Quantum optics. Thermal radiation.

Frame half-timbered houses

Characteristics of thermal radiation:

The glow of bodies, i.e. the emission of electromagnetic waves by bodies, can be achieved through various mechanisms.

Thermal radiation is the emission of electromagnetic waves due to the thermal movement of molecules and atoms. During thermal motion, atoms collide with each other, transfer energy, go into an excited state, and when transitioning to the ground state, they emit an electromagnetic wave.

Thermal radiation is observed at all temperatures other than 0 degrees. Kelvin, at low temperatures long infrared waves are emitted, and at high temperatures visible waves and UV waves are emitted. All other types of radiation are called luminescence.

Let's place the body in a shell with an ideal reflective surface and pump out the air from the shell. (Fig. 1). Radiations leaving the body are reflected from the walls of the shell and are again absorbed by the body, i.e. there is a constant exchange of energy between the body and the radiation. In an equilibrium state, the amount of energy emitted by a body with a unit volume is in units. time is equal to the energy absorbed by the body. If the balance is disturbed, processes arise that restore it. For example: if a body begins to emit more energy than it absorbs, then the internal energy and temperature of the body decrease, which means it emits less and the decrease in body temperature occurs until the amount of energy emitted becomes equal to the amount received. Only thermal radiation is equilibrium. Energy luminosity - , where shows what it depends on (

- temperature).
Energy luminosity is the energy emitted per unit. area in units time.
. The radiation may be different according to spectral analysis, therefore
- spectral density of energy luminosity:

is the energy emitted in the frequency range
is the energy emitted in the wavelength range

per unit area per unit time.
;
Then
- used in theoretical conclusions, and
- experimental dependence.
corresponds
, That's why

Then
, because
, That
.

. The “-” sign indicates that if the frequency increases, the wavelength decreases. Therefore, we discard “-” when substituting
.

Absolutely black body – This is a body that absorbs all radiation incident on it at any frequency and temperature.
. A gray body is a body whose spectral absorption capacity is less than 1, but is the same for all frequencies
. For all other bodies
, depends on frequency and temperature.

And
depends on: 1) body material 2) frequency or wavelength 3) surface condition and temperature.

Kirchhoff's law.

Between the spectral density of energetic luminosity (
) and spectral absorptivity (
) for any body there is a connection.

Let us place several different bodies in the shell at different temperatures, pump out the air and maintain the shell at a constant temperature T. The exchange of energy between the bodies and the bodies and the shell will occur due to radiation. After some time, the system will go into an equilibrium state, that is, the temperature of all bodies is equal to the temperature of the shell, but the bodies are different, so if one body radiates in units. time, more energy then it must absorb more than the other in order for the temperature of the bodies to be the same, which means
- refers to different bodies.

Kirchhoff's law: the ratio of the spectral density of energetic luminosity and spectral absorptivity for all bodies is the same function of frequency and temperature - this is the Kirchhoff function. Physical meaning of the function: for a completely black body
therefore, from Kirchhoff's law it follows that
for an absolutely black body, that is, the Kirchhoff function is the spectral density of the energy luminosity of an absolutely black body. The energetic luminosity of a black body is denoted by:
corresponds
Since the Kirchhoff function is a universal function for all bodies, the main task is thermal radiation, experimental determination of the type of Kirchhoff function and the determination of theoretical models that describe the behavior of these functions.

There are no absolutely black bodies in nature; soot, velvet, etc. are close to them. You can obtain a black body model experimentally, for this we take a shell with a small hole, light enters it and is repeatedly reflected and absorbed with each reflection from the walls, so the light either does not come out, or a very small amount, i.e. such a device behaves in relation to absorption, it is an absolutely black body, and according to Kirchhoff’s law, it emits as a black body, that is, by experimentally heating or maintaining the shell at a certain temperature, we can observe the radiation coming out of the shell. Using a diffraction grating, we decompose the radiation into a spectrum and, by determining the intensity and radiation in each region of the spectrum, the dependence was determined experimentally
(gr. 1). Features: 1) The spectrum is continuous, i.e. all possible wavelengths are observed. 2) The curve passes through a maximum, that is, the energy is distributed unevenly. 3) With increasing temperature, the maximum shifts towards shorter wavelengths.

Let us explain the black body model with examples, that is, if the shell is illuminated from the outside, the hole appears black against the background of luminous walls. Even if the walls are made black, the hole is still darker. Let the surface of the white porcelain be heated and the hole will clearly stand out against the background of the faintly glowing walls.

Stefan-Boltzmann law

After conducting a series of experiments with various bodies, we determine that the energy luminosity of any body is proportional to
. Boltzmann found that the energy luminosity of a black body is proportional to
and wrote it down.
- Stefan-Boltzmann Institute.

Boltzmann's constant.
.

Wine's Law.

In 1893 Vin received -
- Wien's law.
;
;
;, because
. Let's substitute:
;


;
.
, Then
,
- function from
, i.e.
- solution of this equation relative to
there will be some number at
;
from the experiment it was determined that
- constant Guilt.

Wien's law of displacement.

formulation: this wavelength corresponding to the maximum spectral density of the energy luminosity of an absolutely black body is inversely proportional to temperature.

Rayleigh formula-Jeans.

Definitions: Energy flow is the energy transferred through the site per unit time.
. Energy flux density is the energy transferred through a unit area per unit time
. Volumetric energy density is the energy per unit volume
. If the wave propagates in one direction, then through the area
during
the energy transferred is contained in the volume of the cylinder equal to
(Fig. 2) then

. Let's consider thermal radiation in a cavity with absolutely black walls, then 1) all radiation incident on the walls is absorbed. 2) Energy flux density is transferred through each point inside the cavity in any direction
(Fig. 3). Rayleigh and Jeans considered thermal radiation in a cavity as a superposition of standing waves. It can be shown that infinitesimal
emits a radiation flux into the cavity into the hemisphere
.
.

The energetic luminosity of a black body is the energy emitted from a unit area per unit time, which means that the flux of energy radiation is equal to:
,
; Equated

;
is the volumetric energy density per frequency interval
. Rayleigh and Jeans used the thermodynamic law of uniform distribution of energy over degrees of freedom. A standing wave has degrees of freedom and for each oscillating degree of freedom there is energy
. The number of standing waves is equal to the number of standing waves in the cavity. It can be shown that the number of standing waves per unit volume and per frequency interval
equals
here it is taken into account that 2 waves with mutually perpendicular orientation can propagate in one direction
.

If the energy of one wave is multiplied by the number of standing waves per unit volume of the cavity per frequency interval
we get the volumetric energy density per frequency interval
.
. Thus
we'll find it from here
for this
And
. Let's substitute
. Let's substitute
V
, Then
- Rayleigh-Jeans formula. The formula describes well the experimental data in the long wavelength region.

(gr. 2)
;
and the experiment shows that
. According to the Rayleigh-Jeans formula, the body only radiates and thermal interaction between the body and radiation does not occur.

Planck's formula.

Planck, like Rayleigh-Jeans, considered thermal radiation in a cavity as a superposition of standing waves. Also
,
,
, but Planck postulated that radiation does not occur continuously, but is determined in portions - quanta. The energy of each quantum takes on the values
,those
or the energy of a harmonic oscillator takes discrete values. A harmonic oscillator is understood not only as a particle performing a harmonic oscillation, but also as a standing wave.

For determining
the average value of energy takes into account that energy is distributed depending on frequency according to Boltzmann's law, i.e. the probability that a wave with a frequency takes the energy value equal to
,
, Then







.

;
,
.

- Planck's formula.

;
;


. The formula fully describes the experimental dependence
and all the laws of thermal radiation follow from it.

Corollaries from Planck's formula.

;

1)
Low frequencies and high temperatures

;
;
- Rayleigh Jeans.

2)
High frequencies and low temperatures
;
and that's almost
- Wine's Law. 3)


- Stefan-Boltzmann law.

4)
;
;
;
- this transcendental equation, solving it using numerical methods, we obtain the root of the equation
;
- Wien's law of displacement.

Thus, the formula completely describes the dependence
and all the laws of thermal radiation do not follow.

Application of the laws of thermal radiation.

It is used to determine the temperatures of hot and self-luminous bodies. For this purpose pyrometers are used. Pyrometry is a method that uses the dependence of the energy dependence of bodies on the rate of glow of hot bodies and is used for light sources. For tungsten, the share of energy in the visible part of the spectrum is significantly greater than for a black body at the same temperature.

Light- electromagnetic radiation with wave and quantum properties.

Quantum– particle (corpuscle).

Wave properties.

Light is a transverse electromagnetic wave ().

, E 0 , H 0 - amplitude values,
- circle. Cycle. frequency,
- frequency. Fig.1.

V – speed Distribution waves in a given environment. V=C/n, where C is the speed of light (in vacuum C=3*10 8 m/s), n is the refractive index of the medium (depends on the properties of the medium).

, - the dielectric constant, - magnetic permeability.

- wave phase.

The sensation of light is due to the electromagnetic component of the wave ( ).

- wavelength, equal to the path traveled by the wave during the period (
;
).

Visible light range: =0,40.75 µm.

;

4000 - short (purple); 7500 – long (red).

Quantum properties of light.

From point of view quantum theory light is emitted, spreads and absorbed in separate portions - quanta.

Photon characteristics.

1. Mass.
; m 0 - rest mass.

If m 0 0 (photon), then because V=C,m= - nonsense, therefore m 0 =0 is a moving photon. Therefore, the light cannot be stopped.

Therefore, the photon mass must be calculated from relativistic formula for energy. E=mC 2 , m=E/C 2 .

2. Photon energy.E=mC 2 .

In 1900, Max Planck, a German physicist, derived the following formula for photon energy:
.

h=6.62*10 -34 J*s- Planck's constant.

3. Impulse.

p=mV=mC=mC 2 /C=E/C=h/
; p-characteristic of the particle, - characteristics of the wave.

Wave optics. Interference - redistribution. Light in space.

The superposition of light waves, as a result of which the intensity of light increases in some places in space, and weakens in others. That is, there is a redistribution of light intensity in space.

The condition for observing interference is the coherence of light waves (waves that satisfy the condition: -monochromatic waves;
– the phase of the wave is constant at a given point in space over time).

CALCULATION OF INTERFERENCE PATTERNS.

Sources are coherent waves. ; * - exact source.

Dark and light stripe.

1. If l~d, then
the picture is indistinguishable, therefore, in order to see something, you need 2. l<.

At point M, two coherent waves overlap.

, d1,d2 - meters traveled by the waves; -phase difference.

Darker/lighter - intensity.
(proportional).

If the waves are not coherent:
(average value for the period).

(superposition, imposition).

If – coherent:
;

;
-light interference occurs (light redistribution).

; If
(optical wave path difference); n-refractive index; (d2-d1)-geometric difference in wave path; -wavelength (path that the wave travels during a period).

- the basic formula of interference.

Depending on the path , they come with different . Ires depends on the latter.

1. Ires.max.

This condition maximum interference of light, because in this case the waves arrive in the same phase and therefore reinforce each other.

n-multiplicity factor; - means that the interference pattern is symmetrical relative to the center of the screen.

If the phases coincide, then the amplitudes do not depend on the phases.

- Also the maximum condition.

2 . Ires.min.

; k=0,1,2…;
.

- This condition minimum, because in this case, the waves arrive in antiphase and cancel each other.

Methods for producing coherent waves.

The principle of receiving.

To obtain coherent waves, it is necessary to take one source and divide the light wave coming from it into two parts, which are then forced to meet. These waves will be coherent, because will belong to the same moment of radiation, therefore. .

Phenomena used to split a light wave in two.

1. Phenomenon light reflections(Fresnel bead mirrors). Fig.4.

2 . Phenomenon light refraction(Fresnel biprism). Fig.5.

3 . Phenomenon light diffraction.

This is the deviation of light from rectilinear propagation when light passes through small holes or near opaque obstacles, if their dimensions (both) d are commensurate with the wavelength (d~ ). That: Fig.6. - Jung's installation.

In all of these cases, the real light source was a point one. In real life, light can be extended - a section of the sky.

4.
, n is the refractive index of the film.

There are two possible cases:

H=const, then
. In this case, the interference pattern is called an equal-slope fringe.

H const. A parallel beam of rays falls.
.
- strips of equal thickness.

Installation of "Newton's ring".

It is necessary to consider the interference pattern in reflected and refracted light.

The content of the article

QUANTUM OPTICS– a branch of optics that studies the quantum properties of light. We can say that quantum optics is the quantum physics of light. Interest in quantum optics appeared in the first half of the 20th century, but this area of ​​science received particularly intensive development at the end of the 20th century, when physicists learned to prepare special states of light - the so-called non-classical light. Now non-classical light is successfully used in metrology, spectroscopy, and is used for precise measurements, as well as for secret transmission of information. In addition, approaches and methods of quantum optics can significantly supplement the information provided by various measurements related to the emission and absorption of light.

Quanta.

It was for light, and more precisely, for the electromagnetic field, that the idea of ​​a quantum description was first proposed. This idea was put forward in 1900 by Max Planck, who suggested that the emission of light occurs in portions - quanta. This assumption seemed paradoxical to many, but it became life-saving for the entire branch of optics. It made it possible to explain the shape of the radiation spectrum of heated bodies, which could not be explained previously. Previous attempts to calculate the emission spectrum led to the fact that in the region of small wavelengths, i.e. in the ultraviolet part of the spectrum, unlimitedly large values ​​of divergence arose. Of course, no discrepancies were observed in the experiment, and this discrepancy between theory and experiment was called the “ultraviolet catastrophe.” The assumption that light emission occurs in portions made it possible to remove divergences in theoretically calculated spectra and, thereby, save physics from the “ultraviolet catastrophe.”

In addition to the emission spectra, there was still one unclear place in physics, namely, the phenomenon of the photoelectric effect ( cm. PHOTOELECTRIC EFFECT). It was unclear why the kinetic energy of electrons knocked out of a metal by light depends on the frequency of the light. Moreover, light with a sufficiently low frequency is not capable of causing the photoelectric effect at all. Since the low frequency of light corresponds to the red part of the spectrum, this phenomenon is called the red edge of the photoelectric effect. In 1905, Albert Einstein used the quantum hypothesis to explain the photoelectric effect. Einstein's idea was that each electron gets one single portion of energy - one quantum. And if the energy of this quantum is small, it is simply not enough for the electron to exit the metal. Based on this idea, Einstein developed the theory of the photoelectric effect, which was perfectly confirmed by experimental data.

Now it turns out that light is both emitted and absorbed in portions. This led Einstein to propose that light always has a discrete structure. This wonderful idea was only a hypothesis: after all, from the fact that the absorption and emission of light occurs in portions, it does not follow that light exists only in the form of portions. But it is this idea that justifies the name “quantum optics,” and it was with the development of quantum optics that more compelling arguments in favor of the quantum nature of light appeared.

Particles or waves?

At the beginning of the 20th century. Light quanta began to be called photons, and the statement soon became generally accepted: “Light consists of photons.” The idea of ​​light as a flow of corpuscles, i.e. particles, appeared. However, wave phenomena observed for light, such as interference and diffraction, could not be explained in terms of the corpuscular structure of light. It turned out that light, and indeed electromagnetic radiation in general, are waves and at the same time a flow of particles ( cm. QUANTUM MECHANICS). The reconciliation of these two points of view made it possible to develop in the mid-20th century. quantum approach to the description of light. From the point of view of this approach, the electromagnetic field can be in one of various quantum states. In this case, there is only one distinguished class of states with a given number of photons - Fock states, named after V.A. Fock. Therefore, the phrase “light consists of photons” should not be taken literally - so, for example, light can be in such a state that with a 99% probability it contains no photons, and with a 1% probability it contains two photons. This is one of the differences between a photon and other elementary particles - for example, the number of electrons in a limited volume is specified absolutely precisely, and it can be determined by measuring the total charge and dividing by the charge of one electron . The number of photons located in a certain volume of space for some time can be accurately measured in very rare cases, namely, only when the light is in Fock states. An entire section of quantum optics is devoted to various methods of preparing light in various quantum states; in particular, preparing light in Fock states is an important and not always feasible task.

Brown–Twiss experiment.

Single and correlated photons . Could there be non-quantum physics of light? Of course, yes, and in most cases optical phenomena can be explained without the help of quantum theory. But there are many cases where this is not the case and where it is important to consider the quantum nature of light.

The first experiment in quantum optics is believed to be that of Brown and Twiss, performed in 1956. Brown and Twiss showed that if light from some sources was directed at two photodetectors that "clicked" when detecting photons, the receivers would often click simultaneously. In the experiment, Brown and Twiss used radiation from a mercury lamp, and later light from a star. This experiment was considered for quite a long time to be proof of the photonic nature of light: after all, the simultaneity of clicks of the photodetectors means that both of them register portions of light that actually exist, and do not just randomly click from time to time. However, it turns out that when registering light from a mercury lamp or a star, simultaneous clicks occur, at best, only twice as often as would occur with random clicks of photodetectors. This result is fully explainable classically and does not yet prove the photonic structure of light. However, very soon (in the sixties of the 20th century) light sources were discovered, which in such an experiment lead to strictly simultaneous clicks of photodetectors. The simultaneity of some events at different spatial points in physics is usually called correlation. For example, if two friends talk on the phone only to each other, then their phone is always busy at the same time, and we can talk about the correlation of phone calls in their apartments. Accordingly, light that causes two photodetectors to click exactly simultaneously can be called pair-correlated light, or photon grouping. Two-photon light exhibits such properties. On the other hand, there are light sources that never produce simultaneous photodetector clicks. Such light is called photon antibunching light.

Non-classical light.

Experiments to detect light with bunching and antibunching of photons have indeed proven the photonic structure of light, and they can be considered “true quantum optical” experiments. But in both cases, the light was prepared in special quantum states with a given number of photons. In experiments of the first type, two-photon light was recorded, in experiments of the second type, single-photon light was recorded. Thus, we can again come to the conclusion that only in special states does light exhibit properties that cannot be explained from classical positions. Such states of light are called non-classical.

Two-photon light has another remarkable property. It turned out that such light can be used to experimentally test the basic idea of ​​quantum mechanics - the idea of ​​the probabilistic behavior of individual quantum particles ( cm. BELL'S INEQUALITIES).

What non-classical states of light can be prepared in laboratories today? It turns out there are very few species. Physicists know how to prepare one-photon light and two-photon light with an admixture of a vacuum state, i.e. states of light without photons. What does this mean? In the case of single-photon light, this means that even an ideal photodetector turned on at a certain moment will not necessarily register the photon; it will click only with some probability. (An ideal photodetector is a receiver that operates with a 100% probability if there is a photon at the input.) However, the photodetector will never register two photons, even if it is, in principle, capable of distinguishing one photon from two. Likewise, triplets of photons, quadruples of photons, etc. will not be registered. Accordingly, if a photodetector (or pair of photodetectors) detects a mixture of vacuum and two-photon states, clicks will occur only in pairs, but at random times. Triples, quadruples of photons, etc. will also not be registered.

Single-photon light can be prepared without the admixture of a vacuum state - in this case, the moments when you need to turn on the photodetector will be known exactly, and it will click with 100% probability. But experimenters do not know how to prepare three-photon and, even more so, four-photon light even with an admixture of vacuum!

And finally, the last of the “available” types of non-classical light is the so-called squeezed light, such light contains only an even number of photons, and when it is registered, photodetectors can detect pairs of photons, fours, sixes, etc., but never triplets, quints and other odd numbers of photons.

Applications of non-classical light.

Nonclassical light attracts the attention of physicists not only as an interesting object of study. It turns out to be very useful in terms of various applications. Thus, two-photon light is used to accurately calibrate photodetectors. Each photodetector is imperfect, i.e. triggers with a probability of less than 100%. This probability is called the quantum efficiency of the photodetector. Calibrating a photodetector is a measurement of its quantum efficiency; Previously, reference light sources or receivers were used for this, and this made the measurement not very accurate. However, two-photon light makes it possible to do without such standards. Indeed, if two photodetectors detect two-photon light, then ideally they should always click simultaneously. In reality, the number of simultaneous clicks will be less than the number of clicks of any of the photodetectors. By dividing the number of simultaneous clicks by the number of clicks of one of the photodetectors, we can obtain the quantum efficiency of the second photodetector. In this case, no standards are required, and the measurement accuracy can be significantly improved compared to traditional methods.

Squeezed light, like two-photon light, is useful for making precise measurements. Its use makes it possible to reduce experimental errors associated with quantum uncertainty. It is known that quantum objects most often do not have precisely specified parameters; their properties can be called “smeared”, just as their position in space is “smeared”. In high-precision measurements, when experimental errors are reduced to a minimum, this smearing of properties becomes a fundamental limitation on the accuracy of measurements. Using compressed light circumvents this difficulty and reduces blur at certain times.

Finally, one of the latest applications of non-classical light is the secret transmission of information (quantum cryptography). For this, it is most convenient to use single-photon light. The idea of ​​quantum cryptography is to transmit information in individual photons. For example, the numbers 0 and 1 are encoded by the polarization of photons: a vertically polarized photon is denoted by “0”, and a horizontally polarized photon is denoted by “1”. Such transmission of information will be secret because it cannot be “overheard.” Any eavesdropper can only intercept some photons entirely - after all, he cannot split off part of a photon and thus find out its polarization. But the intercepted photons simply will not participate in the transmission of information, so the information transmitted by individual quanta is protected from eavesdropping.

Maria Chekhova

Definition 1

Quantum optics is a branch of optics whose main task is the study of phenomena in which the quantum properties of light can manifest themselves.

Such phenomena may be:

• photoelectric effect;
• thermal radiation;
• Raman effect;
• Compton effect;
• stimulated emission, etc.

Fundamentals of Quantum Optics

Unlike classical optics, quantum optics represents a more general theory. The main problem it addresses is to describe the interaction of light with matter, while taking into account the quantum nature of objects. Quantum optics also deals with the description of the process of light propagation under special (specific conditions).

A more accurate solution to such problems requires a description of both matter (including the propagation medium) and light exclusively from the position of the existence of quanta. At the same time, scientists often simplify the task when describing it when one of the components of the system (for example, a substance) is described in the format of a classical object.

Often in calculations, for example, only the state of the active medium is quantized, while the resonator is considered classical. However, if its length is an order of magnitude higher than the wavelength, it can no longer be considered classical. The behavior of an excited atom placed in such a resonator will be more complex.

The tasks of quantum optics are aimed at studying the corpuscular properties of light (that is, its photons and corpuscular particles). According to M. Planck's hypothesis about the properties of light, proposed in 1901, it is absorbed and emitted only in separate portions (photons, quanta). A quantum represents a material particle with a certain mass $m_ф$, energy $E$ and momentum $p_ф$. Then the formula is written:

Where $h$ represents Planck's constant.

$v=\frac(c)(\lambda)$

Where $\lambda$ is the frequency of light

$c$ will be the speed of light in vacuum.

The main optical phenomena explained by quantum theory include light pressure and the photoelectric effect.

Photoelectric effect and light pressure in quantum optics

Definition 2

The photoelectric effect is a phenomenon of interactions between photons of light and matter, in which the radiation energy will be transferred to the electrons of the substance. There are such types of photoelectric effect as internal, external and valve.

The external photoelectric effect is characterized by the release of electrons from the metal at the moment of its irradiation with light (at a certain frequency). The quantum theory of the photoelectric effect states that each act of absorption of a photon by an electron occurs independently of the others.

An increase in radiation intensity is accompanied by an increase in the number of incident and absorbed photons. When energy is absorbed by a substance of frequency $ν$, each of the electrons turns out to be capable of absorbing only one photon, while taking away energy from it.

Einstein, applying the law of conservation of energy, proposed his equation for the external photoelectric effect (an expression of the law of conservation of energy):

$hv=A_(out)+\frac(mv^2)(2)$

$A_(out)$ is the work function of an electron leaving the metal.

The kinetic energy of the emitted electron is obtained by the formula:

$E_k=\frac(mv^2)(2)$

From Einstein’s equation it turns out that if $E_k=0$, then it is possible to obtain the very minimum frequency (red limit of the photoelectric effect) at which it will be possible:

$v_0 = \frac (A_(out)) h$

The pressure of light is explained by the fact that, as particles, photons have a certain momentum, which they transfer to the body through the process of absorption and reflection:

Such a phenomenon as light pressure is also explained by the wave theory, according to which (if we refer to de Broglie’s hypothesis), any particle also has wave properties. The relationship between the momentum $P$ and the wavelength $\lambda$ is shown by the equation:

$P=\frac(h)(\lambda)$

Compton effect

Note 1

The Compton effect is characterized by incoherent scattering of photons by free electrons. The very concept of incoherence means the non-interference of photons before and after scattering. The effect changes the frequency of photons, and after scattering the electrons receive part of the energy.

The Compton effect provides experimental evidence of the manifestation of the corpuscular properties of light as a stream of particles (photons). The phenomena of the Compton effect and the photoelectric effect are important proof of quantum concepts of light. At the same time, phenomena such as diffraction, interference, and polarization of light confirm the wave nature of light.

The Compton effect represents one of the proofs of wave-particle duality of microparticles. The law of conservation of energy is written as follows:

$m_ec^2+\frac(hc)(\lambda)=\frac(hc)(\lambda)+\frac(m_ec^2)(scrt(1-\frac(v^2)(c^2)) )$

The inverse Compton effect represents an increase in the frequency of light when scattered by relativistic electrons with higher than photon energy. In this interaction, energy is transferred to the photon from the electron. The energy of scattered photons is determined by the expression:

$e_1=\frac(4)(3)e_0\frac(K)(m_ec^2)$

Where $e_1$ and $e_0$ are the energies of the scattered and incident photons, respectively, and $k$ is the kinetic energy of the electron.

QUANTUM OPTICS, a branch of optics in which the principles of quantum mechanics (wave-particle duality, state vectors, Heisenberg and Schrödinger ideas, etc.) are used to study the properties of light and its interaction with matter.

The origin of the quantum theory of light dates back to 1900, when M. Plath, to explain the spectral distribution of electromagnetic energy emitted by a thermal source, postulated the absorption and emission of it in discrete portions. The idea of ​​discreteness formed the basis for the derivation of the formula that bears his name and served as the impetus for the creation of quantum mechanics. However, it remained unclear whether the source of discreteness was the matter or the light itself. In 1905, A. Einstein published the theory of the photoelectric effect, in which he showed that it can be explained if light is considered as a stream of particles (light quanta), later called photons. Photons have energy E = hv (h is Planck's constant, v is the frequency of light) and propagate at the speed of light. Later, N. Bohr showed that atoms can emit light in discrete portions. Thus, light is considered both as an electromagnetic wave and as a stream of photons. A quantized light field is a statistical object and its state is determined in a probabilistic sense.

The creation in 1960 of a laser - a fundamentally new source of radiation compared to thermal one - stimulated research into the statistical characteristics of its radiation. These studies involve measuring the distribution of laser photons and field coherence. Non-laser light sources are essentially sources of random light fields with Gaussian field statistics. While studying the statistics of laser radiation, R. Glauber introduced the concept of a coherent state, which corresponds well to the radiation of a laser operating in a regime above the lasing threshold. In 1977, the American physicist J. Kimble first observed the so-called antibunching of photons (see below), which could be explained using quantum theory.

Since the end of the 20th century, quantum optics has been intensively developing. It is closely related to nonlinear and atomic optics, quantum information theory. One of the most convenient ways to determine the state of the light field is to measure correlation functions. The simplest of them is the field correlation function, which characterizes the connection of fields at different spatiotemporal points. It fully characterizes the field of a thermal radiation source, but does not allow one to distinguish sources with other statistical properties from thermal ones. In this regard, an important role is played by the correlation function of the number of photons (intensities) of the second order g (2) (τ), which contains information on the distribution of delay times τ of photon emission. It is used to measure the effects of bunching and antibunching of photons. Light from the source enters the beam splitter plate (Fig. 1), after which it is fed to two photodetectors. Registration of a photon is accompanied by the appearance of a pulse at the detector output. Pulses from the detectors enter a device that measures the delay time between them. The experiment is repeated many times. In this way, the distribution of delay times, which is associated with the function g (2) (τ), is measured. Figure 2 shows the dependence g (2) (τ) for three typical light sources - thermal, laser and resonant fluorescence. As τ → ∞, the values ​​of the functions for the thermal source and resonant fluorescence approach unity. For laser radiation g (2) (τ) = 1 and photon statistics are Poisson. For a thermal source g(2)(0) = 2 and it is more likely to detect two photons arriving immediately after each other (photon grouping effect). In the case of resonant fluorescence, the probability of an atom emitting two photons at once is zero (photon antibunching). The value g (2) (0) = 0 is due to the fact that there is a delay time between two successive acts of photon emission by one atom. This effect is explained by the complete quantum theory, which describes both the medium and the electromagnetic field from a quantum point of view.

Closely related to the antibunching effect is sub-Poisson photon statistics, for which the distribution function is narrower than the Poisson distribution. Therefore, the level of fluctuations in photon beams with sub-Poisson statistics is less than the level of fluctuations of coherent radiation. In the limiting case, such nonclassical fields have a strictly defined number of photons (the so-called Fock state of the field). In quantum theory, the number of photons is a discrete variable.

Nonlinear optics methods can be used to create non-classical light fields in which, compared to coherent fields, the level of quantum fluctuations of some continuous variables, for example, quadrature components or Stokes parameters characterizing the state of field polarization, is reduced. Such fields are called compressed. The formation of compressed fields can be interpreted in classical language. Let us express the electric field strength E through the quadrature components a and b: E(t) = a(t)cosωt + b(t)sinωt, where a(t) and b(t) are random functions, ω = 2πν is the circular frequency, t - time. By applying such a field to a degenerate optical parametric amplifier (OPPA) with a pump frequency of 2ω, one quadrature component (for example, a) can be amplified due to its phase sensitivity, and the other quadrature (b) can be suppressed. As a result, fluctuations in quadrature a increase, and in quadrature b decrease. The transformation of the noise level in the VOPU is shown in Figure 3. In Figure 3, b, the area of ​​fluctuations is compressed compared to the input state (Figure 3, a). Quantum fluctuations of the vacuum and coherent states behave in a similar way under parametric amplification. Of course, in this case the quantum-mechanical uncertainty relation is not violated (there is, as it were, a redistribution of fluctuations between quadratures). In parametric processes, as a rule, radiation is formed with super-Poisson photon statistics, for which the level of fluctuations exceeds that for coherent light.

To record compressed fields, balanced homodyne detectors are used, which can record only one quadrature. Thus, the level of fluctuations during photodetection of compressed light can be below the level of the standard quantum limit (shot noise) corresponding to the detection of coherent light. In squeezed light, fluctuations can be suppressed by up to 90% relative to the coherent state. Nonlinear optics methods also produce polarization-squeezed light in which fluctuations in at least one of the Stokes parameters are suppressed. Compressed light is of interest for precision optical-physical experiments, in particular for recording gravitational waves.

From a quantum point of view, the considered parametric process is the process of decay of a pump photon with a frequency of 2ω into two photons with a frequency of ω. In other words, photons in compressed light are created in pairs (biphotons), and their distribution function is radically different from Poisson (there are only an even number of photons). This is another unusual property of compressed light in the language of discrete variables.

If pump photons in a parametric process decay into two photons that differ in frequencies and/or polarizations, then such photons are correlated (connected) with each other. Let us denote the frequencies of the generated photons as ω 1 and ω 2, and let the photons have vertical (V) and horizontal (H) polarizations, respectively. The state of the field in this case is written in quantum language as |ψ) = |V) 1 |H) 2. It turns out that at a certain orientation of a nonlinear optical crystal in which a parametric process is observed, photons of the same frequency propagating in the same direction can be produced with orthogonal polarizations. As a result, the field state takes the form:

(*)

(The appearance of the coefficient in front of the bracket is due to the normalization condition.)

The state of photons described by the relation (*) is called entangled; this means that if a photon of frequency ω 1 is polarized vertically, then a photon of frequency ω 2 is horizontally polarized, and vice versa. An important property of the entangled state (*) is that measuring the polarization state of one photon projects the state of a photon of another frequency into an orthogonal one. States of type (*) are also called Einstein-Podolsky-Rosen pairs and entangled Bell states. The quantum states of atomic systems, as well as the states of atoms and photons, can be in an entangled state. Experiments have been carried out using photons in entangled states to test Bell's inequality, quantum teleportation and quantum dense coding.

Based on parametric optical interactions, as well as the effect of cross-interactions, quantum non-destructive measurements of the quadrature components and the number of photons, respectively, were carried out. The use of quantum optics methods in processing optical images makes it possible to improve their recording, storage and reading (see Quantum image processing).

Quantum fluctuations of the electromagnetic field in a vacuum state can manifest themselves in a unique way: they lead to the appearance of an attractive force between conducting uncharged plates (see Casimir effect).

Quantum optics also includes the theory of fluctuations of laser radiation. Its consistent development is based on quantum theory, which gives correct results for photon statistics and laser radiation linewidth.

Quantum optics also studies the interaction of atoms with a light field, the effect of light on two- and three-level atoms. At the same time, a number of interesting and unexpected effects associated with atomic coherence were discovered: quantum beats (see Interference of states), Hanle effect, photon echo, etc.

Quantum optics also studies the cooling of atoms when interacting with light and the production of a Bose-Einstein condensate, as well as the mechanical effect of light on atoms for the purpose of capturing and controlling them.

Lit.: Klyshko D.N. Non-classical light // Advances in physical sciences. 1996. T. 166. Issue. 6; Bargatin I.V., Grishanin B.A., Zadkov V.N. Entangled quantum states of atomic systems // Ibid. 2001. T. 171. Issue. 6; Physics of Quantum Information / Edited by D. Bouwmeister et al. M., 2002; Scully M. O., Zubairi M. S. Quantum optics. M., 2003; Shleikh V. P. Quantum optics in phase space. M., 2005.