3,144 research outputs found
TBM pressure models: observations, theory and practice
Mechanized tunnelling in soft ground has evolved significantly over the last 20 years. However, the interaction between the tunnel boring machine (TBM) and the ground is often understood through idealized concepts, focused mostly on the machine actions in detriment of the reactions from the ground. These concepts cannot be used to explain several mechanisms that have been observed during the construction of mechanized tunnels. Therefore, this paper presents the path from field observations to the theoretical developments to model the TBM-ground interaction more realistically. Some ideas on how these developments can be applied into practice are presented. Finally, a discussion is proposed about how an effective collaboration between academia and industry can alleviate the current concentration of knowledge in the state of practice
A Silicon Surface Code Architecture Resilient Against Leakage Errors
Spin qubits in silicon quantum dots are one of the most promising building
blocks for large scale quantum computers thanks to their high qubit density and
compatibility with the existing semiconductor technologies. High fidelity
single-qubit gates exceeding the threshold of error correction codes like the
surface code have been demonstrated, while two-qubit gates have reached 98\%
fidelity and are improving rapidly. However, there are other types of error ---
such as charge leakage and propagation --- that may occur in quantum dot arrays
and which cannot be corrected by quantum error correction codes, making them
potentially damaging even when their probability is small. We propose a surface
code architecture for silicon quantum dot spin qubits that is robust against
leakage errors by incorporating multi-electron mediator dots. Charge leakage in
the qubit dots is transferred to the mediator dots via charge relaxation
processes and then removed using charge reservoirs attached to the mediators. A
stabiliser-check cycle, optimised for our hardware, then removes the
correlations between the residual physical errors. Through simulations we
obtain the surface code threshold for the charge leakage errors and show that
in our architecture the damage due to charge leakage errors is reduced to a
similar level to that of the usual depolarising gate noise. Spin leakage errors
in our architecture are constrained to only ancilla qubits and can be removed
during quantum error correction via reinitialisations of ancillae, which ensure
the robustness of our architecture against spin leakage as well. Our use of an
elongated mediator dots creates spaces throughout the quantum dot array for
charge reservoirs, measuring devices and control gates, providing the
scalability in the design
Influence of two-level fluctuators on adiabatic passage techniques
We study the process of Stimulated Raman Adiabatic Passage (STIRAP) under the
influence of a non-trivial solid-state environment, particularly the effect of
two-level fluctuators (TLFs) as they are frequently present in solid-state
devices. When the amplitudes of the driving-pulses used in STIRAP are in
resonance with the level spacing of the fluctuators the quality of the
protocol, i.e., the transferred population decreases sharply. In general the
effect can not be reduced by speeding up the STIRAP process. We also discuss
the effect of a structured noise environment on the process of Coherent
Tunneling by Adiabatic Passage (CTAP). The effect of a weakly structured
environment or TLFs with short coherence times on STIRAP and CTAP can be
described by the Bloch-Redfield theory. For a strongly structured environment a
higher-dimensional approach must be used, where the TLFs are treated as part of
the system.Comment: 8 pages, 8 figure
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