Prof Dr Surjit Singh Bhatti, Calgary (Canada)

Quantum means a discrete and quantifiable unit. A quantum of light is called a photon, and quantum of electricity is an electron. Quantum computing is a multidisciplinary field (based on computer science, quantum physics, and mathematics), that utilizes the quantum mechanical phenomena. Quantum computers perform calculations based on the probability of an object’s state before it is measured.  The  classical computers are based on one of the two positions, such as ‘on or off’, or  ‘up or down’, represented by 1 or 0, the binary bits. Quantum computing operations instead use the quantum states of  objects to produce what are known as  qubits (quantum bits). These states are the undefined properties of an object (such as the spin of an electron or  photon),  before they have been detected. Instead of having a clear position, the unmeasured quantum states occur in a mixed ‘superposition‘, like a coin spinning in the air before it drops down. These superpositions can be connected with those of other objects in space. Their final outcomes are mathematically related.

The  qubits that exist in a superposition of states and can be connected with other qubits. The qubits perform multiple computations in parallel, to solve problems that were intractable earlier.  Such computers work by preparing superpositions of all possible computational states. A quantum circuit, prepared by the user, uses interference selectively according to an algorithm. They can be used to factor very large and exponentially increasing data, simulate complex systems, analyze molecular structures, and optimize complex processes. This is revolutionizing many areas, such as Materials science, Drug discovery and Finance. About 20% organizations today are budgeting for their secure quantum computing projects.  

Entanglement

Quantum computing is based on a new concept called entanglement, first given by a Chinese scientist, Madame Wu, who suggested that pairs of photons resulting from ‘particle-antiparticle (matter-antimatter) collisions’ remained connected.  Entangled particles are always correlated, no matter how far apart they stray. In 2022, the Physics Nobel Prize was awarded to Alain Aspect (University of Paris), John Clauser and Anton Zeilinger (University of Vienna) who produced convincing evidence for this phenomenon. They showed that  these photons can be linked with one another.

This feature of the universe had baffled even Albert Einstein. It connects matter and light in a complex way. Qubits of such distant particles have a relationship that can  help encrypt and teleport information. Satellites have demonstrated this fact, and it can be used to design extremely fast computers and in the synthesis of superconductors (materials with negligible resistance to the flow of electricity through them). It shows  that very distant parts of the universe are connected. The Quantum Computers based on such photons can interact, no matter how distant they are.

Coherence and de-coherence

Coherence occurs if all light waves hitting a detector have the same frequency and are in phase with one another. Quantum computing requires ‘coherence’ of the qubits, which are highly sensitive to their environment. Their interaction with  environment causes ‘de-coherence’ and introduces errors. This requires development of reliable ‘error correction’ techniques, appropriate  hardware, software, and  multi-dimensional algorithms. Besides, it is necessary to have highly stabilized,  very low-noise, narrow-linewidth lasers (the best coherent light sources), with temperature-controllers (to avoid de-coherence), without external perturbation. To retain their states, these quantum processors need very low temperatures of about 0.01 degree above absolute zero (- 273.15 ⁰C). Cooling is essential as quantum computers must hold an object in a superposition state long enough to carry out various processes. The special ‘in-between states’ are lost even if a very minor de-coherence occurs.

Encoding the Qubits

Qubits are created by encoding quantum data in the internal states of charged ions, trapped using electric and magnetic fields. A randomly moving cloud of charged ions is created by splitting gas  atoms using lasers. A Magneto-Optical Trap (MOT)  is used in which a series of lasers “cool” the atoms by slowing down their movement. They are confined within a small region and thus become more susceptible to applied fields. Different energy levels of ions are manipulated using suitable laser-controlled pulses. Qubits can also be created by encoding information in neutral (un-ionized) atoms, which have long coherence times and are immune to fairly environmental noise. Here, an array of 3-D optical traps is created by focusing laser beams to a small spot. These traps can pick up and manipulate the encoded atoms. Another approach uses exciting high-energy states in neutral atoms using strong interactions  between the atoms.

Artificial Intelligence (AI) and spatial computing are already changing the world. Quantum computing will power, enhance and accelerate the speed and accuracy of these technologies. The change will be startling and dramatic. Everyone will have access to leverage the power of quantum computing through the cloud.