Research

8 pages, 7100 KiB

Open AccessArticle

Correcting Coherent Errors by Random Operation on Actual Quantum Hardware

by Gabriele Cenedese, Giuliano Benenti and Maria Bondani

Entropy 2023, 25(2), 324; https://doi.org/10.3390/e25020324 - 10 Feb 2023

Cited by 1 | Viewed by 1000

Characterizing and mitigating errors in current noisy intermediate-scale devices is important to improve the performance of the next generation of quantum hardware. To investigate the importance of the different noise mechanisms affecting quantum computation, we performed a full quantum process tomography of single [...] Read more.

Characterizing and mitigating errors in current noisy intermediate-scale devices is important to improve the performance of the next generation of quantum hardware. To investigate the importance of the different noise mechanisms affecting quantum computation, we performed a full quantum process tomography of single qubits in a real quantum processor in which echo experiments are implemented. In addition to the sources of error already included in the standard models, the obtained results show the dominant role of coherent errors, which we practically corrected by inserting random single-qubit unitaries in the quantum circuit, significantly increasing the circuit length over which quantum computations on actual quantum hardware produce reliable results. Full article

(This article belongs to the Special Issue Selected Feature Papers from Italian Quantum Information Science Conference 2022)

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12 pages, 12900 KiB

Open AccessArticle

A Novel Approach to Parameter Determination of the Continuous Spontaneous Localization Collapse Model

by Kristian Piscicchia, Alessio Porcelli, Angelo Bassi, Massimiliano Bazzi, Mario Bragadireanu, Michael Cargnelli, Alberto Clozza, Luca De Paolis, Raffaele Del Grande, Maaneli Derakhshani, Diósi Lajos, Sandro Donadi, Carlo Guaraldo, Mihai Iliescu, Matthias Laubenstein, Simone Manti, Johann Marton, Marco Miliucci, Fabrizio Napolitano, Alessandro Scordo, Francesco Sgaramella, Diana Laura Sirghi, Florin Sirghi, Oton Vazquez Doce, Johann Zmeskal and Catalina Curceanu Show full author list Hide full author list

Entropy 2023, 25(2), 295; https://doi.org/10.3390/e25020295 - 04 Feb 2023

Cited by 1 | Viewed by 1339

Abstract

Models of dynamical wave function collapse consistently describe the breakdown of the quantum superposition with the growing mass of the system by introducing non-linear and stochastic modifications to the standard Schrödinger dynamics. Among them, Continuous Spontaneous Localization (CSL) was extensively investigated both theoretically [...] Read more.

Models of dynamical wave function collapse consistently describe the breakdown of the quantum superposition with the growing mass of the system by introducing non-linear and stochastic modifications to the standard Schrödinger dynamics. Among them, Continuous Spontaneous Localization (CSL) was extensively investigated both theoretically and experimentally. Measurable consequences of the collapse phenomenon depend on different combinations of the phenomenological parameters of the model—the strength

λ

and the correlation length

r_{C}

—and have led, so far, to the exclusion of regions of the admissible (

λ - r_{C}

) parameters space. We developed a novel approach to disentangle the

λ

and

r_{C}

probability density functions, which discloses a more profound statistical insight. Full article

(This article belongs to the Special Issue Selected Feature Papers from Italian Quantum Information Science Conference 2022)

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14 pages, 576 KiB

Open AccessArticle

Coarse-Grained Effective Hamiltonian via the Magnus Expansion for a Three-Level System

by Nicola Macrì, Luigi Giannelli, Elisabetta Paladino and Giuseppe Falci

Entropy 2023, 25(2), 234; https://doi.org/10.3390/e25020234 - 27 Jan 2023

Viewed by 1166

Abstract

Quantum state processing is one of the main tools of quantum technologies. While real systems are complicated and/or may be driven by non-ideal control, they may nevertheless exhibit simple dynamics approximately confined to a low-energy Hilbert subspace. Adiabatic elimination is the simplest approximation [...] Read more.

Quantum state processing is one of the main tools of quantum technologies. While real systems are complicated and/or may be driven by non-ideal control, they may nevertheless exhibit simple dynamics approximately confined to a low-energy Hilbert subspace. Adiabatic elimination is the simplest approximation scheme allowing us to derive in certain cases an effective Hamiltonian operating in a low-dimensional Hilbert subspace. However, these approximations may present ambiguities and difficulties, hindering a systematic improvement of their accuracy in larger and larger systems. Here, we use the Magnus expansion as a systematic tool to derive ambiguity-free effective Hamiltonians. We show that the validity of the approximations ultimately leverages only on a proper coarse-graining in time of the exact dynamics. We validate the accuracy of the obtained effective Hamiltonians with suitably tailored fidelities of quantum operations. Full article

(This article belongs to the Special Issue Selected Feature Papers from Italian Quantum Information Science Conference 2022)

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11 pages, 341 KiB

Open AccessArticle

Superradiant Quantum Phase Transition for an Exactly Solvable Two-Qubit Spin-Boson Model

by Roberto Grimaudo, Davide Valenti, Alessandro Sergi and Antonino Messina

Entropy 2023, 25(2), 187; https://doi.org/10.3390/e25020187 - 17 Jan 2023

Cited by 8 | Viewed by 1541

Abstract

A spin-boson-like model with two interacting qubits is analysed. The model turns out to be exactly solvable since it is characterized by the exchange symmetry between the two spins. The explicit expressions of eigenstates and eigenenergies make it possible to analytically unveil the [...] Read more.

A spin-boson-like model with two interacting qubits is analysed. The model turns out to be exactly solvable since it is characterized by the exchange symmetry between the two spins. The explicit expressions of eigenstates and eigenenergies make it possible to analytically unveil the occurrence of first-order quantum phase transitions. The latter are physically relevant since they are characterized by abrupt changes in the two-spin subsystem concurrence, in the net spin magnetization and in the mean photon number. Full article

(This article belongs to the Special Issue Selected Feature Papers from Italian Quantum Information Science Conference 2022)

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11 pages, 1348 KiB

Open AccessArticle

On the Fidelity Robustness of CHSH–Bell Inequality via Filtered Random States

by Antonio Mandarino and Giovanni Scala

Entropy 2023, 25(1), 94; https://doi.org/10.3390/e25010094 - 03 Jan 2023

Cited by 2 | Viewed by 1481

Abstract

The theorem developed by John Bell constituted the starting point of a revolution that translated a philosophical question about the nature of reality into the broad and intense field of research of the quantum information technologies. We focus on a system of two [...] Read more.

The theorem developed by John Bell constituted the starting point of a revolution that translated a philosophical question about the nature of reality into the broad and intense field of research of the quantum information technologies. We focus on a system of two qubits prepared in a random, mixed state, and we study the typical behavior of their nonlocality via the CHSH–Bell inequality. Afterward, motivated by the necessity of accounting for inefficiency in the state preparation, we address to what extent states close enough to one with a high degree of nonclassicality can violate local realism with a previously chosen experimental setup. Full article

(This article belongs to the Special Issue Selected Feature Papers from Italian Quantum Information Science Conference 2022)

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15 pages, 393 KiB

Open AccessArticle

A p-Adic Model of Quantum States and the p-Adic Qubit

by Paolo Aniello, Stefano Mancini and Vincenzo Parisi

Entropy 2023, 25(1), 86; https://doi.org/10.3390/e25010086 - 31 Dec 2022

Cited by 2 | Viewed by 1859

Abstract

We propose a model of a quantum

N

-dimensional system (quNit) based on a quadratic extension of the non-Archimedean field of p-adic numbers. As in the standard complex setting, states and observables of a p-adic quantum system are implemented by suitable [...] Read more.

We propose a model of a quantum

N

-dimensional system (quNit) based on a quadratic extension of the non-Archimedean field of p-adic numbers. As in the standard complex setting, states and observables of a p-adic quantum system are implemented by suitable linear operators in a p-adic Hilbert space. In particular, owing to the distinguishing features of p-adic probability theory, the states of an

N

-dimensional p-adic quantum system are implemented by p-adic statistical operators, i.e., trace-one selfadjoint operators in the carrier Hilbert space. Accordingly, we introduce the notion of selfadjoint-operator-valued measure (SOVM)—a suitable p-adic counterpart of a POVM in a complex Hilbert space—as a convenient mathematical tool describing the physical observables of a p-adic quantum system. Eventually, we focus on the special case where

N = 2

, thus providing a description of p-adic qubit states and 2-dimensional SOVMs. The analogies—but also the non-trivial differences—with respect to the qubit states of standard quantum mechanics are then analyzed. Full article

(This article belongs to the Special Issue Selected Feature Papers from Italian Quantum Information Science Conference 2022)

15 pages, 7638 KiB

Open AccessArticle

Multi-Dimensional Quantum Capacitance of the Two-Site Hubbard Model: The Role of Tunable Interdot Tunneling

by Andrea Secchi and Filippo Troiani

Entropy 2023, 25(1), 82; https://doi.org/10.3390/e25010082 - 31 Dec 2022

Cited by 2 | Viewed by 1174

Abstract

Few-electron states confined in quantum-dot arrays are key objects in quantum computing. The discrimination between these states is essential for the readout of a (multi-)qubit state, and can be achieved through a measurement of the quantum capacitance within the gate-reflectometry approach. For a [...] Read more.

Few-electron states confined in quantum-dot arrays are key objects in quantum computing. The discrimination between these states is essential for the readout of a (multi-)qubit state, and can be achieved through a measurement of the quantum capacitance within the gate-reflectometry approach. For a system controlled by several gates, the dependence of the measured capacitance on the direction of the oscillations in the voltage space is captured by the quantum capacitance matrix. Herein, we apply this tool to study a double quantum dot coupled to three gates, which enable the tuning of both the bias and the tunneling between the two dots. Analytical solutions for the two-electron case are derived within a Hubbard model, showing the overall dependence of the quantum capacitance matrix on the applied gate voltages. In particular, we investigate the role of the tunneling gate and reveal the possibility of exploiting interdot coherences in addition to charge displacements between the dots. Our results can be directly applied to double-dot experimental setups, and pave the way for further applications to larger arrays of quantum dots. Full article

(This article belongs to the Special Issue Selected Feature Papers from Italian Quantum Information Science Conference 2022)

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12 pages, 903 KiB

Open AccessArticle

Entangled States Are Harder to Transfer than Product States

by Tony J. G. Apollaro, Salvatore Lorenzo, Francesco Plastina, Mirko Consiglio and Karol Życzkowski

Entropy 2023, 25(1), 46; https://doi.org/10.3390/e25010046 - 27 Dec 2022

Cited by 2 | Viewed by 1467

Abstract

The distribution of entangled states is a key task of utmost importance for many quantum information processing protocols. A commonly adopted setup for distributing quantum states envisages the creation of the state in one location, which is then sent to (possibly different) distant [...] Read more.

The distribution of entangled states is a key task of utmost importance for many quantum information processing protocols. A commonly adopted setup for distributing quantum states envisages the creation of the state in one location, which is then sent to (possibly different) distant receivers through some quantum channels. While it is undoubted and, perhaps, intuitively expected that the distribution of entangled quantum states is less efficient than that of product states, a thorough quantification of this inefficiency (namely, of the difference between the quantum-state transfer fidelity for entangled and factorized states) has not been performed. To this end, in this work, we consider n-independent amplitude-damping channels, acting in parallel, i.e., each, locally, on one part of an n-qubit state. We derive exact analytical results for the fidelity decrease, with respect to the case of product states, in the presence of entanglement in the initial state, for up to four qubits. Interestingly, we find that genuine multipartite entanglement has a more detrimental effect on the fidelity than two-qubit entanglement. Our results hint at the fact that, for larger n-qubit states, the difference in the average fidelity between product and entangled states increases with increasing single-qubit fidelity, thus making the latter a less trustworthy figure of merit. Full article

(This article belongs to the Special Issue Selected Feature Papers from Italian Quantum Information Science Conference 2022)

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11 pages, 777 KiB

Open AccessArticle

Feasibility of a Novel Quantum Communication Protocol in Jerlov Type I Water

by Alessia Allevi and Maria Bondani

Entropy 2023, 25(1), 16; https://doi.org/10.3390/e25010016 - 22 Dec 2022

Cited by 4 | Viewed by 1036

Abstract

Underwater communication based on the use of optical quantum resources has attracted a lot of attention in the last five years due to the potential advantages offered by quantum states of light. In this context, we propose to operate in the mesoscopic intensity [...] Read more.

Underwater communication based on the use of optical quantum resources has attracted a lot of attention in the last five years due to the potential advantages offered by quantum states of light. In this context, we propose to operate in the mesoscopic intensity regime, where the optical states are well populated and the employed detectors have photon-number resolution. By exploiting these features, we demonstrate that a novel communication protocol based on the experimental quantification of nonclassicality of mesoscopic twin-beam states can be used to transmit binary signals encoded in two single-mode pseudothermal states with different mean values through a Jerlov type I water channel. The experimental results are in perfect agreement with the developed theoretical model, and the feasibility of the protocol is also investigated as a function of the data samples corresponding to each one of the two signals. The good quality of the results encourages a more realistic implementation of the protocol, also exploring the maximum distance at which the quantum states remain nonclassical and thus can be still properly discriminated. Full article

(This article belongs to the Special Issue Selected Feature Papers from Italian Quantum Information Science Conference 2022)

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