Search results for "QUBIT"

Quantum superpositions of clockwise and counterclockwise supercurrent states in the dynamics of a rf-SQUID exposed to a quantized electromagnetic fie…

2002

The dynamical behavior of a superconducting quantum interference device (a rf-SQUID) irradiated by a single mode quantized electromagnetic field is theoretically investigated. Treating the SQUID as a flux qubit, we analyze the dynamics of the combined system within the low lying energy Hilbert subspace both in the asymmetric and in the symmetric SQUID potential configurations. We show that the temporal evolution of the system is dominated by an oscillatory behavior characterized by more than one, generally speaking, incommensurable Rabi frequencies whose expressions are explicitly given. We find that the external parameters may fixed in such a way to realize a control on the dynamical repla…

Molecular spins for quantum computation

2019

Spins in solids or in molecules possess discrete energy levels, and the associated quantum states can be tuned and coherently manipulated by means of external electromagnetic fields. Spins therefore provide one of the simplest platforms to encode a quantum bit (qubit), the elementary unit of future quantum computers. Performing any useful computation demands much more than realizing a robust qubit—one also needs a large number of qubits and a reliable manner with which to integrate them into a complex circuitry that can store and process information and implement quantum algorithms. This ‘scalability’ is arguably one of the challenges for which a chemistry-based bottom-up approach is best-s…

Exact quantum dynamics of interacting spin systems subjected to controllable time dependent magnetic fields

2020

On the validity of non-Markovian master equation approaches for the entanglement dynamics of two-qubit systems

2010

In the framework of the dissipative dynamics of coupled qubits interacting with independent reservoirs, a comparison between non-Markovian master equation techniques and an exact solution is presented here. We study various regimes in order to find the limits of validity of the Nakajima–Zwanzig and the time-convolutionless master equations in the description of the entanglement dynamics. A comparison between the performances of the concurrence and the negativity as entanglement measures for the system under study is also presented.

A Quantum Lovasz Local Lemma

2012

The Lovasz Local Lemma (LLL) is a powerful tool in probability theory to show the existence of combinatorial objects meeting a prescribed collection of "weakly dependent" criteria. We show that the LLL extends to a much more general geometric setting, where events are replaced with subspaces and probability is replaced with relative dimension, which allows to lower bound the dimension of the intersection of vector spaces under certain independence conditions. Our result immediately applies to the k-QSAT problem: For instance we show that any collection of rank 1 projectors with the property that each qubit appears in at most $2^k/(e \cdot k)$ of them, has a joint satisfiable state. We then …

Nonlocality threshold for entanglement under general dephasing evolutions: A case study

2015

Determining relationships between different types of quantum correlations in open composite quantum systems is important since it enables the exploitation of a type by knowing the amount of another type. We here review, by giving a formal demonstration, a closed formula of the Bell function, witnessing nonlocality, as a function of the concurrence, quantifying entanglement, valid for a system of two noninteracting qubits initially prepared in extended Werner-like states undergoing any local pure-dephasing evolution. This formula allows for finding nonlocality thresholds for the concurrence depending only on the purity of the initial state. We then utilize these thresholds in a paradigmatic …

Coexistence of unlimited bipartite and genuine multipartite entanglement: Promiscuous quantum correlations arising from discrete to continuous-variab…

2006

Quantum mechanics imposes 'monogamy' constraints on the sharing of entanglement. We show that, despite these limitations, entanglement can be fully 'promiscuous', i.e. simultaneously present in unlimited two-body and many-body forms in states living in an infinite-dimensional Hilbert space. Monogamy just bounds the divergence rate of the various entanglement contributions. This is demonstrated in simple families of N-mode (N >= 4) Gaussian states of light fields or atomic ensembles, which therefore enable infinitely more freedom in the distribution of information, as opposed to systems of individual qubits. Such a finding is of importance for the quantification, understanding and potenti…

Transfer of arbitrary two-qubit states via a spin chain

2015

We investigate the fidelity of the quantum state transfer (QST) of two qubits by means of an arbitrary spin-1/2 network, on a lattice of any dimensionality. Under the assumptions that the network Hamiltonian preserves the magnetization and that a fully polarized initial state is taken for the lattice, we obtain a general formula for the average fidelity of the two qubits QST, linking it to the one- and two-particle transfer amplitudes of the spin-excitations among the sites of the lattice. We then apply this formalism to a 1D spin chain with XX-Heisenberg type nearest-neighbour interactions adopting a protocol that is a generalization of the single qubit one proposed in Ref. [Phys. Rev. A 8…

Modeling the properties of uranium-based single ion magnets

2013

We analyze the magnetic behavior of the five uranium-based SIMs reported in the literature. By combining a corrected crystal field model with the magnetic experimental data, we obtain the lowest-lying magnetic levels and the associated wave functions of the nanomagnets, which are found to be compatible with the observed SMM behavior. Additionally, this approach has allowed us to propose some geometrical considerations and practical advice for experimentalists aiming for the rational design of SIMs and spin qubits based on uranium.

Peptides as Versatile Platforms for Quantum Computing

2018

The pursuit of novel functional building blocks for the emerging field of quantum computing is one of the most appealing topics in the context of quantum technologies. Herein we showcase the urgency of introducing peptides as versatile platforms for quantum computing. In particular, we focus on lanthanide-binding tags, originally developed for the study of protein structure. We use pulsed electronic paramagnetic resonance to demonstrate quantum coherent oscillations in both neodymium and gadolinium peptidic qubits. Calculations based on density functional theory followed by a ligand field analysis indicate the possibility of influencing the nature of the spin qubit states by means of contro…