What Has Quantum Mechanics Ever Done For Us?

The best Applications for Quantum Computing technology /  In a different corner of the social media universe, someone left comments on a...

The best Applications for Quantum Computing technology / 

In a different corner of the social media universe, someone left comments on a link to Tuesday's post about quantum randomness declaring that they weren't aware of any practical applications of quantum physics. There's a kind of Life of Brian absurdity to posting this on the Internet, which is a giant world-spanning, life-changing practical application of quantum mechanics. But just to make things a little clearer, here's a quick look at some of the myriad everyday things that depend on quantum physics for their operation.

Computers and Smartphones

At the bottom, the entire computer industry is built on quantum mechanics. Modern semiconductor-based electronics rely on the band structure of solid objects. This is fundamentally a quantum phenomenon, depending on the wave nature of electrons, and because we understand that wave nature, we can manipulate the electrical properties of silicon.

Intel Corp. CEO Paul Otellini show off chips on a wafer built on so-called 22-nanometer technology at the Intel Developers' Forum in San Francisco, Tuesday, Sept. 22, 2009. Those chips are still being developed in Intel's factories and won't go into production until 2011. Each chip on the silicon "wafer" Otellini showed off has 2.9 billion transistors. (AP Photo/Paul Sakuma)

Mixing in just a tiny fraction of the right other elements changes the band structure and thus the conductivity; we know exactly what to add and how much to use thanks to our detailed understanding of the quantum nature of matter.

Stacking up layers of silicon doped with different elements allows us to make transistors on the nanometer scale. Millions of these packed together in a single block of material make the computer chips that power all the technological gadgets that are so central to modern life. Desktops, laptops, tablets, smartphones, even small household appliances and kids' toys are driven by computer chips that simply would not be possible to make without our modern understanding of quantum physics.

Lasers and Telecommunications

Unless my grumpy correspondent was posting from the exact server hosting the comment files (which would be really creepy), odds are very good that comment took a path to me that also relies on quantum physics, specifically fiber optic telecommunications. The fibers themselves are pretty classical, but the light sources used to send messages down the fiber optic cables are lasers, which are quantum devices.

Green LED lights and rows of fibre optic cables are seen feeding into a computer server inside a comms room at an office in London, U.K., on Tuesday, Dec. 23, 2014. Vodafone Group Plc will ask telecommunications regulator Ofcom to guarantee that U.K. wireless carriers, which rely on BT's fiber network to transmit voice and data traffic across the country, are treated fairly when BT sets prices and connects their broadcasting towers. Photographer: Simon Dawson/Bloomberg

The key physics of the laser is contained in a 1917 paper Einstein wrote on the statistics of photons (though the term "photon" was coined later) and their interaction with atoms. This introduces the idea of stimulated emission, where an atom in a high-energy state encountering a photon of the right wavelength is induced to emit a second photon identical to the first.

This process is responsible for two of the letters in the word "laser," originally an acronym for "Light Amplification by Stimulated Emission of Radiation." Any time you use a laser, whether indirectly by making a phone call, directly by scanning a UPC label on your groceries, or frivolously to torment a cat, you're making practical use of quantum physics.

Atomic Clocks and GPS

One of the most common uses of Internet-connected smartphones is to find directions to unfamiliar places, another application that is critically dependent on quantum physics. Smartphone navigation is enabled by the Global Positioning System, a network of satellites each broadcasting the time. The GPS receiver in your phone picks up the signal from multiple clocks, and uses the different arrival times from different satellites to determine your distance from each of those satellites.

TO GO WITH AN AFP STORY BY ISABELLE TOUSSAINT A woman holds her smartphone next to her dog wearing a GPS system on its collar in La Celle-Saint-Cloud on July 1, 2015. The Global Positioning System (GPS) collar help owners to track their pets remotely. AFP PHOTO / MIGUEL MEDINA (Photo credit should read MIGUEL MEDINA/AFP/Getty Images)

The computer inside the receiver then does a bit of math to figure out the single point on the surface of the Earth that is that distance from those satellites, and locates you to within a few meters.
This trilateration relies on the constant speed of light to convert time to distance. Light moves at about a foot per nanosecond, so the timing accuracy of the satellite signals needs to be really good, so each satellite in the GPS constellation contains an ensemble of atomic clocks.

These rely on quantum mechanics-- the "ticking" of the clock is the oscillation of microwaves driving a transition between two particular quantum states in a cesium atom (or rubidium, in some of the clocks). Any time you use your phone to get you from point A to point B, the trip is made possible by quantum physics.

Magnetic Resonance Imaging

The transition used for atomic clocks is a "hyperfine" transition, which comes from a small energy shift depending on how the spin of an electron is oriented relative to the spin of the nucleus of the atom. Those spins are an intrinsically quantum phenomenon (actually, it comes in only when you include special relativity with quantum mechanics), causing the electrons, protons, and neutrons making up ordinary matter behave like tiny magnets.

Leila Wehbe, a Ph.D. student at Carnegie Mellon University in Pittsburgh, talks about an experiment that used brain scans made in this brain-scanning MRI machine on campus, Wednesday, Nov. 26, 2014. Volunteers where scanned as each word of a chapter of "Harry Potter and the Sorcerer's Stone" was flashed for half a second onto a screen inside the machine. Images showing combinations of data and graphics were collected. (AP Photo/Keith Srakocic)

This spin is responsible for the fourth and final practical application of quantum physics that I'll talk about today, namely Magnetic Resonance Imaging (MRI). The central process in an MRI machine is called Nuclear Magnetic Resonance (but "nuclear" is a scary word, so it's avoided for a consumer medical process), and works by flipping the spins in the nuclei of hydrogen atoms. A clever arrangement of magnetic fields lets doctors measure the concentration of hydrogen appearing in different parts of the body, which in turn distinguishes between a lot of softer tissues that don't show up well in traditional x-rays.

So any time you, a loved one, or your favorite professional athlete undergoes an MRI scan, you have quantum physics to thank for their diagnosis and hopefully successful recovery. So, while it may sometimes seem like quantum physics is arcane and remote from everyday experience (a self-inflicted problem for physicists, to some degree, as we often over-emphasize the weirder aspects when talking about quantum mechanics), in fact it is absolutely essential to modern life.

Semiconductor electronics, lasers, atomic clocks, and magnetic resonance scanners all fundamentally depend on our understanding of the quantum nature of light and matter. But, you know, other than computers, smartphones, the Internet, GPS, and MRI, what has quantum physics ever done for us?

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