75 research outputs found
Unravelling quantum dot array simulators via singlet-triplet measurements
Recently, singlet-triplet measurements in double dots have emerged as a powerful tool in quantum information processing. In parallel, quantum dot arrays are being envisaged as analog quantum simulators of many-body models. Thus motivated, we explore the potential of the above singlet-triplet measurements for probing and exploiting the ground state of a Heisenberg spin chain in such a quantum simulator. We formulate an efficient protocol to discriminate the achieved many-body ground state with other likely states. Moreover, the transition between quantum phases, arising from the addition of frustrations in a J1-J2 model, can be systematically explored using the same set of measurements. We show that the proposed measurements have an application in producing long distance heralded entanglement between well separated quantum dots. Relevant noise sources, such as nonzero temperatures and nuclear spin interactions, are considered
Atomic force microscope nanolithography of graphene: cuts, pseudo-cuts and tip current measurements
We investigate atomic force microscope nanolithography of single and bilayer
graphene. In situ tip current measurements show that cutting of graphene is not
current driven. Using a combination of transport measurements and scanning
electron microscopy we show that, while indentations accompanied by tip current
appear in the graphene lattice for a range of tip voltages, real cuts are
characterized by a strong reduction of the tip current above a threshold
voltage. The reliability and flexibility of the technique is demonstrated by
the fabrication, measurement, modification and re-measurement of graphene
nanodevices with resolution down to 15 nm
Reading and writing charge on graphene devices
We use a combination of charge writing and scanning gate microscopy to map
and modify the local charge neutrality point of graphene field-effect devices.
We give a demonstration of the technique by writing remote charge in a thin
dielectric layer over the graphene-metal interface and detecting the resulting
shift in local charge neutrality point. We perform electrostatic simulations to
characterize the gating effect of a realistic scanning probe tip on a graphene
bilayer and find a good agreement with the experimental results
Large-scale on-chip integration of gate-voltage addressable hybrid superconductor-semiconductor quantum wells field effect nano-switch arrays
Stable, reproducible, scalable, addressable, and controllable hybrid
superconductor-semiconductor (S-Sm) junctions and switches are key circuit
elements and building blocks of gate-based quantum processors. The
electrostatic field effect produced by the split gate voltages facilitates the
realisation of nano-switches that can control the conductance or current in the
hybrid S-Sm circuits based on 2D semiconducting electron systems. Here, we
experimentally demonstrate a novel realisation of large-scale scalable, and
gate voltage controllable hybrid field effect quantum chips. Each chip contains
arrays of split gate field effect hybrid junctions, that work as conductance
switches, and are made from In0.75Ga0.25As quantum wells integrated with Nb
superconducting electronic circuits. Each hybrid junction in the chip can be
controlled and addressed through its corresponding source-drain and two global
split gate contact pads that allow switching between their (super)conducting
and insulating states. We fabricate a total of 18 quantum chips with 144 field
effect hybrid Nb- In0.75Ga0.25As 2DEG-Nb quantum wires and investigate the
electrical response, switching voltage (on/off) statistics, quantum yield, and
reproducibility of several devices at cryogenic temperatures. The proposed
integrated quantum device architecture allows control of individual junctions
in a large array on a chip useful for the development of emerging cryogenic
nanoelectronics circuits and systems for their potential applications in
fault-tolerant quantum technologies
Statistical evaluation of 571 GaAs quantum point contact transistors showing the 0.7 anomaly in quantized conductance using millikelvin cryogenic on-chip multiplexing
The mass production and the practical number of cryogenic quantum devices producible in a single chip are limited to the number of electrical contact pads and wiring of the cryostat or dilution refrigerator. It is, therefore, beneficial to contrast the measurements of hundreds of devices fabricated in a single chip in one cooldown process to promote the scalability, integrability, reliability, and reproducibility of quantum devices and to save evaluation time, cost and energy. Here, we use a cryogenic on-chip multiplexer architecture and investigate the statistics of the 0.7 anomaly observed on the first three plateaus of the quantized conductance of semiconductor quantum point contact (QPC) transistors. Our single chips contain 256 split gate field effect QPC transistors (QFET) each, with two 16-branch multiplexed source-drain and gate pads, allowing individual transistors to be selected, addressed and controlled through an electrostatic gate voltage process. A total of 1280 quantum transistors with nano-scale dimensions are patterned in 5 different chips of GaAs heterostructures. From the measurements of 571 functioning QPCs taken at temperatures T= 1.4 K and T= 40 mK, it is found that the spontaneous polarisation model and Kondo effect do not fit our results. Furthermore, some of the features in our data largely agreed with van Hove model with short-range interactions. Our approach provides further insight into the quantum mechanical properties and microscopic origin of the 0.7 anomaly in QPCs, paving the way for the development of semiconducting quantum circuits and integrated cryogenic electronics, for scalable quantum logic control, readout, synthesis, and processing applications
Unraveling quantum Hall breakdown in bilayer graphene with scanning gate microscopy
We use low-temperature scanning gate microscopy (SGM) to investigate the
breakdown of the quantum Hall regime in an exfoliated bilayer graphene flake.
SGM images captured during breakdown exhibit intricate patterns of "hotspots"
where the conductance is strongly affected by the presence of the tip. Our
results are well described by a model based on quantum percolation which
relates the points of high responsivity to tip-induced scattering between
localized Landau levels.Comment: 6 pages, 4 figure
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