1,720,975 research outputs found
Principles of quantum computation and information, Vol. II: Basic Tools and Special Topics
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Quantum operations: A Fano-representation approach
No full text The Fano representation is a particularly simple and physically appealing description of quantum operations, in that it directly provides the evolution of the expectation value of polarization measurements. The usefulness of this approach is illustrated in quantum process tomography and in computing the distance between two quantum channels. </jats:p
A bird's eye view of quantum computers
No full textQuantum computers are discussed in the general framework of computation, the
laws of physics and the foundations of quantum mechanics
Entanglement in helium
No full textUsing a configuration-interaction variational method, we accurately compute the reduced, single-electron von Neumann and linear entropy for several low-energy, singlet and triplet eigenstates of helium atom. We estimate the amount of electron-electron orbital entanglement for such eigenstates and show that it decays with energy
Exotic states in the dynamical Casimir effect
No full textWe consider the interaction of a qubit with a single mode of the quantized electromagnetic field and show that, in the ultrastrong coupling regime and when the qubit-field interaction is switched on abruptly, the dynamical Casimir effect leads to the generation of a variety of exotic states of the field, which cannot be simply described as squeezed states. Such effect is a consequence of the intrinsic nonlinearity of the qubit and also appears when initially both the qubit and the field are in their ground state. The non-classicality of the obtained exotic states is characterized by means of a parameter based on the volume of the negative part of the Wigner function. A transition to non-classical states is observed by changing either the interaction strength or the interaction time. The observed phenomena appear as a general feature of nonadiabatic quantum gates, so that the dynamical Casimir effect can be the origin of a fundamental upper limit to the maximum speed of quantum computation and communication protocol
Landau-Zener quantum tunneling in disordered metallic nanomagnets
No full textWe study Landau-Zener macroscopic quantum transitions in ferromagnetic metal nanoparticles containing on the order of 100 atoms. The model that we consider is described by an effective giant-spin Hamiltonian, with a coupling to a random transverse magnetic field mimicking the effect of quasiparticle excitations and structural disorder on the gap structure of the spin collective modes. We find different types of time evolutions depending on the interplay between the disorder in the transverse field and the initial conditions of the system. In the absence of disorder, if the system starts from a low-energy state, there is one main coherent quantum tunneling event where the initial-state amplitude is completely depleted in favor of a few discrete states, with nearby spin quantum numbers; when starting from the highest excited state, we observe complete inversion of the magnetization through a peculiar “backward cascade evolution.” In the random case, the disorder-averaged transition probability for a low-energy initial state becomes a smooth distribution, which is nevertheless still sharply peaked around one of the transitions present in the disorder-free case. On the other hand, the coherent backward cascade phenomenon turns into a damped cascade with frustrated magnetic inversion
Quantum simulation of the single-particle Schrodinger equation
No full textThe nature of a quantum computer is described in the concrete context of a quantum simulator of the single-particle Schrodinger equation. We show that a register of 6-10 qubits is sufficient to realize a useful quantum simulator capable of efficiently solving standard quantum mechanical problem
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