How to subdue the quantum nature of matter?

Matt Trusheim includes a switch in a dark laboratory, and a powerful green laser highlights a tiny diamond held in place under the microscope objective. An image appears on the computer screen, a diffuse gas cloud dotted with bright green dots. These luminous points are tiny defects inside the diamond, in which two carbon atoms are replaced by one tin atom. The light of the laser, passing through them, passes from one shade of green to another.

Later this diamond will be cooled to the temperature of liquid helium. Controlling the crystal structure of a diamond atom by atom, bringing it up to several degrees above absolute zero and applying a magnetic field, researchers from the Laboratory of Quantum Photonics under the guidance of physicist Dirk Englund at the Massachusetts Institute of Technology think that they can choose with such precision the quantum mechanical properties of photons and electrons , that they will be able to pass unbreakable secret codes.

Trusham is one of many scientists who are trying to find out which atoms are enclosed in crystals, under what conditions they will be able to gain control of this level. In fact, scientists around the world are trying to learn to control nature at the level of atoms and below, to electrons or even to the fraction of an electron. Their goal is to find the nodes that control the fundamental properties of matter and energy, and tighten or untangle these nodes, changing matter and energy, create super-powerful quantum computers or superconductors operating at room temperature.

These scientists face two major problems. At the technical level, it is very difficult to carry out such work. Some crystals, for example, should be at 99.99999999% clean in vacuum chambers cleaner than space. An even more fundamental problem is that the quantum effects that scientists want to curb, for example, the ability of a particle to be in two states simultaneously, like the Schrodinger cat, appear at the level of individual electrons. In the macrocosm, this magic is crumbling. Consequently, scientists have to manipulate matter on the smallest scale, and they are limited to the limits of fundamental physics. On their success depends on how our understanding of science and technological capabilities will change in the coming decades.
 
The Alchemist’s Dream

Manipulation of matter, to a certain extent, consists in controlling electrons. In the end, the behavior of electrons in matter determines its properties as a whole – this substance will be a metal, a conductor, a magnet or something else. Some scientists are trying to change the collective behavior of electrons, creating a quantum synthetic substance. Scientists see how “we take an insulator and turn it into a metal or semiconductor, and then into a superconductor. We can turn a non-magnetic material into a magnetic material, “says physicist Eva Andrew from Rutgers University. “This is the fulfillment of the dream of an alchemist.”

And this dream can lead to real breakthroughs. For example, scientists have tried for decades to create superconductors that work at room temperature. With the help of these materials, it would be possible to create power lines that do not lose energy. In 1957, physicists John Bardeen, Leon Cooper and John Robert Schrieffer demonstrated that superconductivity appears when free electrons in a metal like aluminum are aligned in so-called Cooper pairs. Even being relatively far away, each electron corresponded to another, possessing the opposite spin and momentum. Like couples dancing in a crowd at a disco, paired electrons move in coordination with others, even if other electrons pass between them.

This equalization allows the current to flow through the material, without encountering resistance, and hence without loss. The most practical superconductors developed to the present moment should be at a temperature just above absolute zero, so that this state persists. However, there can be exceptions.

Recently, researchers have found that firing a material with a high-intensity laser can also knock down electrons in Cooper pairs, albeit for a short while. Andrea Cavalleri of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, and his colleagues discovered signs of photoinduced superconductivity in metals and insulators. Light, hitting the material, causes the atoms to vibrate, and the electrons briefly enter a state of superconductivity. “The shake-up should be fierce,” says David Essey, a condensed matter physicist at the California Institute of Technology, who uses the same laser technique to display unusual quantum effects in other materials. “For an instant the electric field becomes very strong – but only for a short time.”
 
Non-breaking codes

Electron management – that’s how Trusheim and Englund intend to develop non-corrupt quantum encryption. In their case, the goal is not to change the properties of materials, but to transfer the quantum properties of electrons in designer diamonds to photons that transmit cryptographic keys. In the diamond color centers in the Englund laboratory there are free electrons, the spins of which can be measured with a strong magnetic field. The spin that is aligned with the field can be called spin 1, the spin that is not aligned with spin 2, which is equivalent to 1 and 0 in the digital bit. “It’s a quantum particle, so it can be in both states at the same time,” says Englund. A quantum bit, or qubit, is capable of performing many calculations simultaneously.

It is here that a mysterious property is born – quantum entanglement. Imagine a box containing red and blue balls. You can take one without looking and put in your pocket, and then leave for another city. Then take the ball out of the pocket and find that it is red. You immediately realize that the box was a blue ball. This is confusion. In the quantum world, this effect allows you to transmit information instantly and over long distances.

The color centers in the diamond in Englund’s laboratory transmit the quantum states of the electrons enclosed in them, the photons through confusion, creating “flying qubits”, as Englund calls them. In ordinary optical communications, a photon can be transmitted to a recipient – in this case another vacant void in a diamond – and its quantum state will be transferred to a new electron, so the two electrons will be connected. The transmission of such confusing bits will allow two people to share a cryptographic key. “Everyone has a string of zeros and ones, or upper and lower spins that seem completely random, but they are identical,” says Englund. Using this key to encrypt the transmitted data, you can make them absolutely secure. If someone wants to intercept the transmission, the sender will know about it, because the act of measuring the quantum state will change it.

Englund is experimenting with a quantum network that sends photons over the fiber through his lab, an object down the road at Harvard University and another laboratory at the Massachusetts Institute of Technology in the nearby city of Lexington. Scientists have already succeeded in the transfer of quantum cryptographic keys over long distances – in 2017, Chinese scientists reported that they handed over such a key from a satellite in Earth’s orbit to two ground stations in 1200 kilometers from each other on the mountains of Tibet. But the bitrate of the Chinese experiment was too low for practical communications: scientists recorded only one confusing pair of six million. The innovation that makes cryptographic quantum networks on the earth practical is quantum repeaters, devices placed at intervals in the network that amplify the signal without changing its quantum properties. The purpose of Englund is to find materials with suitable atomic defects so that they can be used to create these quantum repeaters.

The trick is to create enough entangled photons for data transfer. The electron in the nitrogen-substituted vacancy maintains its spin long enough – about a second – which increases the chances that the laser light will pass through it and produce an intricate photon. But the nitrogen atom is small and does not fill the space created by the lack of carbon. Therefore, successive photons can be slightly different colors, and therefore lose their match. Other atoms, tin, for example, fit tightly and create a stable wavelength. But they can not hold the spin long enough – therefore, work is underway to find the ideal balance.
 
Scattered ends

While Englund and others are trying to cope with individual electrons, others dive even deeper into the quantum world and are trying to manipulate already the shares of electrons. This work goes back to the 1982 experiment, when scientists from the Bell Lab and Lawrence Livermore National Laboratory made a sandwich of two layers of different semiconductor crystals, cooled them to almost absolute zero and applied a strong magnetic field to them, pinching the electrons in the plane between the two layers of crystals . Thus, a kind of quantum broth was formed in which the motion of any individual electron was determined by the charges that it felt from other electrons. “These are not separate particles by themselves,” says Michael Manfra of Purdue University. “Imagine a ballet in which each dancer not only makes his own pas, but also responds to the movement of a partner or other dancers. This is in a way a common response. ”

Strange in all this is that this collection can have fractional charges. Electron is an indivisible unit, it can not be divided into three parts, but a group of electrons in the right state can produce a so-called quasiparticle with 1/3 charge. “As if the electrons are divided into parts,” says Mohammed Hafezi, a physicist at the Joint Quantum Institute. “It is very strange”. Hafezi created this effect in super-cold graphene, a monatomic carbon layer, and has recently shown that it can manipulate the motion of quasiparticles by highlighting graphene by a laser. “Now it’s controlled,” he says. “External nodules, such as a magnetic field and light, can be controlled, pulled up or disbanded. The nature of collective changes is changing. ”

Manipulations with quasiparticles allow us to create a special type of qubit – a topological qubit. Topology is a field of mathematics that studies the properties of an object that do not change, even if this object is twisted or deformed. A standard example is a donut: if it was perfectly elastic, it could be transformed into a coffee cup, without changing anything particularly; The hole in the donut will play a new role in the hole in the cup handle. However, in order to turn a donut into a pretzel, you will have to add new holes to it, changing its topology.

The topological qubit retains its properties even under changing conditions. Usually, particles change their quantum states, or “decode” when something is disturbed in their environment, like small vibrations caused by heat. But if you make a qubit of two quasiparticles separated by some distance, say, at opposite ends of the nanowire, you essentially split the electron. Both “halves” will have to experience the same violation to decode, and this is unlikely to happen.

This property makes topological qubits attractive for quantum computers. Because of the ability of a qubit to be in a superposition of multiple states simultaneously, quantum computers must be able to produce almost impossible calculations without them, for example, to simulate the Big Bang. Manfra, in fact, is trying to create quantum computers from topological qubits in Microsoft. But there are also simpler approaches. Google and IBM, in fact, are trying to create quantum computers based on supercooled wires that become semiconductors, or ionized atoms in a vacuum chamber held by lasers. The problem with such approaches is that they are more sensitive to environmental changes than topological qubits, especially if the number of qubits increases.

Thus, topological qubits can lead to a revolution in our ability to manipulate tiny things. However, there is one significant problem: they do not exist yet. The researchers are struggling to create them from the so-called Majorana particles. Proposed by Ettore Majorana in 1937, this particle is itself an antiparticle. The electron and its antiparticle, the positron, have identical properties except the charge, but the charge of the Majorana particle will be zero.

Scientists believe that certain configurations of electrons and holes (the absence of electrons) can behave like Majorana particles. They, in turn, can be used as topological qubits. In 2012, physicist Leo Kouvenhoven from Delft University of Technology in the Netherlands and his colleagues measured what seemed to them Majorana particles in a network of superconducting and semiconductor nanowires. But the only way to prove the existence of these quasiparticles is to create a topological qubit based on them.

Other experts in this area are more optimistic. “I think that without any questions, someone will one day create a topological qubit, just for the sake of interest,” says Steve Simon, a condensed matter theorist at Oxford University. “The only question is whether we can make them a quantum computer of the future.”

Quantum computers – as well as high-temperature superconductors and unbreakable quantum encryption – may appear many years or never appear. But at the same time, scientists are trying to decipher the riddles of nature on the smallest scale. While no one knows how far to go. The deeper we penetrate into the smallest components of our universe, the more they push us out.