Think small

A quantum computer is a kind of holy grail that stands at the center of the ambitions of many scientists, in different parts of the world.

The fact that no one knows what a quantum computer will look like and exactly how it will work does not relax the hands of the scientists. The benefit that could be derived from such a computer is so great that it justifies the effort and dealing with the uncertainty (if it is permissible to use the phrase in this context). Quantum computers will be able, among other things, to perform calculations that cannot be performed on normal computers, such as decomposing a very large number, created as a product of two prime numbers, into its components. Such an ability means the possibility of cracking information encrypted in the most common and reliable encryption systems that exist today, and which are used for economic, security and private communications.

The great advantage of the quantum computer, and the difficulty in building it, stem from the fundamental difference between bits of a normal computer and quantum bits. A normal bit is a kind of switch that is at any given moment in one of two possible states (for example, "off" and "on"), which can be described by the digits zero (0) and one (1), respectively. On the other hand, a quantum bit ("qubit") can be found in both "zero" and "one" at the same time. Therefore, it will be able to perform many calculations at the same time. This ability stems from one of the basic properties that quantum theory attributes to both matter and light: the dual existence as particles and waves.

In the world of big things, classical physics rules, according to which lumps of matter are found in defined places. But in the world of small things, the subatomic particles, quantum physics rules, according to which the particles of matter sometimes appear as waves that are everywhere at the same time. For example, the waves of matter can move simultaneously in several possible paths - as long as no one observes or measures them. As soon as someone or something watches them - the dual existence collapses, the material "chooses" only one path, and in this one path it appears in the show

Its material, particle. This is, in a sense, the essence of the difficulty in building a quantum bit. On the one hand we want to enjoy the parallelism of its existence. On the other hand, if you want to change its state (for example, from the "zero" state to the "one" state), or read it, it may collapse into its particle instance, and lose the wave property and parallel existence.

Prof. Adi Stern, from the Department of Condensed Matter Physics, previously invented a method for testing the ability of a system to serve as a special type of qubit - a topological quantum bit. It is a system of the quantum Hall effect, in which particles

Objects move in a plane with a strong magnetic field acting perpendicular to it. These are simulated particles whose electrical charges are equal to one-third, one-fourth or one-fifth of the charge of the electron. These particles, which are not found in nature, were created in the laboratory of Prof. Motti Highblom from the same department.

Such a system must meet certain criteria to fit the definition of a qubit. The particles must be able to change places, and these changes of place must leave traces that can be followed. In other words, the method must allow information to be stored through the actions of the particles. In Prof. Stern's theoretical experiment, two electric current lines flow in the system with a separation "fence" between them containing simulated particles. If the number of simulated particles (having a charge less than the charge of the electron) is not even, the electrons in the current will behave as particles,

and pass the material in a direct line. But if the number of simulated particles is even - the electrons moving in the system will behave as waves, and will interfere at the end of the track. In fact, not only the number of the simulated particles affects the system, but also the size of their electric charge (a third, a quarter or a fifth of the charge of the electron). to particles

What Prof. Highblom measured in the 90s were odd denominators, and these do not leave marks when they change places in the plane. Therefore they are not suitable to be used as bits to store information. In contrast, the simulated particles whose charge

Having an even denominator (a quarter of the electron charge), may be more suitable for the role of the quantum bit. Particles of this type were produced about a year ago, for the first time in the world, by a team of scientists from the Weizmann Institute, including Prof. Motti Haiblom, Prof. Adi Stern, Dr. Vladimir Amansky, Dr. Diana Mehlo and research student Merav Dolev. .

Wrong number

The ability of quantum particles to be in several states and places at the same time (called "superposition") may give them the much sought-after parallel computing ability. But "superposition" collapses into a situation

One's existence is defined as soon as someone or something observes or measures it (this is what the founders of quantum theory meant when they said that "the observer changes the result"). For example, "superposition" has never been observed in the world of the "big things". The accepted explanation for this is that in lumps of matter containing many particles, everyone is "watching" everyone else and therefore the "superposition" of everyone collapses into the defined state of material existence (a phenomenon called "discoherence"). This is how the scientists found themselves in a situation where they can describe a simple quantum computer, but are unable to build such a computational system based on millions of bits.

This challenge is at the center of the ambitions of Dr. Roi Ozari and research students Nitzan Akerman, Yanon Glickman, Shlomi Kotler, Yoni Dalal and Anna Kesselman, from the Department of Physics of Complex Systems at the Weizmann Institute of Science. Dr. Ozari is mainly interested in the ability to perform error corrections in quantum calculations. The computers that exist today prevent most mistakes through duplication and special protocols for correcting mistakes.

Such protocols protect, for example, the music files stored on CDs from being scratched. Similar protocols, if implemented in quantum systems, may serve as tools for detecting and preventing dicoherence, thus

Maintain "superpositions" of devices built from many material particles. Thus, by way of active protection, it may be possible to apply the principle of superposition in the world of the great things.

Dr. Ozari is also researching ways to produce complex quantum logic gates (logic gates perform the basic operations of electronic calculations). These are the ways in which actions performed on one qubit will, in some cases, change the state of another qubit. But how do you overcome the difficulty arising from the basic quantum property, according to which "the observer changes the result"? How can one measure or change a qubit without causing the system to collapse into a defined material state?

Dr. Ozari tries to answer these questions - and overcome the difficulty described in them - using an experimental system based on strontium ions (ions are atoms that have undergone "laser surgery" to remove some of their electrons). He shoots some of these ions into an empty chamber (vacuum), where they are trapped in an array of electric fields, and cooled using laser beams to a temperature several millionths of a degree above absolute zero. Although it is a few single ions, Dr. Ozari succeeds in examining the effect of dicoherence using

Activating an electromagnetic field that creates "noise" in the ions' environment. To activate the ions in the empty cell in which they are trapped, he uses additional lasers that are tuned to move the strontium ions from one electronic state

to another The precision required in matching the properties of these lasers to the transitions between the different states of the strontium ions is a real challenge. Dr. Ozari: "It's like calibrating the length of a rod that reaches from the earth to the moon with an accuracy of the thickness of a hair."

What do we get out of it?

The safety of the encryption method commonly used today is based on the fact that decomposing a product of two prime numbers into its components requires a very long time (sometimes much longer than a human lifetime). A quantum computer is made

to change this state of affairs, and make it possible to break codes in much shorter periods of time.

In the picture: eight ions are trapped in an empty chamber and cooled using a laser. At a temperature close to absolute zero, the ions organize themselves in a crystal structure similar to a chain of pearls. Each ion in the crystal functions as one quantum bit

in brief:

The question: Is it possible to bypass the well-known principle of quantum theory according to which "the observer changes the result"? Is it possible to use quantum measurement to "save the result"?

The findings: this is exactly what the institute's scientists are trying to do.

תקשורת

Prof. Ran Raz, from the Department of Computer Science and Applied Mathematics at the Weizmann Institute of Science, investigates, among other things, the question of whether communication between computers will become more efficient thanks to the use of quantum methods.

The era of quantum computing may still be a long way off, but the development of quantum communication is an easier task, and in fact it has already been successfully demonstrated in laboratory experiments. An example of a communication problem is software for scheduling a meeting between two people. The smallest number of bits needed today for communication between computers in order to find a common free time between the two calendars of the two people who wish to meet, is equal to the number of slots being tested (n). In contrast, a quantum communication system would be able to perform the task using an amount of bits equal to the square root of n only.

Prof. Raz found that in certain communication problems, a quantum system may offer a much greater improvement, sometimes even logarithmic. In other words, as the value of n increases, the advantage of quantum communication will be more significant. He also found that quantum communication may also be effective in one-way communication systems, where the receiving party must make decisions based on the message sent by the sender.