Quantum computing in the 21st Century – with David Jamieson

The Apollo missions represented the dawn of the computer age rather than the space age, because for the first time humans had built machines that were too complicated for a human being to control, and the lunar module, Command Module, and Saturn V rocket were all controlled by advanced computer technology, with the lunar lander computer containing 2800 silicon chips each with six transistors and 70 kilobytes of core memory.

“it wasn't the dawn of the Space Age it was the dawn of the computer age because For the First Time humans had built machines which were too complicated for a human being to control and so the lunar module and the Command Module and the Saturn V rocket were all controlled by some Advanced for the day computer technology and in indeed the computer on the lunar lander here had a massive 2800 silicon chips each chip was crammed with six transistors and there was a whopping 70 kilobytes of core memory”

Schrodinger's equation, which describes quantum mechanical wave functions for particles in different energy states, is not a mysterious concept but rather a translation of classical ideas—kinetic energy and potential energy—into functions that can describe things in multiple places at once.

“Schrodinger's equation there's nothing mysterious about it it's simply what we're already familiar in the classical world that the energy of a particle such as this cricket ball propagating through space or this Moon orbiting that planet is just given by the sum of the energy due to motion the kinetic energy plus the energy due to any interactions between the two objects such as gravity or electrostatic forces but for quantum mechanics we need to be able to describe things that are in many places at the same time a wave and so we just translate these classical ideas the kinetic energy and the potential energy into functions”

Modern Mars rovers like the Curiosity Rover experience upsets in computer memory when energetic protons from space radiation pass through the electronics, changing bits from zero to one or one to zero, requiring hardware and software to guard against these random perturbations from outside to maintain normal operation.

“the Martian atmosphere is very thin and radiation from space is able to reach the surface and that radiation consists of energetic ions protons the height nucleus of a hydrogen atom which is able to make it down through the thin atmosphere and pass through the electronics on the Rover and this inset here shows upsets in the memory of the computer every time a proton goes zapping through it and maybe changes a bit from a zero to a one or a one from to zero causes other upsets and the hardware and the software has to guard against those random perturbations from outside”

A phosphorus atom naturally embeds beautifully into a silicon crystal lattice and possesses a nuclear spin inside the phosphorus nucleus as well as a single electron orbiting around it which also has spin that can be harnessed for technological applications.

“this is a phosphorus atom that goes into a silicon crystal beautifully and we can see it's got a nuclear spin inside the phosphorus nucleus and there's a single electron orbiting around it which also has a spin which can be harnessed for technological applications”

Another group in Austria performed the double-slit experiment with a buckyball (60 carbon atoms) as the particle, finding the molecule can be in two or three places at the same time and create a Young's two-slit interference pattern, demonstrating that wave-particle duality extends beyond single electrons to complex molecules.

“the more a group in Austria has done the same experiment with a bucky ball 60 carbon atoms in this giant molecule fired at a grating and the molecule can be in two or three places at the same time and make a Young's two-slit interference pattern I find that very spooky but a wonderful aspect of the real world”

Removing silicon-29 isotopes and replacing them with silicon-28 (the zero-spin isotope) creates something like a vacuum with nothing going on except what has been engineered into it with phosphorus atoms, making it the next best thing to a real vacuum for doing quantum mechanics experiments.

“so that's like having a little bar magnet inside our chip which is interacting with our phosphorus atoms and destroying the quantum coherence so step one is to remove the Silicon 29s and replace them with silicon 28 the zero spin isotope and this is like a vacuum with nothing going on except what we've engineered into it with our phosphorus atom it's the next best thing to a real vacuum for doing quantum mechanics”

Standard semiconductor manufacturing techniques including iron implantation involve 15-20 different implantation steps to create silicon chips. After implantation, the crystal is heated to push the implanted phosphorus atoms onto crystal lattice sites, but this process is stochastic with atoms landing at random positions, not suitable for large-scale quantum devices.

“we implant phosphorus atoms into it using the standard techniques of the semiconductor industry there's probably 15 or 20 different Iron implantation steps that have gone to make the Silicon chip in here then we heat up the Crystal and we push the implanted phosphorus atoms onto the crystal lattice site but the way that's done in industry it's like raindrops on a window the raindrops fall at random points on the window this is not good enough for making a large-scale device”

The ability to build large-scale devices from tiny tiny components using imperfect manufacturing techniques while still having them work reliably is a key concept demonstrated by USB drives containing onboard error correction algorithms that detect and wall off failing memory elements.

“the key takeaway from this technology is that it's possible to build large-scale devices from tiny tiny components with imperfect techniques and still have them work just like the Rover on Mars is imperfect because the radiation from space is constantly causing upsets in the memory so you have on boards on onboard algorithms to detect and correct those those errors”

A quantum bit (qubit) differs from a classical bit by existing as a superposition of being in two places at the same time, where the qubit can be in the sum of the zero state and the one state simultaneously, which determines its orientation in space. However, as soon as you observe the qubit, it instantly decoheres into its classical values of zero or one.

“a Quantum bit exists as a superposition of being in two places at the same time like Einstein's Photon being a wave and it can be in a the sum of the zero State and the one state which determines its orientation in space there's only one catch you can exploit this remarkable uh uh large versatility providing you don't observe the qubit because as soon as you observe it it'll instantly Deco here just like the photon changes from a wave to a particle when it interacts with just one electron into its classical values of zero or one”

Paul Dirac observed in 1928 that the underlying physical laws necessary for the mathematical theory of Schrödinger's equation for a large part of physics and the whole of chemistry are completely known, but the difficulty is that the exact application of these laws leads to equations much too complicated to be soluble by available mathematical methods.

“but in 1928 he made the somewhat depressing observation the underlying physical laws necessary for the mathematical Theory Schrodinger's equation of a large part of physics and the whole of chemistry athas completely known I don't know if the chemists would have agreed but that's the physicist View and the difficulty is only that the exact application of these laws leads to equations much more much too complicated to be soluble”

In 1959, Richard Feynman gave a remarkable lecture titled 'There's Plenty of Room at the Bottom' proposing the idea of building a computer employing quantum spins as bits instead of classical bits, which would allow using a quantum system to model another quantum system—an idea with no technology to implement it in 1959.

“but in 1959 Feynman gave this remarkable lecture there's plenty of room at the bottom where he put forward this idea for a new type of calculation not just using classical circuits but a system involving the quantized energy level or the interaction of quantized spins in other words to build a computer that employs Quantum spins as its bits instead of classical bits that would allow you to use a Quantum system to model another quantum system but there was no technology in 1959 to do anything with that idea”

Richard Feynman documented in his famous textbook that when you take an electron gun emitting particles of matter directed at a wall with two slits and measure electron positions at a detector, you will get a Young's two-slit interference pattern rather than a classical shadow of the two slits, though he noted this experiment had never been done when the lectures were published because there was no technology for detecting single electrons at the time.

“in his uh famous textbook he uh described the way Mata behaves using Young's to Slit interference experiment he said you can take an electron gun emitting particles of matter electrons you can direct them to a wall that contains two slits and over here on the backstop there's a detector for measuring the position of all the electrons that pass from the electron gun through the barrier to the detector and when you do this he said you will end up with a Young's two-slit interference pattern not the classical shadow of these two slits but he comments this experiment has never been done in this way because when these lectures were published in the early 60s there was no technology for detecting single electrons at a time”

A spinning top balances due to the velocity vector constantly going around in circles at the rim of the top, which causes the top to process around the gravitational field direction (vertical), and as friction slows the spinning, the procession becomes more extreme until gravity finally causes the top to fall over.

“you can see in the gravitational field of the earth gravity is trying to tip that over and for reasons to do with the velocity Vector constantly going around in circles of the um rim of the top it processes around the gravitational field Direction which is vertical and as it slows down due to friction with the table that procession or that wobble becomes more and more extreme but this is oh until finally it succumbs to gravity”

The Stern-Gerlach experiment of 1922 found that when silver atoms (whose spin came entirely from one unpaired electron in the outer shell) passed through a strong inhomogeneous magnetic field, the beam split into exactly two lines, demonstrating that spin angular momentum is quantized.

“when you boil silver atoms in a vacuum out of a furnace and pass them through a very strong magnetic field you find that the beam of silver atoms splits in two... when this experiment was done first of all this is the result of a beam of silicon atoms hitting this postcard without the magnetic field you just see a line of silver atoms piling into the card then when you turn on the magnetic field you find the line splits in two”

Einstein's 1905 paper on the photoelectric effect proposed that when a light ray spreads from a point, the energy is not distributed continuously over ever-increasing space but consists of a finite number of energy quanta that are localized in points in space, move without dividing, and can be absorbed or generated only as a whole—a sentence widely attributed as the most revolutionary sentence written by a physicist in the 20th century.

“Einstein wrote in the introduction to one of his 1905 papers according to the Assumption to be contemplated here in his paper when a light Ray is spreading from a point the energy is not spread distributed continuously over ever increasing spaces but consists of a finite number of energy quanta that are localized in points in space move without dividing and can be absorbed or generated only as a whole and this sentence in Einstein's 1905 paper is widely attributed to be the most revolutionary sentence written by a physicist in the 20th century”

In 1998, Bruce Kane published a single-author paper in Nature proposing a silicon-based nuclear spin quantum computer based on phosphorus atoms embedded in a silicon crystal lattice, which has been cited thousands of times and triggered a reappraisal of the idea of qubits in engineered devices.

“in 1998 young postdoc at the University of New South Wales Bruce Kane published this single author paper in nature the journal a silicon-based nuclear spin quantum computer based on phosphorus atoms embedded in a silicon crystal lattice this paper has been cited thousands of times and triggered a reappraisal of the idea of qubits in an engineered device”

The caffeine molecule has approximately 100 spinning electrons orbiting the carbon, hydrogen, and oxygen atoms that comprise it, and the number of electron-electron interactions across those 100 outer shell electrons is governed by Schrödinger's equation, but there are so many interactions that even the world's best supercomputers are powerless to calculate the structure of the caffeine molecule.

“the caffeine molecule what a wonderful molecule it docks with all the physics receptors in the brain and makes them go around faster and you get new ideas and you it's great to talk about quantum mechanics and spin and directs depressing observation but the caffeine molecule has got 100 of spinning electrons orbiting the carbon the hydrogen and the oxygen that make up the molecule that's a little electron I made as well which can only have two orientations in space um and the number of electron electron interactions across those 100 outer shell electrons in the molecule are governed by Schrodinger's equation but there are so many interactions that even the world's best supercomputer is powerless to calculate the structure of the caffeine molecule it's just too complicated for a classical computer to solve”