Nobody understands quantum mechanics,” Richard Feynman famously said. Long after Max Planck’s discovery in 1900 that energy comes in separate packets or quanta, quantum physics remains enigmatic. It is vastly different from how things work at bigger scales, where objects from baseballs to automobiles follow Newton’s laws of mechanics and gravitation, consistent with our own bodily experiences. But at the quantum level, an electron is a particle and a wave, and light is a wave and a particle (wave-particle duality); an electron in an atom takes on only certain energies (energy quantization); electrons or photons can instantaneously affect each other over arbitrary distances (entanglement and teleportation); a quantum object exists in different states until it is measured (superposition, or popularly, Schrödinger’s cat); and a real physical force emerges from the apparent nothingness of vacuum (the Casimir effect).
For a theory that nobody understands, quantum physics has changed human society in remarkable ways.1 It lies behind the digital technology of integrated circuit chips, and the new technology of light-emitting diodes moving us toward a greener world. Scientists are now excited by one of the more elusive notions in quantum physics, the idea of ephemeral “virtual” photons, which could make possible non-invasive medical methods to diagnose the heart and brain. These connections illustrate the flow of ideas from scientific abstraction to useful application. But there is also a counter flow, where pragmatic requirements generate deep insight. The universal laws of thermodynamics have roots in efforts by 19th-century French engineer Sadi Carnot to make the leading technology of the time, the steam engine, more efficient. Similarly, the growth of quantum technology leads to deeper knowledge of the quantum. The interplay between pure theory, and its outcomes in the everyday world, is a continuing feature of science as it develops. In quantum physics, this interaction traces back to one of its founders, Danish physicist Niels Bohr.
In 1913, Bohr applied quantum ideas to the simplest atom, hydrogen. He found that the lone electron could occupy only certain orbits around the central proton, each with a specific energy, but not the spaces or energies in between. The electron jumps to other orbits by absorbing energy, or by emitting it as a photon at a wavelength set by the orbital energies involved. Bohr’s model scored a huge success when it predicted the exact wavelengths of light emitted by energized hydrogen gas. Quantum jumps across the gaps between energy levels also produce the vividly colored light from advertising signs filled with neon and other gases, from lasers, and from LEDs.
Energy gaps are also at the heart of digital electronics and computing, which depend on the semiconducting transistor, invented in 1947. Semiconductors lie between metals, with many free electrons that carry electrical current, and insulators, whose electrons are held within its atoms and cannot form a current. The electrons in a semiconductor are also confined to its atoms, but once they jump across a so-called band gap, they can travel freely to form an electrical current. This current can be precisely manipulated to turn on and off as a switch, amplify signals, and perform other electronic functions when the semiconductor is formed into a transistor. Produced in millions within integrated circuit chips made of semiconducting silicon, transistors control the digital technology that defines our world. The band gap underlying all this is a pure quantum effect that arises because the electrons in a semiconductor occupy bands of energy separated by forbidden regions.
The fact that energy is discontinuous at small scales is at odds with our view of the ordinary world, but now we take this aspect of nature for granted. However, superposition, entanglement, and teleportation raise deeper questions about quantum theory, yet also inspire new technology.
Superposition is tied to Erwin Schrödinger’s famous equation, published in 1926. It is the quantum equivalent of Newton’s equation F = ma (that is, force = mass x acceleration, the basic equation of particle motion) but it describes a sub-atomic entity such as an electron as a wave, not a particle (although without telling us what this wave is). However, the equation’s solution, called the wavefunction, can be used to calculate any property of the entity such as its position, but not definitively. The wavefunction only gives the probability that the electron can exist at a given position in an atom. In principle each location is possible according to its probability until an electron is measured, when the wavefunction is said to “collapse” to that value.
One promising idea is to use this technology to examine the human heart and brain without physical contact.
This view of quantum behavior is called the Copenhagen interpretation because of Bohr’s role in developing it. It is like saying a deck of cards contains 52 different states that a selected card could take, in this case, with equal probabilities; but once you pick a card, that becomes the actual state, leaving the other 51 irrelevant. But the analogy is imperfect: We firmly believe that the suit and value printed on each card are real and fixed whether someone picks a card or not. In classical physics too we assume that objects have definite properties even if they are not being measured, a belief called realism. But the Copenhagen view makes it questionable whether an electron or photon has definite values independent of a measurement.
If, reading this, you are wondering how and why the act of measurement somehow deeply affects the nature of the thing measured, you are not alone. This is one of the enduring questions that Einstein, among other physicists, and philosophers too, have asked but not yet answered, leaving the full meaning of quantum theory unsettled. The Copenhagen view is widely used, but other interpretations attempt to resolve the issues it raises.
Regardless, superposition enables a novel approach to computation. For example: A photon has an electric field that can be polarized to point in a given direction. This can be arranged so that under superposition, the photon has a 50 percent probability of pointing either vertically or horizontally, representing binary “1” or “0” respectively. The result is a quantum computer bit (a “qubit”) that is both 1 and 0 until it is measured. An ordinary computer bit is only either 1 or 0, so using qubits enhances computing capacity by a factor of 2n where n is the number of qubits; for instance, four ordinary bits can hold only one of the 16 binary numbers 0000 to 1111 (decimal 0 to 15) but four qubits hold all 16 simultaneously. By handling many pieces of data in parallel, a quantum computer takes computational power to unprecedented levels.
Besides photons, qubits can be based on quantum objects and systems such as electrons,2 ions, and superconductors. Researchers are now testing these approaches to find the best one for commercial quantum computation. IBM projects a 1,000-qubit superconducting chip in two years.3 Even a smaller 11-qubit computer, one based on ion qubits, can in principle juggle 2,048 numbers at once.4
Entanglement provides other new ways to handle data; but when Schrödinger invented the term in 1935 he was thinking about entanglement as “not one but apparently the characteristic trait of quantum mechanics.” This defining feature can be illustrated with a pair of electrons. Due to the property called spin, electrons are like tiny magnets with a north pole that points either up or down. It is possible to create a pair of electrons with total spin zero, with one north pole up and the other down, but we don’t know which is which. Now separate the two electrons as far apart as you like and measure the spin direction of electron A; then whatever the result, a measurement of the spin direction of electron B always gives the opposite value.
This is entanglement, where a measurement of a property of one of two linked quantum objects instantaneously sets the value of that property in the other, regardless of distance. Numerous experiments confirm that quantum objects are correlated in ways that ordinary objects are not, even if far apart. In 2017, a group under Jian-Wei Pan of the University of Science and Technology of China showed that a pair of photons remained entangled over a record distance of 1,200 kilometers. Experiments also show that any interaction between entangled objects occurs more quickly than the speed of light would allow. This violates the condition called locality that arises from special relativity, which is what troubled Einstein when he called entanglement “spooky action at a distance.” With quantum realism already dubious, many physicists are tending to the view that both realism and locality do not apply in the quantum world.
It’s like saying that once you pick a card, that becomes the actual state, leaving the other 51 irrelevant.
Nevertheless, through its role in teleportation, entanglement enables quantum telecommunication, even at a global scale. In 1993, Charles Bennet of IBM and colleagues theorized about how to exactly copy and send the unknown state of a quantum system to a distant receiver, that is, teleport quantum information. This was revolutionary because in quantum theory, one cannot exactly clone, say, a photon with an unknown polarization, since making copies would provide a way to evade the uncertainty principle. But entanglement provides another workaround: If you measure the polarization of an entangled photon, you know the polarization of its partner without a direct measurement. The Bennett paper proposed using entangled photons A and B, located respectively with the sender and the recipient, to transmit the unknown state of a third photon X to photon B.
In 1997 Anton Zeilinger, then at the University of Innsbruck, and colleagues successfully teleported the unknown polarization state of a photon. Such experiments have opened the door to distributing data in the form of polarized photon qubits, with a huge bonus: The quantum nature of teleportation makes the transfer of confidential information secure against tampering or eavesdropping. This is not merely an issue in spy novels: Security is essential in a host of electronic transactions that support internet commerce and financial transfers and carry private information.
For security, these transmissions are encrypted, then decoded by the recipient with a secret key sent via a separate secure channel, which is where the method is vulnerable. In a quantum system, however, the key is a random string of qubits, which is tamper-proof: If a third party reads or alters the key, that amounts to a quantum measurement, changing the key in a way that the sender and recipient can detect. With this feature, quantum teleportation raises the possibility of an utterly secure worldwide network implemented through space satellite transmissions over large distances. The 1,200 km transmission of entangled photons was carried out between a space satellite and ground stations.5 It is a first step toward a global quantum internet.
The quantum effects that translate into technology are anti-intuitive and hard to visualize, but one recent development brings us back to the familiar world of Newtonian mechanics with an actual physical force—except that it arises from the quantum vacuum, which unlike the classical idea of vacuum, is not empty. Instead, in the extension of quantum mechanics called quantum field theory, the vacuum of space is the lowest energy state of the universe and supports “virtual” elementary particles that briefly pop into existence. These include virtual photons with varied wavelengths. In 1948, the Dutch theorist Hendrik Casimir predicted that two barely separated parallel metal plates placed in this quantum vacuum would be attracted to each other. This happens because only waves that fit exactly into the gap between the plates survive there, producing a lower energy density than exists outside the plates and hence an inward force. This force is extremely small, and so is the necessary spacing between the plates, around 100 nanometers.
Apart from its usefulness, quantum technology shows that applied quantum physics can broaden fundamental understanding.
Finally in 1997 delicate measurements quantitatively confirmed Casimir’s prediction, and other results show that the effect also occurs for non-metals in different geometries. Since then, researchers in micro- and nano-electromechanical systems—chip-size devices that combine electrical and mechanical functions—have begun exploiting the Casimir force. Its sensitivity to small distances allows ultra-precise measurements by tracking mechanical movements. One promising idea is to use this technology to examine the human heart and brain without physical contact, by measuring the extremely small magnetic fields they generate. At present this is done with bulky equipment that needs cryogenic cooling. An electromechanical chip would instead use the Casimir effect to analyze a heart or brain right in a physician’s office at room temperature.
Apart from its usefulness, quantum technology shows that applied quantum physics can broaden fundamental understanding. Ingenious quantum scientists showed that teleportation can make an end run around the uncertainty principle and achieve a real-world result, secure telecommunications. In this case, researchers learned how to manipulate quantum effects to evade one basic principle; but also appreciate that the secure nature of teleported keys made up of qubits arises from other basic quantum properties, randomness, and sensitivity to measurement.
But will we ever fully understand why quantum theory works so well? Feynman’s graduate adviser, the distinguished theorist John Archibald Wheeler, did not have the answer but clearly stated the goal when he wrote in 1984 that “the most revolutionary discovery in science is yet to come! … not by questioning the quantum, but by uncovering that utterly simple idea that demands the quantum.”
Sidney Perkowitz is Candler Professor of Physics Emeritus, Emory University. His latest books are Physics: A Very Short Introduction and Science Sketches (forthcoming, 2022).
1. Adesso, G., Lo Franco, R., & Parigi, V. Foundations of quantum mechanics and their impact on contemporary society. Philosophical Transactions of the Royal Society A 376, 20180112 (2018).
2. Valich, L. One small step for electrons, one giant leap for quantum computers. www.rochester.edu (2019).
3. Gambetta, J. IBM’s roadmap for scaling quantum technology. www.research.ibm.com (2020).
4. Wright, K., et al. Benchmarking an 11-qubit computer. Nature Communications 10, 5464 (2019).
5. Popkin, G. China’s quantum satellite achieves “spooky action” at record distance. Science (2017).
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