35 research outputs found
Quantum fluctuation theorem to benchmark quantum annealers
Near term quantum hardware promises unprecedented computational advantage.
Crucial in its development is the characterization and minimization of
computational errors. We propose the use of the quantum fluctuation theorem to
benchmark the performance of quantum annealers. This versatile tool provides
simple means to determine whether the quantum dynamics are unital, unitary, and
adiabatic, or whether the system is prone to thermal noise. Our proposal is
experimentally tested on two generations of the D-Wave machine, which
illustrates the sensitivity of the fluctuation theorem to the smallest
aberrations from ideal annealing.Comment: 5 pages, 4 figur
Riccati equation in studies of spin-boson systems
W pracy wskazujemy na zwi膮zek pomi臋dzy Hamiltonianem danego uk艂adu z艂o偶onego typu qubitu-otoczenie a blokowymi macierzami operatorowymi. Nast臋pnie wykorzystujemy ich zwi膮zek z r贸wnaniem Riccatiego do
badania m.in uk艂ad贸w typu spin-bozon oraz dynamiki zredukowanej qubitu. W szczeg贸lno艣ci rozwa偶amy istnienie symetrii w s艂ynnym, wci膮偶 nie rozwi膮zanym ca艂kowicie modelu Rabiego. Dowodzimy, i偶 pr贸cz zachowania ca艂kowitej energii istnieje nielokalna symetria Z2 (w pewnym sensie ukryta) zadana przez uog贸lniony
operator parzysto艣ci. Prezentujemy warunki istnienia tej symetrii, jej posta膰 oraz spos贸b jawnej konstrukcji z wykorzystaniem rozwi膮zania r贸wnania Riccatiego. Dla otwartych uk艂ad贸w dwustanowych wprowadzamy szczeg贸ln膮 klas臋 stan贸w, zwanych stanami defazingowymi (ang. dephasing states). W pierwszej
kolejno艣ci pokazujemy, i偶 specyficzna dla modeli defazingowych symetria pomi臋dzy oddzia艂uj膮cymi uk艂adami nie jest konieczna aby zapewni膰 bezstratn膮 energetycznie ewolucj臋 poduk艂ad贸w. Dowodzimy, 偶e wystarczaj膮ce s膮 odpowiednio dobrane warunki pocz膮tkowe stany defazingowe oraz przedstawiamy adekwatn膮 technik臋 ich konstrukcji. Nast臋pnie pokazujemy w jaki spos贸b przygotowa膰 stan pocz膮tkowy uk艂ad qubit-otoczenie, tak aby informacja zakodowana w zadanej na przestrzeni qubitu obserwabli nie zosta艂 utracona na rzecz otoczenia w wyniku ewolucji. Na zako艅czenie rozwa偶amy qubit w kontakcie z fermionowym otoczeniem oraz poddany dzia艂aniu zewn臋trznego pola magnetycznego. Wyprowadzamy dok艂adn膮 posta膰 dynamiki zredukowanej oraz analizujemy przybli偶enie adiabatyczne
Dynamics of a quantum phase transition in the 1D Bose-Hubbard model: excitations and correlations induced by a quench
The ground state of the one-dimensional Bose-Hubbard model at unit filling
undergoes the Mott-superfluid quantum phase transition. It belongs to the
Kosterlitz-Thouless universality class with an exponential divergence of the
correlation length in place of the usual power law. We present numerical
simulations of a linear quench both from the Mott insulator to superfluid and
back. The results satisfy the scaling hypothesis that follows from the
Kibble-Zurek mechanism (KZM). In the superfluid-to-Mott quenches there is no
significant excitation in the superfluid phase despite its gaplessness. Since
all critical superfluid ground states are qualitatively similar, the excitation
begins to build up only after crossing the critical point when the ground state
begins to change fundamentally. The last process falls into the KZM framework.Comment: To appear in PR
Defects in Quantum Computers
The shift of interest from general purpose quantum computers to adiabatic
quantum computing or quantum annealing calls for a broadly applicable and easy
to implement test to assess how quantum or adiabatic is a specific hardware.
Here we propose such a test based on an exactly solvable many body system --
the quantum Ising chain in transverse field -- and implement it on the D-Wave
machine. An ideal adiabatic quench of the quantum Ising chain should lead to an
ordered broken symmetry ground state with all spins aligned in the same
direction. An actual quench can be imperfect due to decoherence, noise, flaws
in the implemented Hamiltonian, or simply too fast to be adiabatic.
Imperfections result in topological defects: Spins change orientation, kinks
punctuating ordered sections of the chain. The number of such defects
quantifies the extent by which the quantum computer misses the ground state,
and is, therefore, imperfect.Comment: 8 pages, 7 figures, to appear in Scientific Reports, authors' list
complete
Relation Between Purity of an Open Qubit Dynamics and Its Initial Correlation with an Environment
Purity as a quantifier of an impact of environment on an open quantum system
is studied for a qubit dephasingly interacting with its environment. We analyze how time
evolution of the purity depends on initial states of the composite system both in the case of
infinite and finite environments. It is shown that for a certain class of initial preparations, the
purity of an evolving qubit state initially correlated with infinite environment can be greater
than in the case of uncorrelated qubit-environment initial preparations. We identify a class
of initial states leading to such desired outcome