Diamond quantum computer. Fian has developed two methods for creating invisible diamond markings

Scientists used diamond to make a quantum computer. Previous attempts to create such a computing device were hampered by external influences that distort computation. Now physicists from the Netherlands and the United States have found a solution to this problem.

Diamond has started to be used for quantum computing relatively recently. In this case, defects in a gemstone crystal have become its main value. The so-called point defects are "irregular" lattice sites - vacancies that arise when a carbon atom is removed from a lattice site - and nitrogen atoms associated with them. Such defects are also called nitrogen-substituted vacancies in diamond or NV centers. The electronic spins of each center lend themselves to manipulation of magnetic, electric, and microwave fields to record quantum information.

The smallest elements for storing information in a quantum computer are called quantum bits or qubits. They are the spin of the nucleus and the spin of the unpaired electron of each NV center.

Previous attempts to create a quantum computer were hampered by environmental influences that distort computation. It led to decoherence, that is, a violation of the interaction of qubits and subsequent problems during the operation. Scientists were able to achieve only isolation of free quantum bits from the external environment, but they were unable to ensure the protection of matched qubits.

The article, published in the journal Nature, talks about how researchers from the Netherlands and the United States solved the problem. (The article can also be downloaded from the ArXiv.org preprint site.)

“The interactions between the quantum bit and the environment are known to result in the loss of information being carried. However, dynamic control of the qubits is possible,” said lead researcher physicist David Awschalom, professor at the University of California, Santa Barbara. environment, we can ensure the execution of the quantum information processing algorithm. "

Physicists have found that by synchronizing the rotation (the same spin) of the unpaired electron and the nucleus of the nitrogen atom, it is possible to achieve the protection of the qubits. The electron is much smaller and faster than the nucleus, but it more easily becomes a "victim" of decoherence. To synchronize the qubits, the specialists used microwave pulses, forcing the electron to constantly change the direction of its spin. As a result, there was no qubit mismatch, and the calculations were carried out without failures.

Scientists have demonstrated the work of the new "protected" diamond computer by solving a problem based on Grover's algorithm. The algorithm was created in 1996, before the idea of ​​creating quantum computers appeared. But it is precisely for demonstrating the "capabilities" of quantum computing systems that it is best suited.

The test is a task of finding information in an unsorted database. To make it clearer, the search can be compared to an ordinary situation: the computer, knowing the phone number, must find the name of the subscriber in the phone book.

A person (or an ordinary computer) in this situation, using the usual enumeration of numbers, can accidentally find the desired name on the first page, or, conversely, on the very last. If you search an infinite number of times, then on average the caller's name will be found in the middle of the phone book.

Moving on to mathematical concepts, this means that the correct choice will be found with X / 2 attempts, where X is the number of search attempts made. That is, in the case of 4 attempts, the name will be found on average after 2 attempts.

A quantum computer, using the principle of superposition, will find the right answer much faster. The mathematics behind this process is difficult to understand, but in practice this means that a quantum computing device, in the process of searching through an unsorted database, will always find the right name on the first try.

A two-qubit computer of physicists from the Netherlands and the USA sometimes made mistakes (interacted with the environment), but in 95% of cases it found the right answer on the first try, which, according to the developers, is a good result.

We add that quantum computers are not the only competitors of modern classical computing devices. Recently, another group of specialists on the DNA computer.

In the joint laboratory of FIAN and the Russian Quantum Center (RQC), a model of a quantum computer based on diamond has been created.

Quantum mechanics is one of the main pillars of modern physics research. Elementary particles and microelectronics have long lived according to the quantum laws of the world. Quantum mechanics starts to work at an action size comparable to Planck's constant. The so-called new sciences at the intersection of quantum mechanics and informatics, such as quantum information theory and informatics, have gained great relevance. Classical information is presented in bits of the form 0 and 1. In quantum information theory, the memory cell is a qubit that stores a superposition of states 0 and 1.

Employees of the LPI Gas Laser Laboratory - Sergei Kudryashov, Alexei Levchenko, Leonid Seleznev, and Dmitry Sinitsyn, by exposing the diamond to radiation from a femtosecond laser, were able to create in it an increased concentration of vacancies (defects in the diamond lattice, where there are no carbon atoms). Researcher at FIAN comments Alexey Levchenko:

« Usually, vacancies are created using electron beams or beams of any high-energy particles. This method provides a uniform concentration of vacancies throughout the sample volume. With the help of a femtosecond laser, on the contrary, it is possible to achieve a similar but local effect - to draw the required "picture" with small clusters of vacancies. "

Then these vacancies can bind with impurity nitrogen atoms, which are usually present in the bulk of the diamond in significant concentrations, and form the so-called NV-center (nitrogen-vacancy center) - a very “useful” defect for diamond marking. The fact is that when exposed to visible light, NV centers begin to characteristically fluoresce, and the application of an external microwave field can also change the intensity of this fluorescence.

« In a diamond there may be other impurities that glow under the influence of external radiation (pink, yellow, blue diamonds), therefore, turning on the microwave field, we will be able to see our changing signal against the background of all this noise. If you create an invisible microcluster of NV centers - due to sharp focusing in the volume literally down to a micron, then you can subsequently read the mark of our diamond by fluorescence in the microwave field – Alexey Levchenko.

In the joint laboratory of the Lebedev Physical Institute and the Russian Quantum Center, the study of diamond color centers, NV centers is being carried out. But what is an NV center? Let's consider a matrix of carbon (diamond) in which a nitrogen atom is substituted instead of one carbon in the atom (yellow diamonds are yellow due to nitrogen impurity), and the neighboring atom is absent. The resulting system is called the NV center or coloration center. Accordingly, N-nitrogen, and V-vacancy. This center of color has an axis. The projection of the electron spin onto this axis is conserved and can serve as a qubit. This spin is the sum of the spin of all electrons involved in this insert. Accordingly, we can use this spin as quantum memory.

Powerful laser installation "PIKO" for research on the interaction of laser radiation of the nanosecond and picosecond ranges with matter. From left to right: Mikhailov Yu.A. A. V. Kutsenko

« You can create a state with a projection of zero, with a projection of one on this axis, or a superposition of "zero plus one", while zero and one will be represented in superposition with some kind of weight. By the glow of the NV center, you can determine its state. If it is in state zero, then it glows more brightly. If it is in the state of one, then it is less bright. We have the ability to determine where he was, simply by brightness. As if you had two bulbs zero and one"- comments on the work of a senior researcher at FIAN, head of the group of Quantum simulators and integrated photonics RQC, candidate of physical and mathematical sciences Alexey Akimov.

In addition, we have the ability to manipulate the state using a radio frequency field. Between the two states zero and one, by applying an impulse, it is possible to organize intermediate states, or a complete transition from one state to another. It all depends on the pulse duration, usually this duration is of the order of tens of nanoseconds. Thus, we can prepare quantum states very quickly, faster than the relaxation times of our centers. That is, we can always prepare the state we need by shining green light on it and then applying a radio frequency field.

« But it wouldn't be so interesting if we couldn't use nuclear spin. Due to the fact that the color center and the 13C nuclear spin can be close, a magnetic interaction occurs between them, which makes it possible to rewrite information from the electronic to the nuclear spin and vice versa. Since nuclear spin interacts much less with the outside world, it is a more isolated, more long-term memory. In the nuclear back, information can be stored for much longer, as long as this time is brought to a few seconds."- says Alexey Akimov

The possibility of performing calculations according to the laws of quantum mechanics opens up a huge field of new possibilities for mathematicians, physicists and programmers. But new computation algorithms bring new rules of the game into our life, for example, the RSA encryption algorithm, which is strong from the classical point of view, becomes vulnerable to Shor's quantum algorithm. Shor's algorithm is able to factor out a prime number much faster than classical algorithms, in time comparable to the multiplication of these very prime numbers. And one of the most common and secure RSA encryption methods is precisely based on the use of prime factorization. The model of a quantum computer created at FIAN, consisting of several qubits on diamond color centers, is intended to demonstrate the possibility of such quantum algorithms operating.

B. Massalimov, ANI "FIAN-inform"

Well, I, in turn, really hope that the readers of SUN, thanks to this article, will guess what a magic staff is, the one that Santa Claus, and the monarch, and an ordinary wizard have, and how it works.

13:07 17.10.2013

Experts from the LPI Gas Laser Laboratory have succeeded in developing two methods for precision microscale marking of diamonds. According to the institute's website, the tags, invisible to the naked eye, are created here using radiation from a femtosecond laser.

Sergey Kudryashov, Leonid Seleznev, Alexei Levchenko and Dmitry Sinitsyn have developed a way to create a kind of "quality marks". The diamond is exposed to radiation from a femtosecond laser, which creates in it an increased concentration of vacancies (defects in the diamond lattice, in which there are no carbon atoms).

The use of a femtosecond laser instead of electron beams or beams of any high-energy particles (uniform concentration of vacancies throughout the volume) makes it possible to achieve a local effect - to draw the required "picture" with small clusters of vacancies.

These vacancies can then bind to impurity nitrogen atoms, which are usually present in a diamond in significant concentrations, and form an NV-center (nitrogen-vacancy center), which is very useful for diamond marking defect: when irradiated with visible light, such NV- the centers begin to fluoresce, and the application of an external microwave field is capable of changing the fluorescence intensity.

According to Aleksey Levchenko, diamonds may also contain other impurities that glow under the influence of external radiation. Turning on the microwave field, you can see our changing signal against the background of all these noises, and if you create an invisible microcluster of NV centers, this will allow you to read the diamond's fluorescence mark in the microwave field.

The second method of marking gemstones also uses radiation from a femtosecond laser, however, unlike the first, here instead of creating vacancies, disseminations of an amorphous carbon phase are formed.

Glassy carbon test lines formed under the action of highly focused femtosecond laser radiation. (A) on the surface of a diamond, line width - 3 μm; (B) in its volume, the width of the thin line is about 1 μm. Photo from the site fian-inform.ru

Sergey Kudryashov notes that femtosecond laser radiation can be focused at different depths inside transparent materials, and, therefore, this technology makes it possible to create unique three-dimensional markings. In the experiments carried out, volumetric microscale marks were successfully formed on artificial and natural diamonds.

Under normal conditions, the mark is not visible even under a microscope, it does not reduce the value of the stone; under the influence of laser radiation, it begins to fluoresce brightly. The mark is created inside the diamond and cannot be polished or sawed off. Photo from the site fian-inform.ru

MICROELECTRONICS, 2012, Volume 41, No. 2, p. 104-119

QUANTUM INFORMATICS:

NV CENTERS IN DIAMOND. PART I. GENERAL INFORMATION, PRODUCTION TECHNOLOGY, SPECTRUM STRUCTURE

A. V. Tsukanov

Institute of Physics and Technology of the Russian Academy of Sciences E-mail: [email protected] Received March 31, 2011

A quantum system is considered in detail, which is one of the most popular and promising in experimental quantum informatics - the NV center in diamond. We draw the reader's attention to the results obtained over the past several years and covering a wide range of issues related to the manufacture, control, measurement of NV centers and their use as elementary carriers of quantum information. The problem of constructing a full-scale quantum computer is discussed.

1. INTRODUCTION

The idea of ​​quantum information processing was born at the end of the 20th century and by now has become one of the most attractive and intriguing for many researchers working in various fields of science. With the development of the experimental and technological base, the creation of a quantum computer has ceased to be only a speculative theoretical problem, transformed into a complex but interesting practical problem.

As an elementary cell of a quantum computer - a quantum bit or qubit - a two-level system is chosen, the state of which can be effectively controlled. It is assumed that the system representing a qubit has a number of specific properties. These include a) high discreteness of the energy spectrum, which makes it possible to single out two logical states 10) and 11 qubits from the complete Hilbert space of states of the system, b) the existence of physical mechanisms that provide initialization, control and measurement of the state of a qubit, and c) long relaxation and dephasing times of logical states. Building a full-scale quantum computer, consisting of a large number of synchronously operating qubits, also implies the ability to control the interaction between two arbitrary qubits. It is generally accepted that an increase in the number of qubits to a practically useful value (on the order of several thousand) will most likely be implemented in solid-state structures. There are several promising areas considering such quantum systems (superconducting elements, semiconductor quantum dots, implantation

atoms) as qubits. All of them satisfy these requirements only at very low (<100 мК) температурах, когда энергия размерного квантования системы значительно больше, чем энергия тепловых флуктуаций. Указанное обстоятельство накладывает жесткие ограничения на дизайн и качество контроля кубита. В этой связи представляется крайне важным ослабление данного требования за счет выбора такой системы, которая сохраняла бы когерентность, необходимую для квантовых операций, при более высокой (желательно - комнатной) температуре. На сегодняшний день известны две такие системы. Первая из них, раствор молекул некоторых органических веществ (например, раствор ацетона в хлороформе), представляет собой объект, на котором в 1998 году были продемонстрированы принципы квантовых вычислений . Однако количество кубитов - ядерных спинов атомов водорода, углерода и др., входящих в структуру молекулы, ограничено числом атомов в молекуле. Вторая система, являющаяся предметом нашего рассмотрения, есть дефект кристаллической решетки алмаза, который состоит из соседних атома азота (Ы) и вакансии (V). Принятое обозначение такого дефекта - NV - указывает на структурный состав, а название - "NV-центр" - говорит о том, что он представляет собой так называемый центр окраски по отношению к чистому алмазному субстрату. Принципиальное преимущество данной твердотельной системы - возможность создания упорядоченных двумерных массивов, содержащих произвольное количество одиночных NV-центров, т.е. возможность масштабирования.

The main purpose of this work is a brief, but as complete as possible, complete acquaintance of the reader with the NNi centers, their structure and physical properties, as well as the position that they

occupy in modern experimental physics of low-dimensional structures. While focusing on a fairly detailed discussion of the results directly related to quantum computing, we will nevertheless pay attention to other closely related areas related to coherent manipulations of the state of ME centers. In the first part of the review, we will consider the main properties of ME centers, the technology of their fabrication, and group-theoretical analysis of the spectrum. The second part will be devoted to the issues of control of both the orbital and spin states of the center, elementary quantum operations, initialization, measurements and suppression of quantum errors. In the third part, quantum algorithms, hybrid systems and possible options for scaling a quantum computer at MU centers will be presented. In addition, we will discuss the prospects for their practical use as single-photon sources and magnetometers.

2. MU CENTERS IN DIAMOND: GENERAL

INFORMATION AND BASIC PROPERTIES

The structure of the ME center in diamond is shown in Fig. 1a. As can be seen, the nitrogen atom and the vacancy lie on one of the main diagonals ((111)) of the face-centered cubic diamond lattice, which in this case is also the symmetry axis of our center (z axis). This means that there are four possible orientations of the ME center relative to the crystal lattice of the substrate. When a tetravalent carbon atom is replaced by pentavalent nitrogen, an additional electron appears in the lattice, and when a neighboring vacancy is formed, four more electrons are released - three from nearby carbon atoms lying at the vertices of an equilateral triangle in the xy plane, through the center of which the z axis passes, and one from the atom nitrogen. The corresponding four unpaired d-orbitals are oriented towards the formed vacancy. In addition, experiments convincingly indicate that often a sixth electron is attached to these five electrons associated with the center, apparently from another nitrogen atom. This means that the center can be either neutral (MU0, about 30% of their total number) or negatively charged (MU-, about 70%). The isotopic composition of the ME center depends on the relative concentration of various isotopes of nitrogen and carbon in a given crystal. Usually, the nitrogen isotope 14M with a nuclear spin I = 1 prevails in natural diamond, while the fraction of the 15M isotope with a nuclear spin I = 1/2 is only 0.37%. The spinless carbon isotope 12C also dominates, and the 13C isotope with

nuclear spin I = 1/2 occurs in the crystal lattice with a probability of 1%.

The physical properties of the MU center are determined by its structure. Let us briefly list the most important of them. As follows from the results of numerous experiments, the spin wave function in the ground orbital state is concentrated in the vacancy region. In this case, the paramagnetic ground state of the center with a strong polarization of the electron spin (^ = 1, w, = 0, +1, -1) is inherent only in the ME- form. The center actively absorbs green light at a wavelength of X = 532 nm and demonstrates stable fluorescence in the red wavelength range X ~ 630-800 nm with a peak of the zero phonon line at X = 637 nm. Spectroscopic measurements indicate long times of spin relaxation (τ1 ~ 1 ms) and dephasing (τ2 ~ 10 μs) at room temperature. A very important circumstance is the spin-dependent nature of fluorescence, which makes it possible to measure and initialize the electron spin by exciting optical transitions. A theoretical explanation of these and other properties of the ME center, which requires a detailed analysis of its structure, will be given below. We add that fluorescence from single centers can be observed visually using a conventional optical confocal microscope. The first such observation refers to 1997 (see Fig. 1b).

The data accumulated to date make it possible to assert that ME centers satisfy the above requirements and can be considered as qubits. Thus, paramagnetism of a negatively charged center means the splitting of the spin multiplet in the absence of an external magnetic field and makes it possible to separate sublevels with w = 0 and w = -1 (or +1) into a logical subspace. The magnitude of the splitting for the ground orbital state is = 2.87 GHz, which makes it possible to carry out transitions \ m5 = 0 ^ \ m5 = -1 (+1)) between logical states, that is, to perform one-bit quantum operations by acting on the ME center with a resonant microwave impulse. Long lifetimes of the spin state of a center at room temperatures are also ensured by a large number of such elementary quantum operations. All these facts give reason to consider ME centers as very promising solid-state qubits.

Here are the main experimental results obtained with the use of ME-centers and focused on the processing of quantum information. At present, intensive research is being carried out with the aim of creating an ordered matrix of single centers as a basis for full-scale quantum regions.

Rice. 1. Fragment of the crystal lattice of diamond (a), containing the N ^ center, and the electronic structure of the valence shells of carbon and nitrogen; (b) - the first photographic image of fluorescent N ^ centers in diamond.

strov. Further, coherent operations with single spins (both electronic and nuclear) at room temperature were demonstrated, as well as two- and three-qubit operations on one N ^ center with the participation of the electron spin and nuclear spins of nitrogen and carbon. Among the simplest quantum algorithms, mention should be made of the recently implemented Deutsch-Jozsa algorithm, as well as schemes for generating entangled spin states. Correction of quantum errors is achieved through the use of refocusing techniques adapted from EPR spectroscopy and

A. V. TSUKANOV - 2015