#01 Quantum Technologies
When I studied quantum mechanics during my bachelors and masters in physics, I had the impression that all of these are happening in a hypothetical world, which is inaccessible to us. All the mathematical approaches were just for fun or to explain something. I never thought it can be of any particular use. Later I found out that, without quantum mechanics, there is no laser or semiconductors and hence no modern computers or internet. Does it make them quantum technologies? The answer is no.
The word "quantum" came from "quantity", and it can be associated with other words like "discrete" or "packets". Quantum mechanics describes the behaviour of the sub-atomic particles. The primary difference from a "classical" particle is that they cannot be associated with a definite position and velocity at the same time, which is the well-known Heisenberg's uncertainty principle. One reason is that there is a characteristic wave associated with each sub-atomic particle. The principles of quantum mechanics were developed in the first two decades of the 20th century. We can have a detailed discussion on the quantum principle later. But what exactly is quantum technology? And why it was only developed much later, after the 70s?
Many industries seem to use the word "quantum" to make their product fancy and somewhat mystical. However, most of them have nothing to do with quantum principles, and none of them has anything to do with quantum technology. Quantum technologies use individual quantum systems to perform tasks that we're unable to do with classical systems.
Quantum mechanics explained many phenomena, such as hydrogen spectrum, black body radiation, etc. But every matter in the universe is also made up of these quantum particles. Why are we not observing quantum effects in day to day life? The reason is that these effects get smeared off when a quantum system interacts with the environment.
From explanation to complete control
Quantum mechanics explains how the microscopic world is behaving and evolving. Normally, for experimental verification of quantum principles, one has limited areas to probe in a system since most of the quantum effects will be washed out due to the overall classical nature of the system.
For example, for studying the electronic states of an atom we need to excite it with a photon (light particle) of suitable colour and detect the photon originated from it (during the de-excitation). There are many technical challenges for this experiment (at least until some decades ago). Let me try to explain some of these challenges.
Isolating an atom from its environment: Even a small spec of the sample contains millions of atoms. So when the photon is absorbed or emitted, one cannot determine which atom is excited or de-excited.
It requires a source that emits single photons. Like atoms, the faintest light you usually see contains millions of photons. The case of lasers is no different. For probing one and only one atom you need single photons with precise time information.
Single-photon detectors: The photons that are emitted by the atom (even if you manage to excite one and only one atom) you need to detect them as well as their time of arrival.
All of these challenges were technological roadblocks to make these quantum principles into action. This limitation has changed in the 1970s where researchers managed to trap individual ions/atomic systems. This offered somewhat complete control over a quantum system. At the same time, the semiconductor industry has come up with detectors that can sense the arrival of single photons (particles of light) with sub-nanosecond precision. Since we could probe an atom without interacting with the surroundings, quantum technology has become a possibility. So even though the invention of semiconductor devices and lasers were due to quantum mechanics, these don't qualify as quantum technologies.
The term quantum technology is an umbrella term that includes many fields such as quantum computing, quantum communication, quantum sensing etc. I can talk more about each topic in detail if you folks are interested. Let me finish by giving a short introduction to these fields.
Quantum computing: This is, without a doubt, the most anticipated quantum technology that may revolutionize our industries. The fundamental difference of quantum computing from "normal" computing is the use of quantum bits or "qubits" instead of classical bits (which are nothing but series of 0s and 1s). Qubit itself is a topic to be discussed separately. So I will keep it for another week. Digital computing emerged after the invention of transistors in 1947 by John Bardeen, William Shockley and William Brattain. Quantum computers have quantum systems that will have the quantum states corresponding to the qubits. These systems can be trapped ions or atoms, photons, or electrons in a crystal lattice such as diamond or superconducting devices. If you guys are interested to know more, let me know (comment here or send me an email). I can go into detail later.
Quantum communication: This is my home ground! I work in experimental quantum communication. So you can expect more on this topic. While quantum computers, one day, may excel the classical supercomputers for some tasks, quantum communication may not increase the speed of your internet. Quantum communication has two main parts. One is the transfer of classical communication using quantum states. Quantum communication can ensure more secure communication networks (still classical) which are unhackable even using a quantum computer. The second part is the transfer of quantum information through quantum states. This is relevant if you want to interface two quantum computers as you do now with your PC's. Ultimately building a quantum internet!
Quantum metrology: This is a field in which quantum states are used to enhance the sensitivity of measurements.
Quantum sensing/imaging: You take an object by shining light on it. In quantum imaging, you use a very few numbers of photons to image an object and it is even possible to get interaction-free images (take it with a pinch of salt here)
Many countries like the US, UK, Germany, China, India etc are already spending more than $1bn for the research and development of these technologies. By 2024 the field is expected to receive a funding of $10 bn worldwide. Stakes in the field are high and so are the misconceptions and over expectations. I will touch upon these topics later.
Let me conclude this session. As we move forward, we can have more in-depth discussions on the field. I hope this helped you to have a quick introduction to the field. Please let me know (through comment or email) the changes you wish to see in this section, to make it more useful. Sometimes I am normalized with technical terms which I have taken for granted.
Some further reads on quantum technologies are given here
A nice animation of quantum computing
For students of physics and engineering. Quantum technology may look intimidating. However, for exploring most parts of it, you can start without a deep knowledge of high-level mathematics or even quantum physics. You can start your journey by reading the book by Neilson and Chuang: Introduction to quantum information and computation.
IBM offers a free (but limited, I think) course on quantum computing. They also have an 11 qubit (I think!) quantum computer in which you can run your experiment remotely. Here is their course https://www.qubitbyqubit.org/programs, and the cloud quantum computing https://cloud.ibm.com/quantum
If you want to know more about quantum cryptography, here is a good introductory but detailed article. https://arxiv.org/abs/quant-ph/0101098
The first issue was very insightful. Got a lot of information and resources to explore. Thank you :)
Very well written. There is lot of misunderstanding about and abuse of quantum physics. Such articles will go a long way in demystifying quantum physics and explaining it from the perspective of science.