About the Utility of Advanced Physics:
Quantum computers

One day, as we were walking down Pratt Avenue, my friend asked me what I would like to do when I’m older. Obviously, that is no simple question. Like many of us, I’ve been in endless self-doubt about where I want to find myself later in life, though, recently, I’ve been stuck on the idea that I want to be either a physics professor or a theoretical quantum physics researcher, and so that’s what I told him. His immediate response went something along the lines of “why would you do that? Quantum physics have like, zero actual utility in real life, man”, a comment I get quite often. And so, I figured I’d write a piece on how advanced physics can have very real impacts on a macroscopic scale, focusing mostly on the utility of a quantum computer and the properties it functions on. Hopefully, you’ll find it interesting!

So, what is quantum mechanics?

The tale of quantum mechanics starts with German theoretical physicist Max Planck’s frustration at his failed attempts to solve a half-century-old energy problem (Kirchhoff’s blackbody radiation). You see, Planck was at that time convinced that energy in atoms, at an infinitely small scale, was not discrete, but rather was a continuous substance; he believed that you could divide quantities of energy indefinitely, unlike many other famous scientists at the time, who believed that there was a point where you could not divide a quantity of energy anymore (discrete). This point of view rendered him unable to solve the actual problem, and it came only as “an act of despair” for him to reinforce his leading solution in 1900 by considering this energy to be, at its smallest value, condensed into indivisible little packets called quanta (hence, quantum; now known as photons). As it turns out, Planck’s Postulate confirmed the energy values that had been experimentally found, and this newly discovered property of energy thereafter laid the foundations of the modern quantum theory, the branch of physics associated with the description of subatomic-scale properties of matter and energy.

Superposition (Dirac)

Now, an interesting property in quantum mechanics was first mentioned by Paul Dirac, a mathematics professor at Cambridge. Quantum superposition, as he puts it simply, “…requires us to assume that between [quantum] states there exist peculiar relationships such that […] we can consider [a quantum system] as being partly in each of two or more other states”. In other words, superposition is the property that quantum systems (i.e., an electron, a few quarks, etc..) exhibit in being in more than one quantum state at the same time. For example, the spin of an electron could be partly up and down simultaneously. This is the phenomenon that Schrödinger’s famous cat thought experiment is based on! 

To visualize this, imagine a lightbulb in an empty room. Supposing the lightbulb is a quantum system, let us describe it as being in a superposition of “on” (emitting light) and “off” states. Suddenly, the doorknob turns and the door creaks as it lets in your quick, curious glance fixated on the lightbulb. So, what do you see? Well, sadly, nothing weird; it turns out you have a certain probability of seeing it on and another of seeing it off. The truth is, a quantum system in superposition will always take the form of a single one of its states at the tiniest exterior perturbation. This is called the collapse of the quantum system, or, in more technical terms, the “wave function collapse”. For example, for us to measure a particular property of a quantum system, we must interact with it (“observation” or “measurement”), therefore causing its collapse. Additionally, the measurement of a quantum property only causes the collapse of the states for that particular property: measuring an electron’s spin will not necessarily affect the superposition of its angular momentum states, for instance. Crazy, huh? 

Finally, don’t get confused: this phenomenon is not something that is seen at a macroscopic level, but rather a fundamental property of matter at an infinitesimal scale. 

“Spooky action at a distance” (Einstein)

The next quantum property useful in understanding the functioning of quantum computers is called entanglement. It is a brain-wrecker, so I’ll keep it short: entanglement is the property that quantum systems manifest in sometimes sharing a quantum state with one another. It means that measuring one of the systems affects the other, no matter the distance between them. 

For example, imagine two electrons. As you may or may not know, an electron can either spin up or spin down (the spin is just a property of a particle).

Now, let’s suppose their quantum states are entangled and, as seen in the previous section, they’re in a superposed state (both spin up and down) before measurement. So, what happens when you measure one of them? Well, it turns out that they both collapse at the same time into the same state, even if you measured just one of them. It doesn’t matter how far apart they are (the longest distance recorded is 1200km), while entangled, they will share the same exact quantum state. What’s more is that, to this day, although entanglement can be derived from the basic principles of quantum mechanics, physicists cannot fully agree on how to interpret the phenomenon.

Quantum Supremacy 

This term is thrown around quite often when talking about quantum computers, so I’ll make a quick mention of it. The concept is rather straightforward; quantum supremacy will be attained when a quantum computer will have performed an operation in much less time than the most powerful modern classical supercomputer. As we will see later, the idea of a quantum computer is incredible in theory but is experimentally much harder to conceive. In October 2019, Google scientists famously announced their achievement of quantum supremacy, although that achievement is to this day debated.

Classical computer: functioning 

We’re getting there, people! To compare the functioning of a quantum computer to that of a classical computer, we must first understand how classical computers, or rather how their Central Processing Unit (CPU), works. 

The CPU is the part of the computer that performs all operations on the input data, transforming it into the needed output. In modern computers, the CPU is composed of dozens of bites (binary digits), which are little units that can be either “on” (1) or “off” (0) and, since a single bite is much too small to work with, bites are regrouped in groups of 8 (called bytes), forming binary digit sequences. The sequences are the information going in and out of the CPU. Once it receives input sequences, the programming language transforms the 0s and 1s into the desired sequences, the output. At its most basic level, this is how a computer operates; it performs little operations on codes and codes of binary digits. Now, delve into the actual subject of this article: how quantum properties can be applied to a macroscopic level through quantum computers.

Quantum computer: functioning

Introducing the notion of a qubit: a qubit (quantum bit) is a small particle (ex: an electron) that serves as the equivalent of a classical computer bit. Like a bit, it has two states, 0 and 1 (ex.: an electron as a qubit could have spin down as its “0” and spin up as its “1”), and therefore a programming language can be set up to transform these sequences of qubits into a desired output, like in a classical computer.

Now, how does superposition affect qubits? Well, like any quantum system, the qubits will constantly be in a superposition of all their possible states; therefore, the qubits will be both “1” and “0” at the same time. Much in the same way as the earlier lightbulb example, a qubit will have different probabilities of collapsing into either “1” or “0” once “measured”. Therefore, to describe the state of the qubit, you’ll need these two probabilities (they’re both simply some percentage value), as opposed to a bit which is described with one binary digit. Right, but what happens when you add more qubits? Well, with 2 qubits you’ll need 4 probabilities (one for each:  00,01,10,11). For 3, you’ll need 8 values (000, 001, 010, 100, 011, 101, 110, 111); for 4, 16, and so on. To compare, a classical 64-bit computer will be able to form sequences described with up to 64 binary digits; a 64-qubit computer sequence, however, will be described by up to 2⁶⁴ or 18446744073709551616 values. This is where a quantum computer’s enormous computational power emerges from.

Take the situation of a maze, and the computer is tasked to find the way out. A classical computer will test out every possibility and eventually find the solution; a quantum computer will, thanks to superposition, test out every single possibility at once.

Moreover, with entanglement, these qubits will be able to link with one another, creating “chained” qubits that will react instantaneously once one is affected. Upon creating quantum algorithms playing on this entanglement, quantum computers will be in theory able to solve complex calculations at an incredibly faster rate than their modern counterparts. 

Quantum computer: appliances

Okay, enough with the details. How can these properties be applied to macroscopic, everyday situations? Well, like for the maze, the solving of simulations, models, and lengthy calculations such as optimization problems will be significantly improved by a fully functioning quantum computer.

For instance, since 2016, Volkswagen has been testing quantum computing to develop a real-time traffic-routing system, to determine vehicle pricing with customer demand and even to optimize vehicle painting. Airbus is using quantum computing to calculate the most fuel-efficient pathways for aircraft, and pharmaceutical companies are using it to model and analyze compounds for new drugs. Furthermore, its use in cracking encryption is already causing major cybersecurity issues, and even investing firm Goldman Sachs has already announced its plans to use quantum computation to perform complex financial calculations and to price financial instruments!

You see, the true power of quantum computation is its ability to solve complicated mathematic problems quickly. Even at their early stages of development, quantum computers would simply outperform classical computers and reach a whole new level of digital information processing. Well, theoretically. 

Decoherence (Zeh) and cost

Decoherence is a concept that relates to a quantum system’s loss of coherence over time in a non-isolated environment. Coherence is simply what permits quantum systems to display quantum behaviour; it is the balanced relationship between the different states of a system. A type of decoherence, for example, is “measuring” a quantum system (for which we add energy to it), causing its collapse and loss of superposition (quantum behaviour). Any external perturbation can engender decoherence in a quantum system.

For huge technological development companies, decoherence is the major obstacle in building a quantum computer. The fragility of the quantum system needs an immensely complex setup to be constantly maintained (isolated), one that comes with an equally immense cost, as one simple change to qubit changes the whole CPU. Adding the cost of the research and of the materials, the total production cost ranges in the millions; and while there are many prototypes and some minor achievements, the true, fully functioning super-OP quantum computer is still on the horizon. 

To conclude, quantum computers are a very good example of how quantum properties will one day be utilized to solve very “real life” problems. In the end, it is not a question of usefulness; the world, at an infinitesimal scale, is governed by quantum properties, properties which, without, we would be unable to exist. To any curious soul, quantum physics is the doorway to understanding the Universe, it’s beginning, and its end; I believe that, to anyone who has ever wondered how or why they are in this world, quantum physics can provide, by some path or another, an answer to our questionings.