a significant simplification of the corresponding stochastic description .  A. Smirne, M. Caiaffa, and J. Piilo, Phys. Rev. Lett. 124, 190402 (2020)  J. Dalibard, Y. Castin, and K. Mølmer, Phys. Rev. Lett. 68, 580 (1992)  J. Piilo, S. Maniscalco, K. Härkönen, and K.A. Suominen, Phys. Rev. Lett. 100,180402 (2008)  D. Chruscinski, K. Luoma, J. Piilo, A. Smirne, arXiv:2009.11312v2 (2021)
16:40 – 17:10
17:10 – 17:35
Quantum transfer of interacting qubits
The transfer of quantum information between different locations is key to many quantum information processing tasks. Whereas, the transfer of a single qubit state has been extensively investigated, the transfer of a many-body system con- figuration has insofar remained elusive. We address the problem of transferring the state of n interacting qubits. Both the exponentially increasing Hilbert space dimension, and the presence of interactions significantly scale-up the complexity of achieving high-fidelity transfer. By employing tools from random matrix the- ory and using the formalism of quantum dynamical maps, we derive a general expression for the average and the variance of the fidelity of an arbitrary quan- tum state transfer protocol for n interacting qubits. Finally, by adopting a weak- coupling scheme in a spin chain, we obtain the explicit conditions for high-fidelity transfer of 3 and 4 interacting qubits. https://doi.org/10.48550/arXiv.2205.01579
17:35 – 18:00
Exact solution for the quantum and private capacities of bosonic dephasing channels
The capacities of noisy quantum channels capture the ultimate rates of infor- mation transmission across quantum communication lines, and the quantum ca- pacity plays a key role in determining the overhead of fault-tolerant quantum computation platforms. In the case of bosonic systems, central to many applica- tions, no closed formulas for these capacities were known for bosonic dephasing channels, a key class of non-Gaussian channels modelling, e.g., noise affecting superconducting circuits or fiber-optic communication channels. Here we pro- vide the first exact calculation of the quantum, private, two-way assisted quan- tum, and secret-key agreement capacities of all bosonic dephasing channels. We prove that that they are equal to the relative entropy of the distribution underly- ing the channel to the uniform distribution. Our result solves a problem that has been open for over a decade, having been posed originally by [Jiang & Chen, Quantum and Nonlinear Optics 244, 2010].
18:00 – 18:25