In this blog post, we’ll explore the background behind the development of quantum computers, their principles, and their current status, and discuss their significance.
“A quantum computer? Ugh…”
Anyone who studied physics in high school will likely cringe at the mere mention of the word “quantum.” You might think, “The computers we use now are fast and good enough…” but leading research institutions in advanced nations, including IBM in the U.S., are racing to conduct research on quantum computers. In other words, quantum computers are considered an indispensable element for the future development of human civilization. In this article, I will explain the background behind the creation of quantum computers, their principles, and their current status. What is the most important challenge facing human civilization in the 21st century?
There are various computer architectures, but we currently use the von Neumann architecture. The von Neumann architecture is the form that first realized the sequential processing method with internal memory proposed for computers; it is characterized by an instruction set that consists of executing a sequence of instructions and modifying the values at specific memory locations.
Thanks to groundbreaking advancements in electronics, the computational power of these computers has steadily increased. According to Moore’s Law—which states that computer memory capacity doubles every 18 months—it seems that computer memory will continue to grow indefinitely. However, even Moore’s Law has a theoretical limit that cannot be overcome. Currently, the size of a single bit of memory is much larger than an atom, but if Moore’s Law holds true, by 2020, a single atom would need to be used to create a single bit of memory. No matter how well Moore’s Law has held up, it cannot reach this stage. The reason is that quantum phenomena become strongly evident at the atomic scale, and classical physical phenomena—the basis of current computer manufacturing—no longer apply.
In the extremely small world of atomic scale, events that are difficult to comprehend with our common sense often occur; these are called quantum phenomena. Strange things happen, such as particles suddenly appearing in one place after being in another, or existing in two or more places simultaneously. As the size of memory bits shrinks to the atomic scale, von Neumann-style computers face limitations because these quantum phenomena can cause stored data to vanish or make overall control difficult. Consequently, researchers are struggling to increase storage capacity, and their focus is on finding ways to eliminate quantum phenomena.
However, quantum computers were not born out of an effort to eliminate quantum phenomena, but rather from a proactive idea that completely flips the perspective: to utilize quantum phenomena.
Quantum phenomena, the principle behind quantum computers, refer to events occurring in the microscopic world. In particular, quantum computers utilize the property that particles can exist in two or more states simultaneously until they are observed. What this means is that quantum computers use “qubits”—quantum bits—rather than bits as their unit of information. While a bit can represent either 0 or 1, a qubit can be in a superposition of both states, allowing it to perform calculations involving both 0 and 1 simultaneously.
Every electron has a unique spin direction, and quantum computers typically use this spin direction to superimpose 0 and 1. An electron spinning up represents 0, and one spinning down represents 1; through a series of processes, the spin directions of the electrons can be superimposed so that they become entangled.
To understand this more easily, consider the following scenario.
There are two cats, Cat 1 and Cat 2, whose states of being alive or dead are superimposed and entangled, such that the state of one cat affects the state of the other. When the cats’ states are entangled in this way, their quantum state is described by a single wave function, which represents the superposition of four possible quantum states: (live, live), (live, dead), (dead, live), and (dead, dead).
If we were to determine the states of the two cats using a classical computer, we would have to go through a two-step process: first checking the state of Cat 1, and then checking the state of Cat 2. In contrast, using a quantum computer, once the state of Cat 1 is determined, the state of Cat 2 is determined probabilistically. This is because the cats’ states are entangled. Being probabilistically determined means that we are told the probability of being alive (X%) and the probability of being dead (Y%).
Given modern people’s preference for precise information, the idea of a probabilistic outcome may not be appealing, but fortunately, using quantum algorithms can mitigate this drawback to some extent. You might think, “A time difference of about double isn’t a big deal, is it?” but this time difference increases exponentially as the number of qubits grows. By creating a large number of qubits that interact in a quantum-entangled state and altering the state of a single qubit, the quantum states of all other qubits connected via entanglement automatically change as well. Therefore, by using quantum algorithms to process these state changes according to specific objectives, an enormous amount of information processing can be performed simultaneously. This unique information-processing capability of quantum computers, which utilizes the phenomena of superposition and entanglement, is called quantum parallelism. Quantum computers can reduce computation time by performing this type of quantum parallel processing.
Storage capacity is arguably just as important as processing speed in a computer. Quantum computers will likely use quantum semiconductors; however, there are challenges in using them as actual storage devices because it is difficult to input and output information to and from quantum systems, and with current technology, information stored within atoms does not persist for long periods. Nevertheless, if these difficulties are overcome and quantum semiconductors are successfully developed, it might be possible to store all the information from every computer in Seoul within a device the size of a personal computer.
To realize quantum computers, it is necessary to introduce an appropriate quantum system that enables quantum parallel processing; in fact, this is arguably the most difficult challenge. With current technology, it is impossible to maintain qubits—which are composed of quantum particles—for a sufficient amount of time, nor can we prevent qubits from being altered by even minor influences from the external environment. Currently, superconductors are used to keep qubits stable for as long as possible. However, this drives up the manufacturing cost of qubits astronomically, and since the system must maintain a superconducting state during operation, the operating costs are also extremely high. Furthermore, because advanced soundproofing and shielding equipment is required to block external influences on the qubits, the computer itself is very large.
For reference, the most advanced quantum computer currently available is a nuclear magnetic resonance (NMR) quantum computer with a processing capacity of 7 qubits. As an example of what this computer can do, it can identify which numbers between 0 and 7 are prime.
How will the future unfold with the advent of quantum computers? There is no doubt that quantum computers will bring revolutionary changes across all aspects of our civilization, but at this early stage of development, it is impossible to fully foresee the impact they will have on the future world. Furthermore, since quantum computers are not designed to perform operations like addition quickly, they will not completely replace current computers, and so far, only two or three algorithms have been identified as useful.
However, as nanotechnology advances and the ability to control quantum states improves, quantum computer manufacturing technology will advance alongside it, meaning that quantum computers will eventually find their way onto our desks. As explained earlier, quantum computers—which can overcome the limitations of current computers caused by quantum phenomena and possess unparalleled computational power—are one of the most important challenges facing human civilization in the 21st century!
