MEASUREMENT
Application note

Low-temperature, low-noise and high-speed
characterisation of semiconductor nanodevices
Shuxiao Li and Guoping Guo
Quantum Devices Group at the Lab of Quantum Information, Chinese Academy of Science, USTC

Using the Nanonis Tramea quantum transport measurement system (QTMS) combined withan Oxford Instruments HelioxVL refrigerator (Figure 1), the energy levels of a qubit, formed ina Germanium nanowire have been successfully measured. Due to the low-noise and high-speedof this measurement system, high resolution conductance measurements are produced in shortacquisition times.

Introduction
The operation of quantum computers relies on the formation of so called ‘qubits’,
where the state is not constrained to merely 0 and 1 as in traditional binary
computers. Instead, each bit of information is represented by the state of aquantum particle, which can be in a superposition of multiple states simultaneously.
A wide variety of methods to form qubits are being investigated. In this work,
qubits are formed naturally via the spatial confinement of holes. The confinement
is provided by nanowires with dimensions close to the natural wavelength ofthe carrier, as determined by quantum mechanics. The Nanonis Tramea is a
low-noise, fast QTMS, and also a safe system, using a single ground. It also
eliminates the potential for damaging sensitive samples when disconnecting andconnecting cables, and ground lines to a variety of single-function instruments in ameasurement rack.

Figure 1. Nanonis Tramea,and HelioxVL

Experiment
A rough schematic of the typical device is illustrated in Figure 2. A wire composedof Germanium is grown on a Silicon substrate and electrical contact is createdlithographically. A gate is added to allow a potential to be applied in order to shift
the energy levels of the confined hole up or down, to block or permit hole flow
through the wire.
For this experiment, the entire room temperature measurement electronics
requirement was fulfilled using only the Nanonis Tramea instrument. TheNanonis Tramea system is a fully digital, integrated package that provides thefunctionality of an oscilloscope, spectrum analyser, voltage sweepers, voltmeters,
lock-in amplifiers and function generator, in one compact package. It can also
control eight inputs and eight outputs (expandable to 24 and 24) simultaneously
in one user interface. As all of this functionality is housed within one unit, thecommunication speeds and therefore the data acquisition times are much shorter.
Specifically, there is no need to incorporate slow bus connections between
individual devices that traditional instruments rely on. The connection scheme
between the device and the electronics is shown in Figure 3. A current amplifier isconnected to the drain contact and its output is sent to an input on the electronics.
One output is connected to the source side of the wire and a second output isconnected to the top gate along the wire. All of these connections are therefore
sourced from one single unit instead of a range of individual instruments (oftenfrom different suppliers), tied together using custom software.
.

Figure 2. Schematic layout of device.

Figure 3. Connection diagram between
Nanonis Tramea and device.

MEASUREMENT
Application note

Results
The device was mounted to the 3He pot of an Oxford instruments
HelioxVL 3He system and measured at a temperature of
270 mK. Initial results are presented in Figure 3, where the familiar
Coulomb diamonds are visible. As the source-drain potential isswept and a hole can enter the dot, Coulomb repulsion prevents
additional carriers from tunnelling into the dot, until a further
increase in potential causes an additional energy level to become
available for occupation. The fine structure visible in the lower leftof the diagram is where the energy levels line up, formed due tothe excited states of the qubit, allowing the holes to move throughthe device. This fine structure was revealed when switching to theTramea system, compared to our older electronics measurement
system composed of multiple instruments. Furthermore, since
all of the measurement parameters are downloaded into theTramea, the entire acquisition proceeds autonomously and athigh speed, compared to the traditional methods with software
running on a PC, sequentially sending commands to each partof the measurement setup. The Nanonis Tramea is now a key
instrument in our experimental design, as we are now planning
scale up activities to have multiple qubits. As the Nanonis Tramea
can accommodate 24 inputs and outputs using the same softwareinterface, we do not require additional instrumentation.

Conclusion

Figure 4. Differential conductanceof the nanowire device as a functionof the gate (V2) and source-drain(V1) potentials. Note the fine details(yellow ovals) that emerge when using
the Nanonis Tramea measurement
electronics. Note that the excited states(yellow ovals) were not visible with ourprevious measurement electronics.

We have used the Nanonis Tramea to investigate the energy
levels of a qubit formed within a nanowire. Preliminary results
reveal new details in the conduction through the device,previously obscured by the noise level of our old measurement
system. Additionally, the single instrument nature of the Nanonis
Tramea permits a dramatic increase in acquisition speed byeliminating the slow bus protocol we used previously, whencombining individual components (DC voltage sources, lock-in
amplifiers, etc.) with a home-built software package. The next
step in our research will be to perform these experiments at
ultra-low temperatures on a Triton Cryofree dilution refrigerator(down to 50 mK), which will require the very low-noisecharacteristics of the Nanonis Tramea.

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June 2017, Issue 1