Roberto Stassi
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View article: Noise Protected Logical Qubit in a Open Chain of Superconducting Qubits with Ultrastrong Interactions
Noise Protected Logical Qubit in a Open Chain of Superconducting Qubits with Ultrastrong Interactions Open
To achieve a fault-tolerant quantum computer, it is crucial to increase the coherence time of quantum bits. In this work, we theoretically investigate a system consisting of a series of superconducting qubits that alternate between XX and …
View article: Spectral properties of two superconducting artificial atoms coupled to a resonator in the ultrastrong coupling regime
Spectral properties of two superconducting artificial atoms coupled to a resonator in the ultrastrong coupling regime Open
We experimentally investigate a superconducting circuit composed of two flux qubits ultrastrongly coupled to a common LC resonator. Owing to the large anharmonicity of the flux qubits, the system can be described well by a generalized Dick…
View article: One photon simultaneously excites two atoms in a ultrastrongly coupled light-matter system
One photon simultaneously excites two atoms in a ultrastrongly coupled light-matter system Open
We experimentally investigate a superconducting circuit composed of two flux qubits ultrastrongly coupled to a common LC resonator. Owing to the large anharmonicity of the flux qubits, the system can be correctly described by a generalized…
View article: Sudden change of the photon output field marks phase transitions in the quantum Rabi model
Sudden change of the photon output field marks phase transitions in the quantum Rabi model Open
The experimental observation of quantum phase transitions predicted by the quantum Rabi model in quantum critical systems is usually challenging due to the lack of signature experimental observables associated with them. Here, we describe …
View article: Unveiling and veiling an entangled light-matter quantum state from the vacuum
Unveiling and veiling an entangled light-matter quantum state from the vacuum Open
The ground state of an atom interacting with the electromagnetic field in the ultrastrong coupling regime is composed of virtual photons entangled with the atom. We propose a method to promote to real the entire photonic state, while prese…
View article: Spectral Properties of Two Superconducting Artificial Atoms Coupled to a Resonator in the Ultrastrong Coupling Regime
Spectral Properties of Two Superconducting Artificial Atoms Coupled to a Resonator in the Ultrastrong Coupling Regime Open
We experimentally investigate a superconducting circuit composed of two flux qubits ultrastrongly coupled to a common LC resonator. Owing to the large anharmonicity of the flux qubits, the system can be described well by a generalized Dick…
View article: Flying atom back-reaction and mechanically generated photons from vacuum
Flying atom back-reaction and mechanically generated photons from vacuum Open
We investigate the dynamics of a two-level atom flying through a photonic cavity when the light-matter interaction is in the ultrastrong coupling regime. We adopt a closed full quantum description that takes into account the quantization o…
View article: Sudden change of the photon output field marks phase transitions in the quantum Rabi model
Sudden change of the photon output field marks phase transitions in the quantum Rabi model Open
The experimental observation of quantum phase transitions predicted by the quantum Rabi model in quantum critical systems is usually challenging due to the lack of signature experimental observables associated with them. Here, we describe …
View article: Unveiling and veiling a Schrödinger cat state from the vacuum
Unveiling and veiling a Schrödinger cat state from the vacuum Open
Deep in the ultrastrong light-matter coupling regime, it has been predicted that the ground state of a two-level atom interacting with a cavity mode takes the form of a "virtual" Schrödinger cat entangled state between light and matter. We…
View article: Fast binomial-code holonomic quantum computation with ultrastrong light-matter coupling
Fast binomial-code holonomic quantum computation with ultrastrong light-matter coupling Open
We propose a protocol for bosonic binomial-code nonadiabatic holonomic quantum computation in a system composed of an artificial atom ultrastrongly coupled to a cavity resonator. In our protocol, the binomial codes, formed by superposition…
View article: Fault-Tolerant Multi-Qubit Geometric Entangling Gates Using Photonic Cat Qubits
Fault-Tolerant Multi-Qubit Geometric Entangling Gates Using Photonic Cat Qubits Open
We propose a protocol to implement multi-qubit geometric gates (i.e., the Molmer-Sorensen gate) using photonic cat qubits. These cat qubits stored in high-$Q$ resonators are promising for hardware-efficient universal quantum computing. Spe…
View article: Fault-tolerant multiqubit geometric entangling gates using photonic cat-state qubits
Fault-tolerant multiqubit geometric entangling gates using photonic cat-state qubits Open
We propose a theoretical protocol to implement multiqubit geometric gates (i.e., the Mølmer-Sørensen gate) using photonic cat-state qubits. These cat-state qubits stored in high-$Q$ resonators are promising for hardware-efficient universal…
View article: Generation of Fock-State Superpositions and Binomial-Code Holonomic Gates via Dressed Intermediate States in the Ultrastrong Light-Matter Coupling Regime
Generation of Fock-State Superpositions and Binomial-Code Holonomic Gates via Dressed Intermediate States in the Ultrastrong Light-Matter Coupling Regime Open
By using the dressed-state properties of an ultrastrong coupling system, we propose to generate and manipulate, with high fidelities, arbitrary superposition of Fock states. These generated states can form bosonic codes (e.g., binomial cod…
View article: Scully-Lamb quantum laser model for parity-time-symmetric whispering-gallery microcavities: Gain saturation effects and nonreciprocity
Scully-Lamb quantum laser model for parity-time-symmetric whispering-gallery microcavities: Gain saturation effects and nonreciprocity Open
We use a non-Lindbladian master equation of the Scully-Lamb laser model for\nthe analysis of light propagation in a parity-time symmetric photonic system\ncomposed of coupled active and passive whispering-gallery microresonators.\nPerformi…
View article: Quantum bus building-block for a scalable quantum computer architecture
Quantum bus building-block for a scalable quantum computer architecture Open
In superconducting quantum computers, qubits are usually only coupled to their nearest-neighbors. To overcome this limitation, we propose a scalable architecture to simultaneously connect several pairs of distant logical qubits via a dispe…
View article: Scalable quantum computer with superconducting circuits in the ultrastrong coupling regime
Scalable quantum computer with superconducting circuits in the ultrastrong coupling regime Open
So far, superconducting quantum computers have certain constraints on qubit connectivity, such as nearest-neighbor couplings. To overcome this limitation, we propose a scalable architecture to simultaneously connect several pairs of distan…
View article: Interaction of Mechanical Oscillators Mediated by the Exchange of Virtual Photon Pairs
Interaction of Mechanical Oscillators Mediated by the Exchange of Virtual Photon Pairs Open
Two close parallel mirrors attract due to a small force (Casimir effect) originating from the quantum vacuum fluctuations of the electromagnetic field. These vacuum fluctuations can also induce motional forces exerted upon one mirror when …
View article: Long-lasting quantum memories: Extending the coherence time of superconducting artificial atoms in the ultrastrong-coupling regime
Long-lasting quantum memories: Extending the coherence time of superconducting artificial atoms in the ultrastrong-coupling regime Open
Quantum systems are affected by interactions with their environments, causing\ndecoherence through two processes: pure dephasing and energy relaxation. For\nquantum information processing it is important to increase the coherence time\nof …
View article: Quantum nonlinear optics without photons
Quantum nonlinear optics without photons Open
Spontaneous parametric down-conversion is a well-known process in quantum nonlinear optics in which a photon incident on a nonlinear crystal spontaneously splits into two photons. Here we propose an analogous physical process where one exc…
View article: Hidden Vacuum Rabi Oscillations: Dynamical Quantum Superpositions of On/Off Interaction between a Single Quantum Dot and a Microcavity
Hidden Vacuum Rabi Oscillations: Dynamical Quantum Superpositions of On/Off Interaction between a Single Quantum Dot and a Microcavity Open
We show that it is possible to realize quantum superpositions of switched-on\nand -off strong light-matter interaction in a single quantum dot- semiconductor\nmicrocavity system. Such superpositions enable the observation of\ncounterintuit…
View article: Feynman-diagrams approach to the quantum Rabi model for ultrastrong cavity QED: stimulated emission and reabsorption of virtual particles dressing a physical excitation
Feynman-diagrams approach to the quantum Rabi model for ultrastrong cavity QED: stimulated emission and reabsorption of virtual particles dressing a physical excitation Open
In quantum field theory, bare particles are dressed by a cloud of virtual\nparticles to form physical particles. The virtual particles affect properties\nsuch as the mass and charge of the physical particles, and it is only these\nmodified…
View article: Quantum Memory in the Ultrastrong-Coupling Regime via Parity Symmetry Breaking
Quantum Memory in the Ultrastrong-Coupling Regime via Parity Symmetry Breaking Open
Quantum systems are affected by interactions with their environments, causing decoherence through two processes: pure dephasing and energy relaxation. For quantum information processing it is important to increase the coherence time of Jos…
View article: Output field-quadrature measurements and squeezing in ultrastrong cavity-QED
Output field-quadrature measurements and squeezing in ultrastrong cavity-QED Open
We study the squeezing of output quadratures of an electro-magnetic field\nescaping from a resonator coupled to a general quantum system with arbitrary\ninteraction strengths. The generalized theoretical analysis of output squeezing\npropo…
View article: One Photon Can Simultaneously Excite Two or More Atoms
One Photon Can Simultaneously Excite Two or More Atoms Open
We consider two separate atoms interacting with a single-mode optical or microwave resonator. When the frequency of the resonator field is twice the atomic transition frequency, we show that there exists a resonant coupling between one pho…
View article: Simultaneous Excitation of Two Atoms with a Single Photon
Simultaneous Excitation of Two Atoms with a Single Photon Open
We consider two separate atoms interacting with a single-mode optical resonator. When the frequency of the resonator field is twice the atomic transition frequency, we show that there exists a resonant coupling between one photon and two a…
View article: Multiphoton quantum Rabi oscillations in ultrastrong cavity QED
Multiphoton quantum Rabi oscillations in ultrastrong cavity QED Open
When an atom is strongly coupled to a cavity, the two systems can exchange a\nsingle photon through a coherent Rabi oscillation. This process enables precise\nquantum-state engineering and manipulation of atoms and photons in a cavity,\nwh…
View article: Quantum control and long-range quantum correlations in dynamical Casimir arrays
Quantum control and long-range quantum correlations in dynamical Casimir arrays Open
The recent observation of the dynamical Casimir effect in a modulated superconductingwaveguide, culminating thirty years ofworldwide research, empowered the quantum technology community with a tool to create entangled photons on chip. In t…