Glossary
This A-Z glossary is your cheat sheet to the key concepts shaping the field!
C
Carbon nanotube
A carbon nanotube is a nanoscale cylindrical structure made by carbon atoms that have unique features because of their small size and the specific arrangement of the atoms. Carbon nanotubes have exceptional strength, electrical and thermal conductivity, which makes them valuable in many different applications.
G
Graphene
Graphene is an atomic-scale structure made of carbon atoms that are arranged in a hexagonal lattice. In 1947, Philip Wallace theorised the idea of a one-atom-thick layer of graphite. The term “graphene” was introduced some years later, in 1986, by chemists Hanns-Peter Boehm, Ralph Setton and Eberhard Stumpp.
Graphene was properly isolated and characterised in 2004 by Andre Geim and Konstantin Novoselov at the University of Manchester, a discovery that awarded them the Nobel Prize a few years later. They pulled graphene layers from graphite with a common adhesive tape, in a process called micro-mechanical cleavage. Once single graphene layers were successfully produced, scientists noticed their remarkable properties: flexible, light, and yet strong. They also turned out to be great conductors of heat and electricity and exhibited a host of rich physical phenomena, making graphene an ideal platform for a plethora of applications, including light confinement, ultrafast and ultrasensitive detection of light, photovoltaics and energy storage, and metrology.
M
Mechanical resonator
Resonance is a condition where a system vibrates or oscillates with maximum amplitude at a particular frequency. A resonator is a device or system that exhibits resonance, which is a phenomenon that occurs when an external force or stimulus is applied at a specific frequency, causing the system to oscillate with increased amplitude. In mechanical resonators, these oscillations involve mechanical vibrations, such as the vibrations of a tuning fork or a guitar string.
Mechanical resonators based on carbon nanotubes
They are the smallest operating mechanical resonators that have been produced thus far. The Q-factor approaches 10 million, and they can be cooled to a few quanta of vibrations, as recently shown by our group. The quantum fluctuation amplitude is about 10 pm, which is orders of magnitude larger than what can be achieved in other mechanical resonators. Because of the large quantum fluctuations, the mechanical vibrations are sensitive to the tiny forces associated with the electrons flowing through the nanotube, and vice versa. Advances in our labs allowed us to produce extraordinarily clean nanotube devices. These suspended nanotubes are essentially free of defects and adsorption contamination, as revealed by quantum electron interference measurements, which are extremely sensitive to scattering centres.
Q
Quantum dots
Quantum dots (QD) are semiconductor particles a few nanometers in size that, due to their small size, show quantum features like the ability to confine electrons in three dimensions, enabling the ability to define discrete energy levels. In nanostructures like carbon nanotubes or two-dimensional semiconductors, these dots can also be gate-defined by applying local electric fields via external electrodes to electrostatically trap electrons in a specific segment of the tube.
Quantum dots were discovered in 1981 by Aleksey Ekimov and Alexander Efros, and now they find application in a wide range of areas, such as optoelectronics, biomedicine, and solar cells.
Qubit
In classical information theory, a bit is the minimal unit of information and can take only two values: either 0 or 1. Analogously, in quantum information theory, the minimal unit of information is a qubit, which has to be encoded in a quantum system with only two possible states, one associated with the value 0 and the other with 1. But, because of the superposition principle, a qubit can be in any superposition of its two available states. This way, by measuring a qubit, one can obtain the result 0 or the result 1, each with a certain probability; in general, the final result cannot be predicted with absolute certainty.
The way in which information can be manipulated in a qubit is fundamentally and qualitatively different from that of a classical bit. There are logical operations, for example, that are possible in a qubit and not in a bit. This, therefore, opened new possibilities for the fields of information theory, computing and communications in their quantum version.
Even though the concept of a qubit was already on the table in the 1960s, when Stephen Wiesner introduced the conjugate coding scheme, and in 1980 Paul Benioff proposed the idea of using quantum states in computation, Benjamin Schumacher coined the term in 1995.
S
Semiconductors
Semiconductors are materials that, depending on the circumstances such as temperature, pressure, magnetic fields, etc., can act as conductors of the electrical current or as insulators. In conductors, the valence and the conduction band overlap, so electrons can easily move from the valence to the conduction band, conducting electricity as a consequence. In insulators, there is a big band gap between the valence and the conduction band, so electrons in the valence band cannot jump into the conduction band – they would need to gain too much energy to do so -, resulting in no electrical current. In semiconductors, there is only a small band gap between the bands, so electrons in the valence band can actually gain the necessary energy to jump into the conduction band and start conducting electricity.
Superposition
In quantum mechanics, superposition is the ability of a system to exist in multiple states simultaneously until a measurement is made. Upon measurement, the system collapses into one of the possible outcomes. Unlike classical systems, which are always in one definite state, quantum systems can be described by a linear combination of multiple basis states. It is what allows quantum computers to process multiple inputs simultaneously, offering exponential speed-ups for specific tasks compared to classical machines.
V
Van der Waals structures
A van der Waals (vdW) heterostructure is formed by combining multiple 2D layers that can be isolated and stacked, creating combinations of different 2D materials, held together by weak van der Waals forces. These structures combine the advantages of the electronic, optical, thermal, and magnetic properties of different 2D materials and have the potential to become the next generation of high-performance functional devices.