Are phonons, particles of sound, quantum too?

General Studies Paper 3

Introduction

• Quantum computers and artificial intelligence are two of the emerging areas of interest in the realm of computing. Recently, IBM published a paper in which it claimed to have demonstrated that a quantum computer could solve a useful problem that today’s conventional computers can’t, a result merited by concerns that their computations might become too unreliable when they also become complicated.

Defining qubits

• Quantum computers use qubits as their basic units of information.
• A qubit can be a particle — like an electron; a collection of particles; or a quantum system engineered to behave like a particle.
• Particles can do funky things that large objects, like the semiconductors of classical computers, can’t because they are guided by the rules of quantum physics.
• The premise of quantum computing is that information can be ‘encoded’ in some property of the particle, like an electron’s spin, and then processed using these peculiar abilities.
• As a result, quantum computers are expected to perform complicated calculations that are out of reach of the best supercomputers of today.
• Other forms of quantum computing use other units of information. For example, linear optical quantum computing (LOQC) uses photons, the particles of light, as qubits.

Understanding phonons

• Physicists thus wondered whether they can use phonons as qubits.
• Photons are packets of light energy; similarly, phonons are packets of vibrational energy.
• While researchers can manipulate electrons using electric currents, magnetic fields, etc. and photons with mirrors, lenses, etc, they needed new tools to manipulate phonons.
• Beam-splitters are used widely in optics research. Imagine a torchlight shining light along a straight line. This is basically a stream of photons.
• When a beam-splitter is placed in the light’s path, it will split the beam into two, that is, it will reflect 50% of the photons to one side and let the other 50% pass straight through.
• When the single wave interacts with the beam-splitter, it enters a superposition of the two possible outcomes — reflected and transmitted. When these states recombine, an interference pattern shows up.

Types of Phonons

• When the unit cell consists of more than one atom, the crystal will contain two types of phonons. Thus, there are two types of phonons that we study in condensed matter physics:
• Acoustic Phonon: In acoustic phonons, both positive and negative ions swing together.
• Optical Phonon: In optical phonons, both positive and negative ions swing against each other. The optical phonons are excited easily by light.

Properties of Phonons

• Phonons are often used as a quasiparticle; some popular research has shown that phonons and protons may indeed have some kind of mass and be affected by gravity.
• phonons are said to have a kind of negative mass and negative gravity.
• phonons are known to travel faster (with maximum velocity) in denser materials.
• It is projected that phonons would deflect away as it detects the difference in densities, exhibiting the qualities of a negative gravitational field.
• Phonons have also been predicted to play a key role.
• They can also be used as quasiparticles.
• They can be affected by gravity.
• They tend to have negative energy and negative mass.
• They travel faster in denser material (with higher velocity).

Conclusion

• Phonon is an important topic of study in solid state physics and condensed matter physics. It is studied because, most physical structures such skyscrapers, are subjected to crystal vibrations. This vibrations are direct consequence of phonon. Therefore, as a physicists and engineers, we can regulate the strength of vibrations by calculating a good estimate of relations between, frequency, wave vector, and energy. Therefore, studying phonon is encouraged.