Catherine Therese J. Quiñones
For many years, computers have doubled in power every year or so, as what Moore’s law predicts. This means that transistors are getting smaller and smaller and will eventually approach the size of an atom. However, in the atomic regime, the physics is completely different from what is observed in the electronic devices of today. In this level we have to consider the strange effects of quantum mechanics (QM).
In the classical model of a computer, the most fundamental building block of information, the bit, can only exist in one of two distinct states, a 0 or a 1 encoded in electronic components such as transistors. A bit is analogous to a head and a tail of a coin. When you toss a coin you can only have one of the two states. However, for a quantum computer, a quantum bit or a ‘qubit’ does not follow these rules. A qubit can be 0 or 1 or 0-1 or 0+1 or 0 and 1, all at the same time! This is where quantum mechanics comes in, i.e., the principle of superposition of states. Such superposition of states can lead to a simultaneous processing of 2N values that are being expressed simultaneously by N qubits. This allows far greater flexibility than the binary system. This means that more qubits you have, more options you can work with, thus, the faster you go.
We may now ask, What is the best way of creating a quantum computer or giving a system a qubit form? Physically, qubits are encoded in ions, photons, atoms/molecules, or electrons. Different qubit systems have its advantages and disadvantages. For instance, charged particle or ion trapped within an electromagnetic field or trapped using optical techniques can serve as a qubit however it is vulnerable from decoherence. It is very much important for a quantum computer to isolate the qubit because any interactions from the environment destroy the superposition of states thus causing decoherence or loss of its quantum character. Now, for a molecule, the up and down directions of the nuclear spin can also act as a qubit. Nuclear spins make excellent quantum memory since they interact with their environment only via their tiny magnetic fields. However, for the same reason, this makes the quantum information hard to access. On the other hand, photons can also be fast and robust carriers of quantum states encoded in polarization state thus making them a good medium by which to transmit quantum information. However, these attributes also mean that they are difficult to localized and store. Another approach is by using a solid state device, either a qubit achieved by a superconducting circuit using the Josephson junction or a qubit achieved by a semiconductor quantum dot. Quantum bits encoded in states with different electrical charges can be manipulated and measured very rapidly but the charges make short-lived qubits since they are strongly coupled to their local electrical environment.
Another problem with quantum computing is that if you observe or measure the quantum state of a qubit, it changes its value. So scientists must devise an indirect method of determining the state of a qubit, that is, by taking advantage of another quantum property called “entanglement.” At the quantum level, if you apply a force to two particles they become “entangled” meaning, a change in the state of one particle is instantly reflected in the other particle’s change to the opposite state. So by observing the state of the second particle, physicists hope to determine the state of the first. Thus, quantum effects can be used to acquire information about the system.
A working quantum computer should contain thousands of qubits in order to solve real-world problems usefully. One must have a technology that enables quantum systems to exist as coherent states for a long period of time. Various methods are being experimented and give promising results. One solution is to use a hybrid approach known as quantum network to maximize the different qubit systems. Basically, this approach involves the transfer of quantum information from one qubit form to another. For instance, quantum states which are stored and manipulated in matter qubits are mapped into photons for long distance transmission. The challenge now is to develop techniques in order to coherently morph quantum bits from matter to light.
So, what is the big deal with the quest for high speed computing and quantum computation? Actually, Mother Nature has endowed us with physical phenomenons which are way way too far complicated to solve using conventional computing. For example, we may want to know the ground state of a particular homogeneous system, such as an array of mutually coupled identical spins, or measure simple correlations between different parts of the system. This will pave the way to understanding condensed-matter systems and understanding of materials such as high-temperature superconductors. Not only that, by using algorithms such as Shor’s algorithm, a quantum computer would be able to crack codes much more quickly than any ordinary computer could. Breaking such encryption standards can however put one’s security at risk. Another breakthrough if quantum computing would be a success is the creation of computers that would be capable of simulating conscious rational thought – the key to achieving true artificial intelligence.
As of now, baby steps were made towards the goal of large scale quantum computers. The future of quantum computing is very promising but the benefits must outweigh the risks it could bring.
About the author:
Catherine Therese J. Quiñones is a physics graduate student of Mindanao State University – Iligan Institute of Technology (MSU-IIT). She is currently doing research in the field of medical physics. Her research interests also include high energy physics and astroparticle physics. ☆