In the quantum realm, it's impossible to define a particle’s definite state such as its position or momentum. All we can ever determine is its probability. But with the help of electron microscopes such as scanning tunneling microscopy (STM), we can see an atom up close with our naked eyes!
STM uses electric voltage and the quantum tunneling effect to image the metal sample down to its atomic resolution. Not only scientists in laboratories, but all of us interact with the atomic world on a regular basis.
Silicon Atoms Imaged Using STM
For example, the digital device you are using right now to read this article uses intricate properties of atoms and electrons to facilitate the transmission and communication of information. But this doesn’t make it a quantum device.
The atomic world emerges at some point at the edge of the classical world. In certain instances, the quantum behavior emanates from it. If you hear that quantum mechanics is weird, it’s because the principles of the quantum realm can be counterintuitive in terms of our everyday common sense.
While the Quantum realm is restricted to its domain, its effects are not.
1. The Quantum Realm is Probabilistic
2. Quantum Objects are Dual and Superposed
3. Entanglement & Teleportation in The Quantum Realm
4. Quantum Tunneling in The Quantum Realm
5. Information and Uncertainty in The Quantum Realm
6. Quantum Realm in a Nutshell
In classical physics, using Newton’s law of motion, a relevant parameter can be used to calculate the particle's state. We can determine the trajectory of the particle including its velocity, acceleration, position at all times, etc. to the utmost precision. However, things are inherently probabilistic in the quantum realm.
Let's say we flip a classical coin. In the midway, we see that the coin is in a superposition of heads and tails with the probability of the respective outcome being 75% and 25%. Apart from the probability, we can also predict the whole trajectory of this coin.
But if we flip a quantum coin, all we will see is the state of the coin when we flip it, and when it lands. It’s not that we don’t know what happened in between, but we just simply can’t. In its true state - when we are not observing - the quantum object is in a superposition of all possible states based on its initial conditions.
Probability in quantum mechanics vs classical mechanics
Talking about the quantum realm in terms of its probability is the statistical interpretation of quantum mechanics. This interpretation is about measurement and determining what happens when we measure the quantum system. No one knows what's going on in the quantum realm, in between the moment of production and detection, except for that the state is in a superposition of all possible states.
No one knows what’s going on in the quantum realm, between the moment of production and detection.
The probabilistic nature of the quantum realm means that the quantum states can be superposed. A superposition that gets destroyed at the moment of measurement.
This gives rise to the idea of an ideal quantum measurement. In an ideal quantum measurement, we should be able to interact with the quantum system without perturbing it too much and causing it to collapse. And, we should be able to see the quantum realm to its approximated state.
Superposition filmed by Stockholm University
Recently scientists have performed such an ideal measurement and filmed the evolution of a quantum state from its superposition to a determined classical state. The film represents the gradual loss of superpositions due to interaction and preservation of others because of the ideal quantum measurement. The argument against such an experiment comes from the interpretation of quantum mechanics and what we constitute as a measurement in quantum mechanics.
Apart from the ability to be in multiple states at the same time, a quantum object can also be both particle and wave at the same time. For example, our classical coin would be a solid object that we can hold in our hands. But a quantum coin can be represented as a wave - like ripples in a pond - and also as a solid object that we are familiar with.
Wave-like interference pattern formed by electrons
Even if we don’t know why duality happens, we kind of know what it could mean. For one, it means that in the quantum realm, the properties of a quantum object are not localized with itself. That is, properties like the spin of an electron are not only with the electron but are rather spread out in the Hilbert space. Duality allows for phenomena like entanglement to occur. And the phenomenon of entanglement is proved time and time again with the bell’s experiment.
Entanglement is strictly a property restricted to the quantum realm. When two quantum objects interact they are said to be entangled, in a way that they no longer can be represented as individual entities. Therefore, what happens to one particle happens to the other in a contrasting manner.
In 2019, Scientists at the University Of Glasgow captured the phenomenon of quantum entanglement.
Let’s go back to the coin-tossing example from earlier. Let's say we are tossing two classical coins this time, and we measure one of the coins states to be tails on landing. We immediately learn nothing about the other coin, as it could either be heads or tails.
But in the case of quantum coins, measurement of the first coin instantaneously renders the state of the entangled second coin. This result isn’t determined until the moment of measurement of one of the entangled quantum coins. Entanglement between quantum objects is correlated regardless of distance in the quantum realm.
Quantum entanglement can be used to teleport qubits of information in the quantum realm. In fact, teleportation is one of the key principles behind quantum cryptography and quantum networks. Teleportation here means, things go woosh in one place and appear in another. With entanglement, we can teleport the information we want from one place to another. With the no-cloning theorem, we can guarantee that there is only one quantum object that exists at the end of the teleportation process.
Quantum tunneling is a consequence of the principles such as duality, probability, and superposition. It is also a phenomenon that is strictly restricted to the quantum realm and it works like this: An electron which is a particle can propagate through a barrier (potential barrier) due to its wave-like nature.
A film by IBM with Atoms using STM
Tunneling is why instruments like STMs can help us see atoms and molecules, and make movies with them (see above). And it is the reason why the sun shines.
The sun wouldn’t shine without quantum tunneling.
The fusion reaction that forms helium atoms causes the sun to shine. This fusion process requires a tremendous amount of energy that the sun does not have despite its enormous amount of temperature. So even though the sun is extremely hot, it is the quantum tunneling effect that allows protons to tunnel through the repulsive barrier and perform fusion reactions.
Therefore, if we want to secure our quantum coin in a vault, we need a potential barrier fine-tuned to stop the quantum coin from tunneling to the outside.
If you are familiar with the uncertainty principle you know that two factors in the quantum realm can’t be measured at the same time. These factors are called observables in quantum mechanics and they are defined by Heisenberg’s uncertainty principle. The uncertainty principle states that it is impossible to measure the position and momentum of a particle at the same time. The same applies to the incompatible pair, energy, and time.
The real baffling fact is that this is not an assumption but a result of the nature of the quantum realm.
An illustration of the uncertainty principle that represents why position and momentum can’t be measured at the same time.
If we have the uncertainty principle forbidding us from measuring certain kinds of results, how would we learn anything about a quantum system? In the quantum realm, a quantum of information is called a qubit which possesses all the above-mentioned properties in contrast to a classical bit. In quantum information theory, this information is defined as the degree of uncertainty or ignorance about the given system. And it is expressed in terms of entropy.
So what is inside the quantum realm? The probability, superposition, and duality of quantum objects make phenomena like tunneling, entanglement, and teleportation possible.
Interpretation comes into play when we ask what exactly the quantum theory implies. If we clear away this need to make sense of a theory that doesn’t and work on what we know, we can build awesome quantum technologies that will radically revolutionize society. In that sense, the quantum realm is intrinsically weird but also of great utility value at the same time.