DSpace Collection:
https://dspace.lboro.ac.uk/2134/93
Sat, 02 Jan 2016 19:09:51 GMT
20160102T19:09:51Z

Modeling an adiabatic quantum computer via an exact map to a gas of particles
https://dspace.lboro.ac.uk/2134/19903
Title: Modeling an adiabatic quantum computer via an exact map to a gas of particles
Authors: Zagoskin, Alexandre M.; Savel'ev, Sergey; Nori, Franco
Abstract: We map adiabatic quantum evolution on the classical Hamiltonian dynamics of a 1D gas (Pechukas gas) and simulate the latter numerically. This approach turns out to be both insightful and numerically efficient, as seen from our example of a CNOT gate simulation. For a general class of Hamiltonians we show that the escape probability from the initial state scales no faster than \dot{\lambda}^{\gamma}, where \dot{\lambda} is the adiabaticity parameter. The scaling exponent for the escape probability is \gamma = 1/2 for all levels, except the edge (bottom and top) ones, where \gamma <~1/3. In principle, our method can solve arbitrarily large adiabatic quantum Hamiltonians.
Description: This article was published in the journal, Physical Review Letters [© American Physical Society] and the definitive version is available at: http://dx.doi.org/10.1103/PhysRevLett.98.120503
Mon, 01 Jan 2007 00:00:00 GMT
https://dspace.lboro.ac.uk/2134/19903
20070101T00:00:00Z

Statedependent photon blockade via quantumreservoir engineering
https://dspace.lboro.ac.uk/2134/19901
Title: Statedependent photon blockade via quantumreservoir engineering
Authors: Miranowicz, Adam; Bajer, J.; Paprzycka, Małgorzata; Liu, Yuxi; Zagoskin, Alexandre M.; Nori, Franco
Abstract: An arbitrary initial state of an optical or microwave field in a lossy driven nonlinear cavity can be changed into a partially incoherent superposition of only the vacuum and the singlephoton states. This effect is known as singlephoton blockade, which is usually analyzed for a Kerrtype nonlinear cavity parametrically driven
by a singlephoton process assuming singlephoton loss mechanisms. We study photon blockade engineering via a nonlinear reservoir, i.e., a quantum reservoir, where only twophoton absorption is allowed. Namely, we analyze a lossy nonlinear cavity parametrically driven by a twophoton process and allowing twophoton loss
mechanisms, as described by the master equation derived for a twophoton absorbing reservoir. The nonlinear cavity engineering can be realized by a linear cavity with a tunable twolevel system via the JaynesCummings interaction in the dispersive limit. We show that by tuning properly the frequencies of the driving field and the
twolevel system, the steady state of the cavity field can be the singlephoton Fock state or a partially incoherent
superposition of several Fock states with photon numbers, e.g., (0,2), (1,3), (0,1,2), or (0,2,4). At the right (now fixed) frequencies, we observe that an arbitrary initial coherent or incoherent superposition of Fock states with an even (odd) number of photons is changed into a partially incoherent superposition of a few Fock states of the same photonnumber parity. We find analytically approximate formulas for these two kinds of solutions for several differentlytuned systems. A general solution for an arbitrary initial state is a weighted mixture of the above two solutions with even and odd photon numbers, where the weights are given by the probabilities of
measuring the even and odd numbers of photons of the initial cavity field, respectively. This can be interpreted
as two separate evolutiondissipation channels for even and oddnumber states. Thus, in contrast to the standard
predictions of photon blockade, we prove that the steady state of the cavity field, in the engineered photon blockade, can depend on its initial state. To make our results more explicit, we analyze photon blockades for some initial infinitedimensional quantum and classical states via the Wigner and photonnumber distributions
Description: This article was published in the journal, Physical Review A [© American Physical Society] and the definitive version is available at: http://dx.doi.org/10.1103/PhysRevA.90.033831
Wed, 01 Jan 2014 00:00:00 GMT
https://dspace.lboro.ac.uk/2134/19901
20140101T00:00:00Z

Toroidal qubits: naturallydecoupled quiet artificial atoms
https://dspace.lboro.ac.uk/2134/19900
Title: Toroidal qubits: naturallydecoupled quiet artificial atoms
Authors: Zagoskin, Alexandre M.; Chipouline, Arkadi; Il’ichev, E.; Johansson, J. Robert; Nori, Franco
Abstract: The requirements of quantum computations impose high demands on the level of qubit protection from perturbations; in particular, from those produced by the environment. Here we propose a superconducting flux qubit design that is naturally protected from ambient noise. This decoupling is due to the qubit interacting with the electromagnetic field only through its toroidal moment, which provides an unusual qubitfield interaction, which is suppressed at low frequencies.
Description: This is an Open Access Article. It is published by Nature under the Creative Commons Attribution 4.0 Unported Licence (CC BY). Full details of this licence are available at: http://creativecommons.org/licenses/by/4.0/
Thu, 01 Jan 2015 00:00:00 GMT
https://dspace.lboro.ac.uk/2134/19900
20150101T00:00:00Z

Liouville invariance in quantum and classical mechanics
https://dspace.lboro.ac.uk/2134/19899
Title: Liouville invariance in quantum and classical mechanics
Authors: Maassen van den Brink, Alec; Zagoskin, Alexandre M.
Abstract: The densitymatrix and Heisenberg formulations of quantum mechanics followfor unitary evolutiondirecty from the Schr"odinger equation. Nevertheless, the symmetries of the corresponding evolution operator, the Liouvillian L=i[.,H], need not be limited to those of the Hamiltonian H. This is due to L only involving eigenenergy_differences_, which can be degenerate even if the energies themselves are not. Remarkably, this possibility has rarely been mentioned in the literature, and never pursued more generally. We consider an example involving mesoscopic Josephson devices, but the analysis only assumes familiarity with basic quantum mechanics. Subsequently, such _Lsymmetries_ are shown to occur more widely, in particular also in classical mechanics. The symmetry's relevance to dissipative systems and quantuminformation processing is briefly discussed.
Description: The final publication is available at Springer via http://dx.doi.org/10.1023/A:1019657303471 .
Tue, 01 Jan 2002 00:00:00 GMT
https://dspace.lboro.ac.uk/2134/19899
20020101T00:00:00Z