2 research outputs found
Magnetization Dynamics, Bennett Clocking and Associated Energy Dissipation in Multiferroic Logic
It has been recently shown that multiferroic logic - where logic bits are
encoded in the magnetization orientation of a nanoscale magnetostrictive layer
elastically coupled to a piezoelectric layer - can be Bennett clocked with
small electrostatic potentials of few tens of mV applied to the piezoelectric
layer. The potential generates stress in the magnetostrictive layer and rotates
its magnetization by a large angle to carry out Bennett clocking. This method
of clocking is far more energy-efficient than using spin transfer torque. In
order to assess if such a clocking scheme can be also reasonably fast, we have
studied the magnetization dynamics of a multiferroic logic array with nearest
neighbor dipole coupling using the Landau-Lifshitz-Gilbert (LLG) equation. We
find that switching delays of ~ 3 ns (clock rates of 0.33 GHz) can be achieved
with proper design provided we clock non-adiabatically and dissipate ~48,000 kT
(at room temperature) of energy per clock cycle per bit flip in the clocking
circuit. This dissipation far exceeds the energy barrier separating the two
logic states, which we assumed to be 32 kT to yield a bit error probability of
. Had we used spin transfer torque to switch with the same ~ 3 ns delay, the
energy dissipation would have been much larger (~ kT). This
shows that spin transfer torque, widely used in magnetic random access memory,
is an inefficient way to switch a magnet, and multiferroic logic clocked with
voltage-induced stress is a superior nanomagnetic logic scheme
Fault tolerant architectures for superconducting qubits
In this short review, I draw attention to new developments in the theory of
fault tolerance in quantum computation that may give concrete direction to
future work in the development of superconducting qubit systems. The basics of
quantum error correction codes, which I will briefly review, have not
significantly changed since their introduction fifteen years ago. But an
interesting picture has emerged of an efficient use of these codes that may put
fault tolerant operation within reach. It is now understood that two
dimensional surface codes, close relatives of the original toric code of
Kitaev, can be adapted to effectively perform logical gate operations in a very
simple planar architecture, with error thresholds for fault tolerant operation
simulated to be 0.75%. This architecture uses topological ideas in its
functioning, but it is not 'topological quantum computation' -- there are no
non-abelian anyons in sight. I offer some speculations on the crucial pieces of
superconducting hardware that could be demonstrated in the next couple of years
that would be clear stepping stones towards this surface-code architecture.Comment: 28 pages, 10 figures. For the Nobel Symposium on Qubits for Quantum
Information, submitted to Physica Scripta. v. 2 Corrections and small changes
to reference