Quantum Transport

QUBITS are the smallest functional unit of a quantum information processing device or, in other words, a quantum computer. The electronic transport properties can be probed by a Nanonis Tramea system connected to a low temperture cryostat.
Nanonis Tramea base configuration
Nanonis Tramea base configuration: TRC real-time controller, TSC signal conversion interface and Nanonis software
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Probing Quantum Transport by Scanning Gate Microscopy (SGM)
Most scanning probe microscopy techniques are ideal to investigate theproperties of the first few atomic layers of a surface, but are useless when the system of interest is buried deeper. For example, scanning tunneling microscopy (STM) yields extremely detailed information on the surface electrons, but fails to operate on electronsburied below oxides. Scanning Gate Microscopy (SGM) overcomes this limitation by scanning the electrically biased tip in a plane parallel to the electron of interest, and simultaneously recording a map of the variations of electrical resistance of the system. In other words, the tip induces a local electrostatic potential perturbation, which affects the electron transport.
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Topological Insulator Josephson junctions
In a topological insulator (TI), the electronic structure of the bulk reveals a unique topology that leads to the formation of conducting surface states with intriguing properties. The topological protection of the surface states and the possible emergence of Majorana fermions in superconductor/TI hybrid devices make these materials a leading candidate for use as a robust platform for future fault-tolerant quantum computation. The main challenge in creating these structures lies in the fabrication of an electrically transparent interface between the conventional superconductor and the topological insulator. A Josephson junction, formed by two closely spaced superconducting electrodes separated by a gap of less than 100 nm, can be used as a measure of the quality of the electrical interface. At low temperatures the superconducting electrodes induce superconductivity in the topological insulator by the proximity effect, with a finite Josephson current observed across a sufficiently narrow gap.
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Measuring Electron Temperature using Nanonis Tramea
The Nanonis Tramea Quantum Transport Measurement System (QTMS) was used to measure electron transport through an electrostatically defined Gallium Arsenide (GaAs) quantum dot in an Oxford Instruments Triton 200 dilution refrigerator. The high-speed demonstrated by Nanonis Tramea allowed for a significant reduction in the measurement acquisition time and the low noise environment of the Oxford Instruments dilution refrigerator combined with the Tramea system, allowed us to measure an electron temperature of 35 mK. This was equal to the base temperature of the cryostat with customised wiring installed.
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Characterization of Germanium Nanowire
Using a combination of the Nanonis Tramea quantum transport measurement system fully integrated with an Oxford Instruments HelioxVL refrigerator, the energy levels of a qubit have been successfully measured in a Germanium nanowire. Due to the lower noise and faster speed of this measurement system, conduct-ance measurements with greater detail are produced in shorter acquisition times.
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Transport and RF-Reflectometry Measurements of CMOS Nanodevices
Pioneering work on semiconductor quantum dots has shown that quantum dots are promising candidates as a building block (qubit) for quantum information and computation. Quantum dots are quasi-zero-dimensional nanostructures which can confine single electrons, whose spin or charge degree of freedom can then be used to represent quantum bits (qubits). Quantum computing approaches based on semiconductors can build upon mature micro/nano-fabrication technologies, which will be invaluable in scaling up to a large number of reproducible qubits with practical yields and integrated electronics. As part of the classical information technology industry, CMOS transistors have reached sufficiently small feature sizes that quantum effects can begin to play a dominant role. This motivates the exploration of quantum effects in such transistors fabricated using CMOS processes.
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Tuning and Operation of Quantum Dots and Related Apparatus
Spin qubits in quantum dots are promising candidates towards quantum computation. However, a considerable amount of technical development is still necessary before the advent of a quantum computer. For quantum dots, this involves independently controlling and sweeping the voltages of a multitude of electrostatic gates and monitoring several charge sensing signals. Thus, a fast measurement device with a high number of inputs and outputs is critical for the efficiency of the development of multiple quantum dot devices. The group of Michel Pioro-Ladrière at Institut Quantique, Université de Sherbrooke, Canada, have recently conducted measurements on quantum dots and related components using the Nanonis Tramea instrument, which will be discussed in this application note.
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Physics on the Level of Single Electrons
When shrinking the size of physical systems, it often becomes unclear in which way the physical laws are still valid. For example, thermodynamics is a phenom-enological theory which relies on the assumption that the system under consideration consists of a large (approximated by infinitely many) number of particles. When this limit - the so-called thermodynamic limit - does not hold, fluctuations dominate the physics, and the thermodynamic laws need to be corrected. Howev-er, when considering smaller physical systems, not only the number of particles, but also the dimensional-ity of the system might change. Many laws have been derived for two-dimensional systems, and can be dif-ferent for the case of one or zero dimensional systems.
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