Matt Trushheim flips the switch in the dark lab, and a powerful green laser illuminates a tiny diamond held in place under a microscope objective. An image appears on the computer screen, a diffuse cloud of gas dotted with bright green dots. These glowing dots are tiny defects inside the diamond, in which two carbon atoms are replaced with one tin atom. Laser light passing through them passes from one shade of green to another.
Later, this diamond will be cooled to the temperature of liquid helium. By controlling the crystal structure of a diamond atom by atom, bringing it to a few degrees above absolute zero and applying a magnetic field, researchers at the Quantum Photonics Laboratory, led by physicist Dirk Englund at MIT, think they can select the quantum mechanical properties of photons and electrons with such precision. that they will be able to transfer unbreakable secret codes.
Trushheim is one of many scientists who are trying to figure out which atoms, enclosed in crystals, under what conditions will allow them to gain control of this level. In fact, scientists around the world are trying to learn how to control nature at the level of atoms and below, to electrons or even a fraction of an electron. Their goal is to find the knots that control the fundamental properties of matter and energy, and tighten or untangle these knots by changing matter and energy, to create super-powerful quantum computers or superconductors that work at room temperature.
These scientists face two major challenges. At the technical level, it is very difficult to carry out such work. Some crystals, for example, must be 99.99999999% pure in vacuum chambers cleaner than space. An even more fundamental challenge is that the quantum effects that scientists want to curb - for example, the ability of a particle to be in two states at the same time, like Schrödinger's cat - appear at the level of individual electrons. In the macrocosm, this magic collapses. Consequently, scientists have to manipulate matter on the smallest scale, and they are limited by the limits of fundamental physics. Their success will determine how our understanding of science and technological capabilities will change in the coming decades.
Alchemist's dream
Manipulating matter, to a certain extent, consists of manipulating electrons. In the end, the behavior of electrons in a substance 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 by creating a quantum synthetic substance. Scientists see how “we take an insulator and turn it into a metal or a semiconductor and then into a superconductor. We can turn a non-magnetic material into a magnetic one,”says physicist Eva Andrew of Rutgers University. "This is an alchemist's dream come true."
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 waste energy. In 1957, physicists John Bardeen, Leon Cooper and John Robert Schrieffer demonstrated that superconductivity occurs when free electrons in a metal like aluminum align into what are called Cooper pairs. Even being relatively far away, each electron corresponded to another, with 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.
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This alignment allows current to flow through the material without encountering resistance, and therefore losslessly. The most practical superconductors developed to date must be at temperatures just above absolute zero for this state to persist. However, there may be exceptions.
Recently, researchers have found that bombarding material with a high-intensity laser can also knock electrons into Cooper pairs, albeit briefly. Andrea Cavalleri of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, and his colleagues have found signs of photoinduced superconductivity in metals and insulators. The light striking the material causes the atoms to vibrate, and the electrons briefly enter a state of superconductivity. "The shake-up has to be fierce," says David Esie, a condensed matter physicist at California Institute of Technology, who uses the same laser technique to manifest unusual quantum effects in other materials. "For a moment, the electric field becomes very strong - but only for a short time."
Unbreakable codes
Controlling electrons is how Trushheim and Englund set out to develop unbreakable 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. The color centers of diamonds in Englund's laboratory contain free electrons, whose spins can be measured using a strong magnetic field. A spin that aligns with the field can be called spin 1, a spin that does not align is 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 at the same time.
This is where a mysterious property is born - quantum entanglement. Imagine a box containing red and blue balls. You can take one without looking and put it in your pocket, and then leave for another city. Then take the ball out of your pocket and find that it is red. You will immediately understand that there is a blue ball in the box. This is confusion. In the quantum world, this effect allows information to be transmitted instantly and over long distances.
The colored centers in the diamond at Englund's laboratory transmit the quantum states of the electrons they contain to photons through entanglement, creating "flying qubits," as Englund calls them. In conventional 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 are bound. Transmitting these obfuscated bits will allow two people to share the cryptographic key. “Each has a string of zeros and ones, or high and low spins, that seem completely random, but they are identical,” Englund says. By 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 a quantum state will change it.
Englund is experimenting with a quantum network that sends photons down optical fiber through his lab, an object down the road at Harvard University, and another MIT lab in the nearby city of Lexington. Scientists have already succeeded in transmitting quantum cryptographic keys over long distances - in 2017, Chinese scientists reported that they had transferred such a key from a satellite in Earth orbit to two ground stations 1200 kilometers apart in the mountains of Tibet. But the bitrate of the Chinese experiment was too low for practical communication: scientists only recorded one confusing pair in six million. An innovation that will make cryptographic quantum networks on earth practical are quantum repeaters, devices placed at intervals on the network that amplify the signal,without changing its quantum properties. Englund's goal is to find materials with suitable atomic defects so that these quantum repeaters can be created from them.
The trick is to create enough entangled photons to carry the data. An electron in a nitrogen-substituted vacancy maintains its spin long enough - about a second - which increases the chances that laser light will pass through it and produce an entangled photon. But the nitrogen atom is small and does not fill the space created by the absence of carbon. Therefore, successive photons can be of slightly different colors, which means they will lose their correspondence. Other atoms, tin, for example, adhere tightly and create a stable wavelength. But they will not be able to hold the spin long enough - therefore, work is underway to find the perfect balance.
Split ends
While Englund and others try to cope with individual electrons, others dive deeper into the quantum world and try to manipulate the fraction of electrons. This work is rooted in an experiment in 1982, when scientists at Bell Laboratories and Lawrence Livermore National Laboratories sandwiched two layers of different semiconductor crystals, cooled them to near absolute zero, and applied a strong magnetic field to them, trapping electrons in a plane between two layers of crystals. … Thus, a kind of quantum soup was formed in which the movement of any individual electron was determined by the charges that it felt from other electrons. “These are no longer individual particles in and of themselves,” says Michael Manfra of Purdue University. “Imagine a ballet in which each dancer not only does his own steps,but also reacts to the movement of a partner or other dancers. It's kind of a general answer."
The strange thing about all this is that such a collection can have fractional charges. An electron is an indivisible unit, it cannot be cut into three parts, but a group of electrons in the desired state can produce a so-called quasiparticle with 1/3 of the charge. "It's like electrons are being split up," says Mohammed Hafezi, a physicist at the Joint Quantum Institute. "It is very strange". Hafezi created this effect in ultracold graphene, a monatomic layer of carbon, and recently showed that he can manipulate the motion of quasiparticles by illuminating graphene with a laser. “It's being monitored now,” he says. “External nodules such as magnetic fields and light can be manipulated, pulled up, or unbound. The nature of collective change is changing."
Quasiparticle manipulation allows you to create a special type of qubit - a topological qubit. Topology is a branch of mathematics that studies the properties of an object that do not change even if that object is twisted or deformed. A typical example is a donut: if it were perfectly elastic, it could be reshaped into a coffee cup without changing anything much; 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.
A topological qubit retains its properties even under changing conditions. Usually, particles change their quantum states, or "decohere", when something in their environment is disturbed, such as small vibrations caused by heat. But if you make a qubit from two quasiparticles separated by some distance, say, at opposite ends of a nanowire, you are essentially splitting an electron. Both halves would have to experience the same violation in order to decohere, which is unlikely to happen.
This property makes topological qubits attractive to quantum computers. Because of the ability of a qubit to be in a superposition of many states at the same time, quantum computers must be able to perform calculations that are practically impossible without them, for example, to simulate the Big Bang. Manfra is essentially trying to build quantum computers from topological qubits at Microsoft. But there are also simpler approaches. Google and IBM are essentially trying to build quantum computers from supercooled wires that become semiconductors, or ionized atoms in a vacuum chamber, held together by lasers. The problem with these approaches is that they are more sensitive to environmental changes than topological qubits, especially if the number of qubits grows.
Thus, topological qubits can revolutionize our ability to manipulate tiny things. However, there is one significant problem: they do not exist yet. Researchers are struggling to create them from so-called Majorana particles. Proposed by Ettore Majorana in 1937, this particle is its own antiparticle. The electron and its antiparticle, the positron, have identical properties, except for charge, but the charge of the Majorana particle will be zero.
Scientists believe that certain configurations of electrons and holes (no electrons) can behave like Majorana particles. They, in turn, can be used as topological qubits. In 2012, physicist Leo Kouvenhoven of the Delft University of Technology in the Netherlands and his colleagues measured what they thought were Majorana particles in a network of superconducting and semiconducting 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 without any questions someone will one day create a topological qubit, just for fun,” says Steve Simon, a condensed matter theorist at Oxford University. "The only question is whether we can make of them the quantum computer of the future."
Quantum computers - as well as high-temperature superconductors and unbreakable quantum encryption - may appear many years from now or never. But at the same time, scientists are trying to decipher the mysteries of nature on the smallest scale. So far, no one knows how far they can go. The deeper we penetrate into the smallest components of our universe, the more they push us out.
Ilya Khel