Saturday, February 4, 2023


We cannot observe them directly, but the behavior of atoms, quarks, photons and everything that makes up reality on a nanometric scale or smaller confirms that we still do not know much about the universe

Quantum theory – which describes these tiny particles – is no longer a rarity previously confined to the laboratory; Now it invades our lives and is found on the smartphone that we carry in our pocket, and even on the credit card number that we use to buy online. “Quantum” appears more and more in terms like “quantum healing” and “quantum politics.” Quantum has become a buzzword. Any scientific relevance to these uses is purely accidental; however, this illustrates that the ” quantum” has a mystique beyond the scientific.

Despite the fact that quantum mechanics arose to solve a scientific problem, more than a century later it still holds some mystery. Quantum physics predicts paradoxical or incredible behaviors. For example, a quantum particle does not have only one value of a physical quantity, but all the values ​​at the same time, something called superposition; two quantum particles can remain linked or ” entangled “, even at unlimited distances and without any physical connection in between; and they can teleport through empty space.

Quantum jumps can be found at your favorite bar and local supermarket

In 2011, the Austrian physicist Anton Zeilinger administered a questionnaire with 16 multiple-choice questions to more than 30 specialists in quantum theorists, about their basic concepts and their interpretation. None of the possible answers received unanimous support, as many of the questions elicited a wide range of opinions. According to researcher Charles Clark, co-director of the Joint Quantum Institute at the University of Maryland, it would be “a great topic to locate where the problem is” that makes quantum theory so difficult to interpret. In part, this is because it is so abstract, for the sake of the smallness of what it describes. When we kick a ball, we gain empirical knowledge of how the world works on a human scale. But we can’t kick a quark or throw a photon; We can only describe these particles with the help of quantum theory.


When Max Planck invented quantum theory in 1900, he thought it was just a mathematical trick. But his “trick” explained why the physicists of the time could not answer this question: “What is the nature of the light emitted by a flame or any other hot body?” They knew that light was an electromagnetic wave generated by electrically charged particles, such as electrons, but the problem was that the calculations they used to apply this theory contradicted the laboratory results of the light spectrum generated by hot objects.

Planck tried several solutions to solve the problem before coming up with the idea that light is emitted by means of “quantum” energies, exact multiples of a certain minimum quantity, or “how much”. He called this “an act of desperation,” but he produced the correct spectrum of light from a hot body and that earned him the Nobel Prize in 1918. Later, Albert Einstein and Niels Bohr won their own Nobel Prizes by extending Planck’s work. . Einstein showed that light comes in discrete packets of energy, later called photons, and Bohr posited that electrons in an atom absorb or emit photons as they jump between quantum energy levels.

It was amazing to find that the world operated in this strange way. Now it is known that quantum jumps and everything else are real. But why didn’t humanity notice the “quanta” until 1900? Because we are talking about a very small amount of energy. Even the feverish glow of a candle represents a torrent of photons (trillions per second). The light that radiates from a fountain is like sand spilling from a bucket; it appears to be a continuous stream, but in reality it is a multitude of tiny grains lost within the larger stream. Similarly, quantum jumps in atoms are extremely small changes in energy, although popular use of “quantum jumps” often incorrectly refers to large changes.


They can be found at your favorite bar or at the local supermarket. Whenever you see a flashing beer sign or a barcode scanner, look closely: you are observing electrical quantum jumps in action through their fingerprints, the emission of light, as Niels Bohr determined.

A neon sign is a glass tube filled with the noble gas neon or another gas that glows when a voltage is applied to it. The “glow discharge”, first seen in the late 1800s, works because the voltage raises the electrons in the gas atoms to a higher energy level; then the electrons drop to lower levels and release photons. Gases have different levels of atomic energy, and these levels define the wavelengths of the photon. Neon produces red light, argon generates blue light… and so on.

The light discharge is also in fluorescent lighting and lasers. In a fluorescent tube, quantum jumps in the mercury vapor create ultraviolet photons, which activate a coating inside the tube, which produces white light. The laser, invented in 1960, is like a discharge tube between two mirrors. As the photons from an atomic quantum jump bounce back and forth, they stimulate more photons from the atoms that pass through it. That produces an enhanced beam of pure light at a single wavelength. A ray whose infinite range of uses makes it clear that quantum energy is real.

Quantum jumps also appear in light-emitting diodes (LEDs). LEDs are made of semiconductors in which electrons must jump through a gap to a higher energy, before moving as an electric current. By applying voltage to the led, the electrons jump across the gap, and then come back producing photons.

In addition to LEDs, quantum behavior is crucial for digital devices. Its integrated circuits are made of semiconductor silicon, whose quantum energy gap allows good control of electrons to manipulate digital bits.


Although quantum leaps were considered radical, they do not contradict existing world views. Overlapping, entanglement, and teleportation, however, produce more strangeness because they oppose our understanding of the universe. These problems arise because quantum theory does not predict definitive values ​​for physical properties, but only probabilities.

Einstein did not believe that nature was random, as expressed in his famous comment “God does not play dice with the universe”, but in quantum theory this does not seem to be the case. A baseball has a certain momentum, but in the quantum world, any particle carries all of its possible physical values ​​at the same time or in “superposition” until it is measured or interacts with the environment.

For example, the property called “spin” causes electrons to behave like little bar magnets with their north pole pointing up (U) or down (D). In quantum theory, the electron is in these states at the same time, since there is a 50% probability that a measurement will show U or D.

The “Schrödinger’s cat” experiment – as envisioned in 1935 by quantum theory pioneer Erwin Schrödinger – illustrates this statistical nature. The cat is dead or alive depending on a random event and can therefore be described in both states at the same time.


We need to understand these rare effects if we want to understand quantum physics; But even without that, quantum is entering digital technology. Integrated circuits in digital devices represent binary bits in small electronic switches that are turned on or off to represent 0 and 1. But any system with two possibilities can also represent 0 and 1, including the U and D states of the electrons and the H and V states of photons; just by way of superposition, these represent 0 and 1 simultaneously.

This is the innovative idea behind the quantum bit, or qubit, a kind of superbit (the name was invented as a joke in 1995). For example, two ordinary bits represent only one of the decimal numbers 0, 1, 2, 3… but two qubits represent all four numbers at the same time. The advantage grows rapidly, such that 20 qubits carry 20 million times more information than 20 bits. It has been estimated that a “quantum” computer using 150 or 300 qubits would have the power of all the conventional supercomputers in the world put together.

The Joint Quantum Institute at the University of Maryland and a dozen other labs around the world are working to use qubits in computing and also in telecommunications, as photons traversing a vast fiber-optic network carry much of the information that travels the world, from phone calls to internet downloads. However, qubit technology is difficult to implement, because the particles must be isolated from the environment and kept at ultra-low temperatures in order for them to remain in overlap. It will be years before we have the 150 qubit computer, but trial versions that use a few qubits of photons have already been built and programmed to solve the problem.


The first step to entangle photons is to create a correlated pair with one of them in state H and the other in state V (which can be obtained by sending light through certain crystals), although we still do not know which is which. If the photons are then widely separated, they will show a surprising property. If photon 1 is measured as H, the measurement of photon 2 will give V; But if photon 1 is measured as V, the second photon gives H. In some way, photon 2 “knows” the measurement result of photon 1 and adjusts accordingly; the two particles are intertwined.

To see how exceptional this is, let’s put it in a more familiar context. A drawer in Mexico City contains an identical number of black and white socks, as does a drawer in Toronto, Canada. If a sock is randomly picked in Mexico City and a friend picks another in Toronto, half the time the choices will coincide. But if the socks are entwined, like photons, no matter what color you choose, your friend will choose the other color every time, despite the distance between the two socks and the absence of any physical connection.

Photon entanglement was demonstrated in the laboratory in 1982; the latest measurements show that it can operate over distances of up to 144 kilometers of empty space. They also point out that any information transmitted between photons travels 10,000 times faster than light and perhaps instantaneously. This contravenes the results of Einstein’s relativity, where it is claimed that nothing can travel faster than light. Worse still, instantaneous transmission will make us completely reconsider our notions of time and space.

Long before these disturbing results occurred, Einstein had a hard time accepting entanglement, calling it “a creepy action at a distance.” But it does exist, with particles somehow connected by an unknown quantum channel that we cannot understand. Even more: researchers have taken this mysterious link further, to the field of teleportation. In this form of transport so common in science fiction, a person or an object is replicated elsewhere while disappearing from its original location, as could be seen in the Star Trek stories. In 1993, IBM’s Charles Bennett and his colleagues theoretically showed how to teleport a photon. Imagining a pair of entangled photons at different locations, A and B, showed that the polarized state of a third photon could be sent from position A to the photon at B, via the entanglement channel, thereby recreating the third photon at the far site. Anton Zeilinger (of the quantum questionnaire) and his colleagues demonstrated the teleportation of a photon in the laboratory in 1997, and in 2012 they reported teleporting photons over distances greater than 143 kilometers.

A quantum computer would have the power of all the conventional supercomputers in the world


These effects go beyond science fiction when polarized photons are controlled like qubits in quantum cryptography, a method designed to transmit information securely over a fiber optic network. In 1984, Charles Bennett and Gilles Brassard invented the quantum key distribution. Like the combination of a padlock, the “key” is a long string of bits that make up the secret password to access a complex of algorithms that encode and decode information. The code is indecipherable without the key, but this, in turn, must be broadcast from the transmitter to the receiver when it runs the risk of being read by a third party.

Bennett and Brassard showed how this security vulnerability could be circumvented by using the quantum randomness of photon qubits to create a single random string of bits that functioned as a coded secret key based on photon entanglement. Quantum keys have been used to secure bank transfers and election results in Switzerland. They are not common yet.


We may never be able to teleport people or large objects, but in 2011, Ian Walmsley of the University of Oxford and his colleagues intertwined macroscopic objects visible to the human eye: two diamonds, each three millimeters long.

Atoms in crystalline solids, such as diamonds, vibrate at quantum energies, which are found in unusual amounts for carbon atoms in diamonds. In the experiment, these outside effects were kept out long enough to preserve quantum states and allow researchers to bond the diamonds at distances of up to 15 centimeters. This is one step in the growing quantum strangeness to get to a point where it is easier to examine and understand.

Max Planck’s idea in 1900 began a journey from the ordinary world to the submicroscopic world. Although we do not fully understand quantum theory yet, it illuminates this world and advances technology. With results like those of the diamond experiment, we continue the journey bringing the submicroscopic universe to the world we occupy. Planck, Einstein and Bohr would be completely fascinated today.

Sidney Perkowitz, the author of this article, is Emeritus Professor of Physics at Emory University. Some of his books are Slow Light and Hollywood Chemistry .



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