Qubit
Unlike a regular bit capable of having only values of 1 and 0, a quantum bit (qubit) can be in a superposition of these states, that is, simultaneously in a value of 1 and 0. In practice, a qubit can exist in a variety of combinations of these values, which in the future will allow creating ultra-fast computers.
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Qubits will become the building blocks of future quantum computers capable of solving problems practically inaccessible to classical digital computers. To perform calculations on a quantum computer, it is necessary to interact several qubits, and in such a way that they form a single quantum system. Then this system should be allowed to develop according to the laws of quantum mechanics and after a certain time find out what state it has come to.
With the increase in the number of combined qubits, the computational power of such a quantum system grows exponentially. In theory, this allows a quantum computer to cope with tasks that would take an ordinary digital computer millions of years. For example, the so-called Shore algorithm has long been known, which allows you to quickly decompose large numbers into prime factors (a task necessary to crack modern ciphers). Conventional computers solve this problem by enumerating possible divisors, so modern computers can process long numbers for years. A quantum computer would cope with such a task in a matter of minutes or even seconds, depending on performance.
Russian studies
2024: Russian scientists have developed a way to accelerate quantum computing
Russian scientists from MISIS University and the Russian Quantum Center have proposed their own approach to implementing a fast three-cube operation on superconducting qubits - flaxoniums. Representatives of NITU MISIS reported this to TAdviser on May 7, 2024. According to them, this method brings closer the creation of a practically useful "noisy" quantum processor and algorithms for efficient multi-cube operations. Quantum effects, in turn, are useful for studying molecules, creating drugs, effectively solving logistics problems and searching the database.
Superposition and state entanglement are key differences between quantum physics and classical physics. Superposition allows you to simultaneously calculate functions at several points, and entanglement allows you to extract useful information from calculation data. Therefore, the implementation of a multi-cube entangled state is one of the most important tasks in quantum computing, the university emphasized.
The proposed approach to the implementation of the three-bit CCZ (Controlled-Controlled-Z) operation is distinguished by the ease of calibration. According to scientists, most existing quantum processors use two-cube quantum operations for entanglement, but errors arise when they are implemented. The three-cube operation allows a large range of algorithms to be executed in fewer steps, thereby reducing the number of errors. Therefore, quantum processors with a natural three-cube operation can bring closer the appearance of a practically useful "noisy" quantum computer.
 | "The main advantage of this approach is to perform the operation with a microwave pulse applied to the connector. The very presence of the connecting element noticeably reduces the undesirable interaction of qubits, and activation by a microwave pulse allows you to realize an effective three-qubit interaction without removing qubits from the points most protected from external noise, "said Ilya Simakov, engineer of the scientific project of the Laboratory of Superconducting Quantum Technologies at MISIS University. |  |
The three-cube operation is implemented on a new type of superconducting qubits - flaxoniums. They significantly exceed the most common qubits, transmons, in such important indicators as the isolation of computational states and coherence time, i.e. the consistency of several vibrational or wave processes, the scientists explained. The connecting element is a qubit-transmon, which is more resistant to technological errors.
| | "Multi-cube operations allow you to speed up and improve the accuracy of algorithms. Our proposal uses the developments already tested in two-cube operations to increase the number of entangled qubits to three in one operation, namely the use of qubits-flaxoniums, a connecting element - a transmon and a microwave activation pulse. This approach makes it possible to obtain a highly coherent system with small parasitic interactions, with a fast and accurate three-cube confusing operation, "said Grigory Majorin, engineer of the scientific project of the Laboratory of Superconducting Quantum Technologies at MISIS University. | |
Scientists are engaged in scaling this approach both in the direction of increasing the number of entangled qubits in one operation, and in the study of the possibility of creating processors with a unit cell supporting a natural three-qubit operation. The study simultaneously opens up new opportunities for the creation and use of not very large quantum devices operating in the presence of quantum noise, and for the implementation of large-scale universal fault-tolerant quantum computers, MISIS noted. The detailed results of the study were published in the scientific journal Physical Review Applied (Q1).
The work was supported by the state corporation Rosatom within the framework of the Roadmap for Quantum Computing.
2023
A method has been developed for the implementation of a fast two-cube operation on superconducting qubits-flaxoniums
Russian scientists at MISIS University and the Russian Quantum Center (RCC) together with colleagues from the Moscow State Technical University named after N.E. Bauman and FSUE "VNIIA named after N.L. Dukhova" proposed and demonstrated a method for implementing a fast two-cube operation on superconducting qubits-flaxoniums, which can form the basis of scalable and error-resistant quantum processors. Thus, Russian scientists have become one step closer to creating a universal quantum computer capable of solving problems in various fields, for example, for modeling molecular and chemical reactions, which will be the key to the further development of pharmaceuticals and materials science. Such information was shared with the TAdviser portal on November 27, 2023 by representatives of MISIS.
The operation of Z-controlled rotation or CZ is a basic operation in quantum computing performed between the two smallest carriers of quantum information - qubits. Such an operation changes the state of one qubit depending on the state of the second so that their states are entangled. It is the ability to operate with such confusing states that allows us to talk about quantum processors as revolutionary devices that will significantly speed up data processing and solving complex problems, the researchers explained.
The main challenge in creating universal quantum computers is the creation of long-lived qubits with high accuracy of operations. Flaxoniums - a type of superconducting qubits with a complex energy structure - are becoming more attractive to scientists every year due to their high life expectancy and accuracy of operation compared to other types of qubits, such as transmons. However, achieving high accuracy of two-cube entanglement operations on flaxoniums while effectively suppressing parasitic interactions' spoiling'the quantum state is still a challenge.
In their study, scientists from the University of MISIS, the Russian Quantum Center, Moscow State Technical University named after N.E. Bauman and FSUE "VNIIA named after N.L. Dukhova" proposed their own approach to performing CZ operations on flaxonium qubits connected through another qubit (connecting element), a single-qubit operation on which allows you to effectively obtain a two-qubit gate that converts the input states of qubits to weekends according to a certain law. The accuracy of the work was 97.6%, and the duration of the operation was only 44 ns.
As Ilya Simakov, an engineer at the Laboratory of Superconducting Quantum Technologies at MISIS University, a junior researcher at the group "Superconducting Qubits and Quantum Circuits" of the RCC, Ilya Simakov, specified, a short execution time is needed for a universal two-cube operation, the absence of intermediate states with poor coherence, a low level of residual interaction when the connection is not activated, and a simple procedure for calibrating the sequence of control signals.
| | "In this case, we combine the architecture in which qubits are connected to each other through an additional degree of freedom (binders) with the activation of the gate by a microwave signal, which allows qubits to be stored at a point with high coherence throughout the operation," added Ilya Simakov. | |
In turn, Ilya Rodionov, director of the REC Functional Micro/Nanosystems of the Moscow State Technical University named after N.E. Bauman and FSUE "VNIIA named after N.L. Dukhova," shared that even a defect of atomic scale can interfere with the high coherence of qubits, as well as with logical operations, including CZ gates. Moreover, when it comes to flaxoniums - the most difficult qubits to manufacture, containing a chain of sub-micrometer Josephson transitions.
| | "We have developed a technology for creating superconducting circuits based on qubits-flaxoniums from more than a hundred technological operations, which ensures high quality of quantum elements and their repeatability, which means that it is possible to further scale on the way to a universal computer," Ilya Rodionov specified. | |
When creating a superconducting quantum processor, the researchers moved away from the concept of direct qubit coupling and proposed a more scalable approach based on the use of special connecting elements. This made it possible to improve the operation of the system and use better approaches to performing quantum operations.
As has been repeatedly noted, flaxoniums, due to their high coherence (ability to transform quantum states) and significant anharmonicity (nonlinearity), can be the key to improving superconducting quantum circuits and in the future replace widely used transmons. The researchers have already begun work on scaling the proposed approach, and are also developing the concept of performing a three-cube operation on flaxoniums using a single connecting element.
The results of the study were published in one of the leading scientific journals PRX Quantum (Q1).
Atoms can be used as qubits in a quantum computer
Physicists from MIPT together with colleagues from France experimentally showed that atoms of impurities in semiconductors can form long-lived stable quantum states. So these atoms can be used as qubits in a quantum computer. The work is published in the journal Communication Physics. This was announced on July 24, 2023 by representatives of the Moscow Institute of Physics and Technology.
As reported, a qubit is a unit of information in a quantum computer, it differs from a regular bit in that it can take any value between 0 and 1 in the process of calculations. This effect arises from the superposition principle in quantum mechanics. Thanks to the superposition, the qubit in the process of calculations is in all states immediately and therefore helps to process much more information than the classic bit. Various quantum systems can act as a qubit: superconducting artificial atoms, quantum dots, atoms in traps, real atoms in a solid, etc. However, the weak point of all existing qubits is instability to noise. For example, a small variation in temperature or magnetic field can disrupt the quantum state of a qubit, and it will be unsuitable for calculation. This problem of quantum state destruction is called decoherence and is one of the main fundamental reasons why quantum computers do not yet have widespread use. Scientists are looking for physical systems in which qubits that are more resistant to noise can be implemented.
For example, if impurities are added to some semiconductors, the electrons of the impurity atoms will last a long time (by quantum standards, this is a few nanoseconds) to maintain the direction of the spin - the proper magnetic moment. Thanking for a long time of spin coherence, such atomic systems can be used as qubits. Physicists from the Center for Advanced Methods of Mesophysics and Nanotechnology MIPT explore such structures and select optimal materials for them.
In the work, the scientists of the center replaced some of the tellurium atoms in the dichalcogenide molybdenum tellurium (2H-MoTe2) with bromine atoms and, using electron parmagnetic resonance and tunnel scanning microscopy, examined the structure of the electrons of the impurity atom and estimated the coherence time of the system.
{{quote 'author
= told Vasily Stolyarov, director of the center, head of the laboratory of superconducting and quantum
technologies, doctor of physical and mathematical sciences' If a separate foreign atom placed in a single crystal leads to the localization of a spinpolarized state, then it can become a qubit. In transition metal dichalcogenides, a strong spin-orbital interaction just creates such conditions. The only question is how to work with such qubits, because this is the most atomic scale, about 0.3 nm. We in our studies added bromine impurities to the semiconductor molybdenum tellurium. This impurity has an energy position within the band gap of the material, that is, its electrons are localized. In our work, we show that the quantum properties of these impurities can be studied by using an electron spin resonance measurement technique and low-temperature scanning tunneling spectroscopy. We have shown that in these atoms there are localized spin-valley states inherited from the material with nanosecond spin coherence times.}}
To understand the effects that physicists studied, you need to turn to the electronic structure of matter. The electrons of each atom, according to quantum mechanics, have a certain energy - they are at the energy level. In crystals, electrons can transition from one atom to another, their energy spectrum becomes almost continuous, without dividing into levels. However, there is a band gap in semiconductors - the range of energies that electrons cannot accept. But, if you add an impurity atom to the semiconductor, the electrons of this atom will have access to levels at the upper or lower edge of the band gap. It turns out that such a secluded place where you can hold for a long time an electron is an excellent platform for a qubit. It is worth noting that this is possible at temperatures below 250 degrees Celsius.
It is important to select the correct semiconductor and impurity to localize electrons. Therefore, physicists drew attention to the dichalcogenides of transition metals - layered two-dimensional semiconductors consisting of a transition metal atom (molybdenum here) and chalcogen (tellurium here). In dichalcogenide crystals, due to symmetry (atoms are arranged in the shape of a hexagon), the most advantageous energy states for electrons are in certain regions spaces - valleys - around atoms. Moreover, electrons are able to retain the projection of the spin - their own magnetic moment - for some time. However, such times are too small for qubit coherence.
For this reason, the researchers replaced tellurium atoms with bromine atoms, "opening" additional levels for electrons near the lower edge of the band gap. In this case, an associated state of electrons and valleys arose, and the projection of the spin at these levels was maintained for several nanoseconds, which is enough to create a qubit.
To study such subtle effects, scientists used several high-precision devices. First, they obtained the electronic structure of the bromine impurity using electron paramagnetic resonance - the splitting of energy levels in the external magnetic field - and estimated the coherence time of the spin state from these data. It was about 5 nanoseconds at temperatures below -258 degrees Celsius (15 Kelvin).
Then a scanning tunneling microscope was used - a device that determines the surface relief with an accuracy of the atom. A voltage was applied to the microscope needle, and electrons from the surface were tunneled onto the needle, creating a current. On change in the current value of physics obtained spatial localization of electrons and their energy. These measurements confirmed that the states of the bromine electrons localize near the valleys, and their energy changes. It is the connection of the valleys and impurities provided a long coherence time. Physicists suggest that it can be increased by taking a single-layer dichalcogenide crystal. The researchers obtained similar experimental data using computer modeling.
Thus, the scientists showed the possibility of using real atoms as qubits and theoretically explained the long coherence time by building the electronic structure of the material.
{{quote 'author
= summed up Vasily Stolyarov|While this is a relatively pioneering work, where it is shown in principle that impurity atoms have signs of long-lived localized electronic states - the alya-qubit atom. The message of the work is that it is necessary to further study the possibility of using real atoms in a solid-state matrix to create qubits. We plan to improve the technique, my graduate student Valeria Sheina, the first author of the work, is also trying to translate impurity atoms into an excited state. To do this, we need to enter the source into the tunnel microscope, right under the needle
high-frequency radiation, which would transfer the qubit from the ground state to the excited state. And this is the next stage. Much of its success depends on the choice of material and impurity.}}
The work was carried out with the support of the Ministry of Science and Higher Education of the Russian Federation and the Federal Academic Leadership Program "Priority 2030."
The study, in addition to employees of the Center for Advanced Methods of Mesophysics and Nanotechnology of the Moscow Institute of Physics and Technology, was attended by their colleagues from the University of Paris-Saclay and the Sorbonne University (France), MISIS, the All-Russian Research Institute of Automation named after N. L. Dukhov, the Institute of Metal Physics named after M.N. Mikheev (Yekaterinburg), the Institute of Physics of ion Beams and Materials Research (Germany) and the University of AAlto (Finland).
Russian scientists have increased the performance of quantum processors using kudits
Scientists at NUST MISIS and the Russian Quantum Center have proposed an approach to the implementation of quantum algorithms using additional levels of the quantum system, which made it possible to increase by an order of magnitude the final quality of execution of quantum algorithms. This was announced on April 7, 2023 to TAdviser by representatives of NUST MISIS.
According to scientists, the main way to improve the performance of quantum processors is to increase the number of their qubits - the smallest unit of information in a quantum computer. However, ions or atoms, which often act as qubits, have more than two levels and can work not only as qubits, but also as kudits, which are an extended version of the qubit and can be in three (cutrites), four (cuckwart), five (cuckwine), and more states.
Additional states allow denser coding data in physical media, which in turn makes it possible to implement increasingly complex and complex quantum algorithms. Thus, the quantum power increases, and operations processor can be carried out much faster, the researchers explained.
As of April 2023, much of the research on quantum operations focuses on qubits - all operations that apply to a quantum system are presented as one- and two-qubit quantum gates that convert qubits' input states at weekends by a certain law. To work with kudits, it is important to find new approaches from a mathematical point of view.
Scientists at MISiS University and the Russian Quantum Center considered one of the ways to use puppets - 5-level kudits - and presented a decomposition model of the generalized Toffoli valve. As an example, Grover's quantum algorithm for searching a disordered database is considered. It is known that by using only this gate, any reversible classical logic circuit, such as an arithmetic device or a classical processor, can be constructed.
| | "Puppets are good because their space can be seen as a two qubit space with a common extra level. Such consideration helps to both reduce the number of physical media and use an additional layer as an auxiliary state to simplify the decomposition of multi-bit gates, or as they are also called gates, complex logic operations with qubits. Thanks to this approach, when implementing quantum algorithms on puppets, it becomes possible to reduce the number of two-part gates, i.e. using two physical systems, "said Alexey Fedorov, head of the laboratory of quantum information technologies at NUST MISIS. | |
As a foldable multi-bit gate, scientists chose Toffoli's multi-bit gate, which is often found in quantum algorithms - a version of the universal controlled reversible gate generalized to n qubits. Its application inverts the state of the nth qubit if all the other n-1 qubits are in state 1. As the researchers noted, by arranging two qubits in each puppet and using the fifth level as an auxiliary, it is possible to significantly reduce the number of two-part gates in its decomposition compared to qubit locations and thus improve the quality of execution of quantum algorithms.
| | "We demonstrate the reduction in the number of two-part gates using the example of Grover's algorithm, which solves the problem of brute force. To demonstrate the processes, it was this algorithm that was chosen, since for its execution it is necessary to repeatedly implement multi-bit gates. We compared three methods of decomposing multi-cube valves as part of the implementation of this algorithm on 2-10 qubits, when qubits, cutrites and cuckoos are used as carriers of information, and demonstrated how the number of two-part gates is reduced, "explained Anastacia Nikolaeva, an expert of the scientific project NUST MISIS, researcher at the RCC. | |
Compared to qubits, the implementation on puppets with a large number (> 5) of qubits involved in the algorithm requires an order of magnitude fewer two-part gates, scientists noted. For example, the 8-qubit Grover algorithm requires more than 1000 two-part gates on qubits, while only 88 will be required to implement it on cuckwints.
In general, the study demonstrated one of the advantages of using kudits for quantum computing and helped to take a fresh look at their potential, emphasized in NUST MISIS and RCC. The results obtained by scientists apply to quantum processors based on various physical platforms, such as ions, neutral atoms, superconducting circuits and others.
The study was carried out by researchers at NUST MISIS and RCC with the support of a grant from the Russian National Research University of 19-71-10091. The article was published in the scientific journal Entropy.
2022: Russian scientists test superconducting low-frequency qubits
Russian physicists of the laboratory "Superconducting Metamaterials" of the University of MISIS and Moscow State Technical University named after N.E. Bauman were among the first in the world to implement a two-cube operation using superconducting flaxonium qubits - an alternative to popular transmons. The peculiarity of flaxoniums consists in a longer life cycle and greater accuracy of operations, which makes it possible to perform longer algorithms. This was reported to TAdviser on November 15, 2022 by representatives of MISIS University.
As you know, one of the main problems of the development of a universal quantum computer is in qubits, namely, from which quantum objects it is best to make processors for quantum computers: electrons, photons, ions, superconductors or other candidates for "quantum transistors." Over the past ten years, superconducting qubits have received a huge boost in development. At the same time, the most commercially successful superconducting qubits as of 2022 are transmons, which are actively studied and used in quantum developments by Google, IBM and other world laboratories, told NUST MISIS.
According to scientists, the main task of the qubit is to holistically store and process information. Random noise and even just observation can lead to loss or change. data For the stable operation of superconducting qubits, an extremely low ambient temperature is often needed - close to zero Kelvin, which is hundreds of times colder than the open temperature. space
In tests to protect qubits from noise, the researchers added a superinductor -- a superconductor element with a high level of resistance to alternating current -- to the circuit, which is a chain of 40 Josephson contacts -- structures of two superconductors separated by a thin layer of dielectric.
| | "Flaxonium qubits are more complex and less studied compared to transmons. The main plus of flaxoniums is that you can work with them at a low frequency - about 600 MHz. It is known that the lower the frequency, the longer the lifetime of qubits, which means more operations with them can be performed. During the tests, it turned out that the dielectric losses of flaxonium qubits make it possible to keep the state of superposition longer than that of transmons, "said Ilya Besedin, one of the authors of the study, engineer of the scientific project of the laboratory" Superconducting metamaterials "NUST MISIS. | |
As an element that converts qubit input states to outputs, scientists used high-precision two-qubit fSim and CZ gates. And in order to bring qubits into resonance with each other, parametric modulation of the flow of one of the qubits of the system was used. As the authors of the study noted, thanks to the tunable communication element, it was simultaneously possible not only to obtain the accuracy of two-bit operations above 99.22%, but also to suppress the residual ZZ interaction between qubits, which allowed performing parallel one-bit operations with an accuracy of 99.97%.
| | "The low frequency of computational qubits opens the way not only to a longer lifetime of qubits and the accuracy of gate operations, which, accordingly, will allow longer algorithms, but also makes it possible to use subhigahertz electronics in qubit control lines, and this significantly reduces the complexity of the quantum processor control system," Ilya Besedin emphasized. | |
Overall, according to the scientists, the findings open up a promising approach to fault-tolerant quantum computing with low-frequency qubits, which, with their improved coherent properties, could be a competitive alternative to widely used qubit-transmon superconductor processors.
In the future, it is planned to continue research with calculations based on qubits-flaxoniums, namely: optimize the qubits management system, improve reading indicators and start developing multi-qubit systems based on them.
An article about research that brings the creation of a quantum computer closer to reality is published in npj Quantum Information - Nature.
2013: Qubit condition measured in Russia
In June 2013, it became known that specialists from the university laboratory MISIS, in cooperation with the Russian Quantum Center (RCC), were the first in Russia to measure the state of the qubit. A team of researchers led by a member of the scientific council of the RCC, Professor Alexei Ustinov, conducted an experiment to measure the state of a superconducting qubit. Scientists managed to observe the periodically changing qubit signal, as well as measure its resonant frequency.
Superconducting qubits are rings of a superconductor with a diameter of several microns. In some places of the rings there are nanometer-sized breaks - they are called Josephson transitions. The superconducting rings are cooled to a very low temperature with a mixture of liquid helium-3 and helium-4 and placed in an ultra-fine-tuned weak magnetic field. As a result, they acquire quantum properties similar to those of atomic spins.
Russian scientists were able to create an experimental chip with 7 superconducting qubits placed in microwave resonators. Interaction with the superconducting qubit affects the microwave spectrum, which makes it possible to judge the current state of the qubit without disturbing this state, that is, to bypass the problem of decoherence. The most stable of the 7 qubits was measured in MISiS.
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