12,552 research outputs found
Efficient variational quantum simulator incorporating active error minimisation
One of the key applications for quantum computers will be the simulation of
other quantum systems that arise in chemistry, materials science, etc, in order
to accelerate the process of discovery. It is important to ask: Can this be
achieved using near future quantum processors, of modest size and under
imperfect control, or must it await the more distant era of large-scale
fault-tolerant quantum computing? Here we propose a variational method
involving closely integrated classical and quantum coprocessors. We presume
that all operations in the quantum coprocessor are prone to error. The impact
of such errors is minimised by boosting them artificially and then
extrapolating to the zero-error case. In comparison to a more conventional
optimised Trotterisation technique, we find that our protocol is efficient and
appears to be fundamentally more robust against error accumulation.Comment: 13 pages, 5 figures; typos fixed and small update
Hierarchical surface code for network quantum computing with modules of arbitrary size
The network paradigm for quantum computing involves interconnecting many
modules to form a scalable machine. Typically it is assumed that the links
between modules are prone to noise while operations within modules have
significantly higher fidelity. To optimise fault tolerance in such
architectures we introduce a hierarchical generalisation of the surface code: a
small `patch' of the code exists within each module, and constitutes a single
effective qubit of the logic-level surface code. Errors primarily occur in a
two-dimensional subspace, i.e. patch perimeters extruded over time, and the
resulting noise threshold for inter-module links can exceed ~ 10% even in the
absence of purification. Increasing the number of qubits within each module
decreases the number of qubits necessary for encoding a logical qubit. But this
advantage is relatively modest, and broadly speaking a `fine grained' network
of small modules containing only ~ 8 qubits is competitive in total qubit count
versus a `course' network with modules containing many hundreds of qubits.Comment: 12 pages, 11 figure
Stabilisers as a design tool for new forms of Lechner-Hauke-Zoller Annealer
In a recent paper Lechner, Hauke and Zoller (LHZ) described a means to
translate a Hamiltonian of spin- particles with 'all-to-all'
interactions into a larger physical lattice with only on-site energies and
local parity constraints. LHZ used this mapping to propose a novel form of
quantum annealing. Here we provide a stabiliser-based formulation within which
we can describe both this prior approach and a wide variety of variants.
Examples include a triangular array supporting all-to-all connectivity, and
moreover arrangements requiring only or spins but providing
interesting bespoke connectivities. Further examples show that arbitrarily high
order logical terms can be efficiently realised, even in a strictly 2D layout.
Our stabilisers can correspond to either even-parity constraints, as in the LHZ
proposal, or as odd-parity constraints. Considering the latter option applied
to the original LHZ layout, we note it may simplify the physical realisation
since the required ancillas are only spin- systems (i.e. qubits,
rather than qutrits) and moreover the interactions are very simple. We make a
preliminary assessment of the impact of this design choices by simulating small
(few-qubit) systems; we find some indications that the new variant may maintain
a larger minimum energy gap during the annealing process.Comment: A dramatically expanded revision: we now show how to use our
stabiliser formulation to construct a wide variety of new physical layouts,
including ones with fewer than Order N^2 spins but custom connectivities, and
a means to achieve higher order coupling even in 2
Quantum state transfer via the ferromagnetic chain in a spatially modulated field
We show that a perfect quantum state transmission can be realized through a
spin chain possessing a commensurate structure of energy spectrum, which is
matched with the corresponding parity. As an exposition of the mirror inversion
symmetry discovered by Albanese et. al (quant-ph/0405029), the parity matched
the commensurability of energy spectra help us to present the novel
pre-engineered spin systems for quantum information transmission. Based on the
these theoretical analysis, we propose a protocol of near-perfect quantum state
transfer by using a ferromagnetic Heisenberg chain with uniform coupling
constant, but an external parabolic magnetic field. The numerical results shows
that the initial Gaussian wave packet in this system with optimal field
distribution can be reshaped near-perfectly over a longer distance.Comment: 5 pages, 2 figure
Screening of cosmological constant in non-local cosmology
We consider a model of non-local gravity with a large bare cosmological
constant, , and study its cosmological solutions. The model is
characterized by a function where
and is a real dimensionless parameter. In the
absence of matter, we find an expanding universe solution with
, that is, a universe with decelarated expansion without any fine-tuning
of the parameter. Thus the effect of the cosmological constant is effectively
shielded in this solution. It has been known that solutions in non-local
gravity often suffer from the existence of ghost modes. In the present case we
find the solution is ghost-free if . This is
quite a weak condition. We argue that the solution is stable against the
includion of matter fields. Thus our solution opens up new possibilities for
solution to the cosmological constant problem.Comment: 7 pages, 1 figure, LaTeX, V2:Some clarifications and references adde
Classical noise assists the flow of quantum energy by `momentum rejuvenation'
An important challenge in quantum science is to fully understand the
efficiency of energy flow in networks. Here we present a simple and intuitive
explanation for the intriguing observation that optimally efficient networks
are not purely quantum, but are assisted by some interaction with a `noisy'
classical environment. By considering the system's dynamics in both the
site-basis and the momentum-basis, we show that the effect of classical noise
is to sustain a broad momentum distribution, countering the depletion of high
mobility terms which occurs as energy exits from the network. This picture
predicts that the optimal level of classical noise is reciprocally related to
the linear dimension of the lattice; our numerical simulations verify this
prediction to high accuracy for regular 1D and 2D networks over a range of
sizes up to thousands of sites. This insight leads to the discovery that
dramatic further improvements in performance occur when a driving field targets
noise at the low mobility components
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