The story of quantum computers begins in 1981 with Richard Feynman, probably the most famous physicist of his time. At a conference on physics and computation at the Massachusetts Institute of Technology, Feynman asked the question: Can we simulate physics on a computer?
The answer was—not exactly. Or, more precisely—not all of physics. One of the branches of physics is
quantum mechanics, which studies the laws of nature on the scale of individual atoms and particles. If we try to simulate quantum mechanics on a computer, we run into a fundamental problem. The full description of quantum physics has so many variables that we cannot keep track of all of them on a computer.
If one particle can be described by two variables, then to describe the most general state of n particles, we need 2n variables. If we have 100 particles, we need 2100 variables, which is roughly 1 with 30 zeros. This number is so big that computers will never have so much memory.
By itself, this problem was nothing new—many physicists already knew that. But Feynman took it one step further. He asked whether we could turn this problem into something positive: If we cannot simulate quantum physics on a computer, maybe we can build a quantum mechanical computer—which would be better than the ordinary computers?
This question was asked by the most famous physicist of the time. Yet, over the next few years, almost nothing happened. The idea of quantum computers was so new and so unusual that nobody knew how to start thinking about it.
But Feynman kept telling his ideas to others, again and again. He managed to inspire a small number of people who started thinking: what would a quantum computer look like? And what would it be able to do?
Quantum mechanics, the basis for quantum computers, emerged from attempts to understand the nature of matter and light. At the end of the nineteenth century, one of the big puzzles of physics was color.
The color of an object is determined by the color of the light that it absorbs and the color of the light that it reflects. On an atomic level, we have electrons rotating around the nucleus of an atom. An electron can absorb a particle of light (photon), and this causes the electron to jump to a different orbit around the nucleus.
In the nineteenth century, experiments with heated gasses showed that each type of atom only absorbs and emits light of some specific frequencies. For example, visible light emitted by hydrogen atoms only consists of four specific colors. The big question was: how can we explain that?
Physicists spent decades looking for formulas that would predict the color of the light emitted by various atoms and models that would explain it. Eventually, this puzzle was solved by Danish physicist Niels Bohr in 1913 when he postulated that atoms and particles behave according to physical laws that are quite different from what we see on a macroscopic scale. (In 1922, Bohr, who would become a frequent Member at the Institute, was awarded a Nobel Prize for this discovery.)
In classical computing, a bit is a term to represent information by computers. Quantum computing uses quantum bits or qubits for a unit of memory. Qubits are comprised of a two-state quantum-mechanical system. A quantum-mechanical system is one that can exist in any two distinguishable quantum states. Seeing the terms superposition and entanglement might be baffling, but it's okay: we don't experience these phenomena in our lives. Superposition is a principle that states while we do not know the state of an object at a given time, it is possible that it is in all states simultaneously, as long as we do not look at it to check its state. The way that energy and mass become correlated to interact with each other regardless of distance is called entanglement.
What can quantum computers can do?
Quantum computers can easily crack the encryption algorithms used today in very less time whereas it takes billions of years to best supercomputer available today. Even though quantum computers would be able to crack many of today’s encryption techniques, predictions are that they would create hack-proof replacements
Quantum computers are great for solving optimization problems.
First of all, if we have a quantum computer, it will be useful for scientists for conducting virtual experiments. Quantum computing started with Feynman’s observation that quantum systems are hard to model on a conventional computer. If we had a quantum computer, we could use it to model quantum systems. (This is known as “quantum simulation.”) For example, we could model the behavior of atoms and particles at unusual conditions (for example, very high energies that can be only created in the Large Hadron Collider) without actually creating those unusual conditions. Or we could model chemical reactions—because interactions among atoms in a chemical reaction is a quantum process.
Another use of quantum computers is searching huge amounts of data. Let’s say that we have a large phone book, ordered alphabetically by individual names (and not by phone numbers). If we wanted to find the person who has the phone number 6097348000, we would have to go through the whole phone book and look at every entry. For a phone book with one million phone numbers, it could take one million steps. In 1996, Lov Grover from Bell Labs discovered that a quantum computer would be able to do the same task with one thousand steps instead of one million.
More generally, quantum computers would be useful whenever we have to find something in a large amount of data: “a needle in a haystack”—whether this is the right phone number or something completely different.
Another example of that is if we want to find two equal numbers in a large amount of data. Again, if we have one million numbers, a classical computer might have to look at all of them and take one million steps. We discovered that a quantum computer could do it in a substantially smaller amount of time.
WHAT ARE QBITS OR QUANTUM BITS?
“Quantum bits” or simply saying “Qubits” differ from regular bits by the ability to operate in both “1” and “0” states simultaneously. It’s called superposition. Superposition is one of the key principles of quantum physics.
To explain it briefly, imagine tossing a coin which is either one of the two possible outcomes, a head or a tail. A ‘Classical computer’ works in the same manner: either “1” or “0” for each bit.
On the contrary, imagine the same coin tossed in the air before falling, and you want to measure its outcome. At this state, the outcome is said to be head, tail, or both at the same time. This is how a qubit functions. It is similar to a coin in the air. The qubit will have two states until you measure it. Once you implement a measurement to the qubit, it will directly get one of the two possibilities either a “0” or a “1”.
Why is it so hard to make a quantum computer?
We have decades of experience building ordinary, transistor-based computers with conventional architectures; building quantum machines means reinventing the whole idea of a computer from the bottom up. First, there are the practical difficulties of making qubits, controlling them very precisely, and having enough of them to do really useful things. Next, there's a major difficulty with errors inherent in a quantum system—"noise" as this is technically called—which seriously compromises any calculations a quantum computer might make. There are ways around this ("quantum error correction"), but they introduce a great deal more complexity. There's also the fundamental issue of how you get data in and out of a quantum computer, which is, itself, a complex computing problem. Some critics believe these issues are insurmountable; othersacknowledge the problems but argue the mission is too important to abandon.
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