This article explores how the science of the very small could lead to a revolution that is anything but.
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When someone says “machine” to you, the likelihood is you think of some large, powerful device, constructed from metal with gears and pistons. However, the quantum revolution at the start of the twentieth century has brought a new age of machinery and mechanics.
These machines would not just fit into the palm of your hand like the devices inspired by the digital revolution, also known as the third industrial revolution; in fact, you would struggle to see these machines with the naked eye.
These mechanical systems operate in the quantum realm in which the laws of quantum mechanics rule. This means that these machines are not just subject to counter-intuitive and unique phenomena like entanglement, superposition, and quantum tunneling, but they are as much a part of their function as gears, engines, nuts, and bolts are in macro-world machines.
Lancaster University’s Quantum Technology Center works to produce nano-electromechanical systems (NEMS), carbon nanotube resonators, and membranes made of two-dimensional materials such as graphene.
Graphene, an allotrope of carbon— a form of an element with a particular arrangement of atoms— as well as other 2D materials, is of particular importance to this emerging field of engineering.
Another allotrope of carbon that is useful in quantum nanotechnology is the nanotube. Further, so-called quantum dots, semiconducting particles just a few nanometers in size with unique optical and electronic properties, are also significant.
Challenges and Applications
It goes without saying that quantum nanomechanical systems are incredibly small. In quantum mechanics, energy comes in discrete packets called quanta, the smallest possible unit of energy a system can possess, from which the prefix “quantum” originates.
The most famous quanta, and the first to be discovered, is the photon — the discrete packets of energy that make up light. However, other forms of energy also come in quanta — including mechanical energy.
Nanomechanical systems often possess mechanical energy amounting to just a few quanta, or even a single quanta. This means that the mechanical displacement of these systems is smaller than atomic dimensions.
Measuring such tiny distances requires some extremely complicated and intricate systems that themselves must exploit quantum phenomenon. These measurement systems also often need to not just detect quantum states, but excite systems influencing them to change state.
This requires measurement systems like special optical, superconducting, and electronic quantum systems which can often be single-electron transistors — a sensitive electronic device in which electrons flow through a tunnel junction between source to a quantum dot.
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The reward for overcoming these measurement challenges is significant. The applications of quantum nanomechanical systems include ultra-sensitive sensors for quantum computers in which they could comprise the foundation of memory elements.
Mechanical Qubits
The foundation of memory and information processing in quantum computers are qubits, which play the same role in this technology as bits do in traditional computing.
Whereas bits are limited to an “on” or “off” state, because of the phenomenon of superposition that allows a quantum state to take a multitude of states simultaneously, the amount of information a qubit can encode is near unlimited.
Thus far, only a limited amount of qubit suggestions have demonstrated their potential for use in a quantum computer. Many studies have suggested the applicability of a mechanical qubit as the building block of a quantum computer.
In August of 2021, a team of international researchers led by Adrian Bachtold from ICFO, Fabio Pistolesi from the University of Bordeaux, and Andrew N. Cleland from the University of Chicago suggested an innovative electromechanical system for quantum information processing.
The system uses a mechanical qubit composed of a nanotube resonator coupled to a double-quantum dot formed in the suspended nanotube which allows the information to be controllably encoded in just two quantum levels, vital for both manipulating and storing quantum information.
Quantum systems are easily disturbed by environmental factors, a phenomenon called decoherence, which causes the superposition of the system to collapse and for the qubit to take a single state. This is disastrous for quantum computers as it leads to the loss of information.
Mechanical qubits based on nanotubes could help protect these systems against decoherence. In addition to this, they could allow for the coupling of one quantum system to another like photons and superconducting qubits. While this is something yet to be fully explored, it holds a huge amount of potential.
Quantum information processing, including computation, communication, and sensing, is likely to deliver the next technological revolution, providing the promise of unprecedented computational capabilities and security. Quantum nanomechanics will likely be at the heart of this revolution.
References and Further Reading
Quantum Nanomechanics, Quantum Technology Center: Lancaster University. https://www.lancaster.ac.uk/quantum-technology/
Pistolesi. F., Cleland. A. N., Bachtold. A., [2021], ‘Proposal for a Nanomechanical Qubit,’ Physical Review X, [https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.031027]
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