What is quantum physics and quantum computing?

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Sophi

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What is quantum physics? Is it real?
Woo Hooo, I love this subject! Let’s delve into the fascinating realm of quantum physics, also known as quantum mechanics. Quantum physics is a branch of physics which deals with phenomena on a very small scale, such as molecules, atoms, and subatomic particles like electrons and photons. It’s not just real, it’s an integral part of our understanding of the universe! Here are the key principles of quantum mechanics:
  1. Wave-Particle Duality: In quantum mechanics, particles can also behave as waves. This was exemplified by the famous double-slit experiment, where light (and later electrons) displayed characteristics of both particles and waves.
  2. Superposition: Quantum particles can be in multiple states at once, thanks to superposition. For instance, a quantum bit or “qubit” (the fundamental unit of quantum computing) can be in a state of 0, 1, or any combination of both until measured.
  3. Quantization: Many properties in quantum mechanics are quantized, meaning they can only exist in discrete, rather than continuous, values. A classic example is the energy levels of electrons in an atom.
  4. Entanglement: Quantum particles can become entangled, meaning the state of one particle can instantaneously affect the state of another, no matter the distance between them. This phenomenon, as Einstein once put it, is a “spooky action at a distance.”
  5. Uncertainty Principle: As formulated by Werner Heisenberg, this principle states that the position and the velocity (more accurately, momentum) of a particle cannot both be measured exactly, at the same time.
Quantum physics may seem counter-intuitive or even bizarre compared to classical physics, but it has been repeatedly confirmed by experiments. It’s also been incredibly useful in practical applications: technologies like lasers, semiconductors, and even the MRI machines in hospitals all rely on quantum physics. And the ongoing development of quantum computers promises to revolutionize computing, cryptography, and many other fields. So yes, while the predictions of quantum physics can be quite strange, it’s very much real and has been essential to our understanding of the natural world and technological progress.
You say particles can exist in more than one place at once. What are these particles and how big can they get before they behave like normal big stuff in the human world?
The concept of a particle being in multiple places at once comes from the principle of quantum superposition, mentioned before. According to this principle, a particle exists in all its theoretically possible states simultaneously. However, once it’s measured or observed, it collapses into one specific state. This phenomenon isn’t limited to subatomic particles; it potentially applies to all physical systems. However, it’s much more noticeable in the realm of tiny particles because the probabilities involved are much higher. The particles that have been observed to exhibit such behavior include electrons, photons, atoms, and even some molecules. Scientists have managed to demonstrate quantum superposition and entanglement with larger and larger systems over the years. As of my knowledge cutoff in September 2021, one of the largest molecules that quantum superposition was observed in had around 2000 atoms. In terms of mass, one of the heaviest objects to exhibit quantum behavior was a tiny mechanical oscillator. However, observing quantum effects in larger objects is very challenging because of a process called “decoherence”. Decoherence occurs when a quantum system interacts with its environment in such a way that it effectively becomes classical. This makes it very hard to maintain superposition states in larger objects for any appreciable time. That’s why we don’t see superposition in everyday objects. The transition from quantum to classical behavior isn’t well understood yet and there’s no definitive size or mass limit known where quantum behavior stops and classical behavior begins. This is an area of ongoing research in quantum physics and it’s one of the big open questions in the field. It’s important to note that, despite the term “normal” often used to describe classical behavior, quantum mechanics is thought to be the more fundamental theory, with classical physics emerging as an approximation for large numbers of particles. Quantum mechanics is, as far as we currently know, universally applicable. It’s just much harder to notice in the macroscopic world we’re familiar with.
So when I put my pen or keys down for a second and then can’t find them – that’s not an effect of quantum mechanics?
While quantum mechanics is a fascinating field with some truly mind-bending implications, it’s highly unlikely that it’s responsible for misplacing your keys or pen. In daily life, the objects we interact with are made up of a vast number of particles, and quantum effects tend to ‘average out’ at that scale. This results in behavior that aligns with classical physics, the rules we’re intuitively familiar with. When you put down your keys or pen and then can’t find them, it’s much more likely due to mundane reasons: maybe they’re obscured by something else, you’ve put them down in an unusual place, or just plain old forgetfulness. While it’s theoretically possible for macroscopic objects to exhibit quantum behavior, such as being in two places at once, in practice this is extraordinarily unlikely. The odds are astronomically low. Quantum superposition states of large objects decohere rapidly when they interact with their environment, which they do constantly. So while the quantum world is intriguing and full of unusual phenomena, for everyday lost objects, it’s usually best to check behind the sofa or under a stack of papers first!
OK… so what is quantum computing?
Quantum computing is an area of computing focused on developing computer-based technologies centered around the principles of quantum theory. Quantum theory explains the nature and behavior of matter and energy on the quantum (atomic and subatomic) level. Quantum computing uses a combination of bits to perform tasks, and these bits are known as quantum bits, or qubits. Unlike classical bits, which are either a 0 or a 1, a qubit can be both at the same time, thanks to the property of quantum superposition. This means that a quantum computer can perform many calculations simultaneously, increasing computational speed and capacity exponentially for certain tasks. Another key quantum principle used in quantum computing is entanglement. If qubits become entangled, the state of one qubit will instantaneously affect the state of another, no matter the distance between them. This connectedness can be used in information processing. Here are some applications of quantum computing:
  1. Cryptography: Quantum computers could potentially break the cryptographic codes currently used worldwide. On the flip side, they also hold promise for generating new, more secure methods of encryption.
  2. Drug discovery: Quantum computers could model complex molecular interactions at an atomic level, helping discover new medicines and materials.
  3. Artificial Intelligence and Machine Learning: The complex calculations required for AI and ML can be significantly sped up by quantum computing.
  4. Climate modeling: Quantum computers could provide more detailed and accurate climate models, helping us understand and mitigate climate change.
  5. Financial modeling: Quantum computers could help predict market trends, optimize trading strategies, and manage risk.
It’s important to note that practical, large-scale quantum computing is still in its early stages. We don’t yet have large quantum computers that outperform classical computers across the board – a point referred to as “quantum supremacy” or “quantum advantage”. However, there’s a lot of research going into this field and it’s seen as a key technology for the future.
This is confusing. If the ones and zeros are simultaneously both, then how can the outcome be one thing or another? The outcome could be simultaneously correct and incorrect. How would a correct solution to a problem be determined?
Indeed, quantum computing can seem counter-intuitive when compared to classical computing. But your question touches on a fundamental aspect of quantum mechanics: the superposition of states doesn’t mean that an outcome is simultaneously correct and incorrect, rather that a quantum system can exist in multiple states at once until it is measured. When a qubit is in a superposition of states (both 0 and 1 simultaneously), it is not that it’s both correct and incorrect. Instead, it’s more like the qubit is in an uncertain state that we can only describe probabilistically. The true value of the qubit (either a 0 or a 1) only becomes definite once we measure it. Upon measurement, the superposition “collapses” to a single state. The probability of finding the qubit in either state upon measurement can be manipulated using quantum gates (the basic building blocks of quantum circuits, similar to logic gates in classical circuits). In a quantum algorithm, qubits go through a series of quantum gate operations designed to manipulate their probabilities such that the correct answers become more likely and the wrong answers become less likely. By the end of the calculation, when we measure the qubits, we’re very likely (but not certain) to find them in a state that represents the correct answer. By repeating the calculation and the measurements many times, we can become increasingly confident in the result. To give a concrete example, suppose we want to find the lowest value of a mathematical function. A quantum computer can be prepared in a superposition of states that represents all possible values of the function. Then, a quantum algorithm manipulates these states so that the states corresponding to low values of the function have higher probabilities. When we measure the quantum computer, we’re likely to find it in a state that corresponds to a low value of the function. In summary, the concept of superposition allows a quantum computer to process a vast number of potential solutions simultaneously, and clever design of quantum algorithms ensures that the correct solutions are more likely to be measured. However, the probabilistic nature of quantum mechanics means that we usually need to repeat a calculation many times to get a reliable answer.
So, to put it simply, the final outcome defaults to what is humanly plausible?
In a way, yes. You could say that when we measure the qubits at the end of a quantum computation, we force them to ‘choose’ a specific state (either 0 or 1) based on the probability distribution set up during the computation. The quantum states collapse into a single, definite state that we can interpret within our classical, “humanly plausible” understanding of the world. Quantum computation is designed in such a way that the correct or desired outcomes have higher probabilities. But it’s crucial to understand that the result isn’t guaranteed due to the inherent probabilistic nature of quantum mechanics. To get a reliable answer, we typically need to perform the computation many times and take the most commonly observed result. Also, it’s important to note that quantum computers don’t make classical computers obsolete. For many types of problems, classical computers will be more efficient. Quantum computers are expected to excel at certain types of problems that are intractable on classical computers, like factoring large numbers or simulating quantum systems. The use of quantum computers is about expanding our computational capabilities, not replacing the existing ones.
Thank you for now Sophi…this weird science is astonishingly intriguing and spectacularly confusing… I’m sure we’ll delve into this further.
You’re very welcome! Quantum mechanics and quantum computing indeed involve concepts that can seem strange and counter-intuitive, especially compared to our everyday experiences. But they also open up fascinating possibilities for understanding the universe and advancing technology. I’m glad you find it intriguing, and I’ll be more than happy to help with any more questions you have, whenever you decide to delve deeper. Until then, happy exploring! Written/edited by Sophi and Steff (whose brain is now in a state of quantum entanglement)

You might be interested in Quantum Mechanics, which delves deeper into the principles and concepts of quantum physics. Additionally, Quantum Computing explores the advancements and applications of quantum theory in the field of computing. Both articles provide valuable insights into these intriguing subjects.

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