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Fractional quantum mechanics

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In physics, fractional quantum mechanics is a generalization of standard quantum mechanics, which comes out when the Brownian-like quantum paths substitute with the Lévy-like ones in the Feynman path integral. This concept was discovered by Nick Laskin who coined the term fractional quantum mechanics.[1]

Fundamentals[edit]

Standard quantum mechanics can be approached in three different ways: the matrix mechanics, the Schrödinger equation and the Feynman path integral.

The Feynman path integral[2] is the path integral over Brownian-like quantum-mechanical paths. Fractional quantum mechanics comes from generalizing the Feynman path integral, from the Brownian-like to the Lévy-like quantum mechanical paths. A path integral over the Lévy-like quantum-mechanical paths results in a generalization of quantum mechanics.[3] If the Feynman path integral leads to the well known Schrödinger equation, then the path integral over Lévy trajectories leads to the fractional Schrödinger equation.[4] The Lévy process is characterized by the Lévy index α, 0 < α ≤ 2. At the special case when α = 2 the Lévy process becomes the process of Brownian motion. The fractional Schrödinger equation includes a space derivative of fractional order α instead of the second order (α = 2) space derivative in the standard Schrödinger equation. Thus, the fractional Schrödinger equation is a fractional differential equation in accordance with modern terminology.[5]

Fractional Schrödinger equation[edit]

The fractional Schrödinger equation in the form originally obtained by Nick Laskin is:[6]

Further,

  • Dα is a scale constant with physical dimension [Dα] = [energy]1 − α·[length]α[time]α, at α = 2, D2 =1/2m, where m is a particle mass,
  • the operator (−ħ2Δ)α/2 is the 3-dimensional fractional quantum Riesz derivative defined by (see, Ref.[2]);

Here, the wave functions in the position and momentum spaces; and are related each other by the 3-dimensional Fourier transforms:

The index α in the fractional Schrödinger equation is the Lévy index, 1 < α ≤ 2. Thus, the fractional Schrödinger equation includes a space derivative of fractional order α instead of the second order (α = 2) space derivative in the standard Schrödinger equation. Thus, the fractional Schrödinger equation is a fractional differential equation in accordance with modern terminology.[7] This is the main point of the term fractional Schrödinger equation or a more general term fractional quantum mechanics.[8] At α = 2 fractional Schrödinger equation becomes the well-known Schrödinger equation.

The fractional Schrödinger equation has the following operator form

where the fractional Hamilton operator is given by

The Hamilton operator, corresponds to the classical mechanics Hamiltonian function introduced by Nick Laskin

where p and r are the momentum and the position vectors respectively.

Time-independent fractional Schrödinger equation[edit]

The special case when the Hamiltonian is independent of time

is of great importance for physical applications. It is easy to see that in this case there exist the special solution of the fractional Schrödinger equation

where satisfies

or

This is the time-independent fractional Schrödinger equation (see, Ref.[2]).

Thus, we see that the wave function oscillates with a definite frequency. In classical physics the frequency corresponds to the energy. Therefore, the quantum mechanical state has a definite energy E. The probability to find a particle at is the absolute square of the wave function Because of time-independent fractional Schrödinger equation this is equal to and does not depend upon the time. That is, the probability of finding the particle at is independent of the time. One can say that the system is in a stationary state. In other words, there is no variation in the probabilities as a function of time.

Probability current density[edit]

The conservation law of fractional quantum mechanical probability has been discovered for the first time by D.A.Tayurskii and Yu.V. Lysogorski [9]

where is the quantum mechanical probability density and the vector can be called by the fractional probability current density vector

and

here we use the notation (see also matrix calculus): .

It has been found in Ref.[5] that there are quantum physical conditions when the new term is negligible and we come to the continuity equation for quantum probability current and quantum density (see, Ref.[2]):

Introducing the momentum operator we can write the vector in the form (see, Ref.[2])

This is fractional generalization of the well-known equation for probability current density vector of standard quantum mechanics (see, Ref.[7]).

Velocity operator[edit]

The quantum mechanical velocity operator is defined as follows:

Straightforward calculation results in (see, Ref.[2])

Hence,

To get the probability current density equal to 1 (the current when one particle passes through unit area per unit time) the wave function of a free particle has to be normalized as

where is the particle velocity, .

Then we have

that is, the vector is indeed the unit vector.

Physical applications[edit]

Fractional Bohr atom[edit]

When is the potential energy of hydrogenlike atom,

where e is the electron charge and Z is the atomic number of the hydrogenlike atom, (so Ze is the nuclear charge of the atom), we come to following fractional eigenvalue problem,

This eigenvalue problem has first been introduced and solved by Nick Laskin in.[10]

Using the first Niels Bohr postulate yields

and it gives us the equation for the Bohr radius of the fractional hydrogenlike atom

Here a0 is the fractional Bohr radius (the radius of the lowest, n = 1, Bohr orbit) defined as,

The energy levels of the fractional hydrogenlike atom are given by

where E0 is the binding energy of the electron in the lowest Bohr orbit that is, the energy required to put it in a state with E = 0 corresponding to n = ∞,

The energy (α − 1)E0 divided by ħc, (α − 1)E0/ħc, can be considered as fractional generalization of the Rydberg constant of standard quantum mechanics. For α = 2 and Z = 1 the formula is transformed into

,

which is the well-known expression for the Rydberg formula.

According to the second Niels Bohr postulate, the frequency of radiation associated with the transition, say, for example from the orbit m to the orbit n, is,

.

The above equations are fractional generalization of the Bohr model. In the special Gaussian case, when (α = 2) those equations give us the well-known results of the Bohr model.[11]

The infinite potential well[edit]

A particle in a one-dimensional well moves in a potential field , which is zero for and which is infinite elsewhere,

 

 

 

 

(i)

 

 

 

 

(ii)

 

 

 

 

(iii)

It is evident a priori that the energy spectrum will be discrete. The solution of the fractional Schrödinger equation for the stationary state with well-defined energy E is described by a wave function , which can be written as

where , is now time independent. In regions (i) and (iii), the fractional Schrödinger equation can be satisfied only if we take . In the middle region (ii), the time-independent fractional Schrödinger equation is (see, Ref.[6]).

This equation defines the wave functions and the energy spectrum within region (ii), while outside of the region (ii), x < −a and x > a, the wave functions are zero. The wave function has to be continuous everywhere, thus we impose the boundary conditions for the solutions of the time-independent fractional Schrödinger equation (see, Ref.[6]). Then the solution in region (ii) can be written as

To satisfy the boundary conditions we have to choose

and

It follows from the last equation that

Then the even ( under reflection ) solution of the time-independent fractional Schrödinger equation in the infinite potential well is

The odd ( under reflection ) solution of the time-independent fractional Schrödinger equation in the infinite potential well is

The solutions and have the property that

where is the Kronecker symbol and

The eigenvalues of the particle in an infinite potential well are (see, Ref.[6])

It is obvious that in the Gaussian case (α = 2) above equations are ö transformed into the standard quantum mechanical equations for a particle in a box (for example, see Eq.(20.7) in [12])

The state of the lowest energy, the ground state, in the infinite potential well is represented by the at n=1,

and its energy is

Fractional quantum oscillator[edit]

Fractional quantum oscillator introduced by Nick Laskin (see, Ref.[2]) is the fractional quantum mechanical model with the Hamiltonian operator defined as

where q is interaction constant.

The fractional Schrödinger equation for the wave function of the fractional quantum oscillator is,

Aiming to search for solution in form

we come to the time-independent fractional Schrödinger equation,

The Hamiltonian is the fractional generalization of the 3D quantum harmonic oscillator Hamiltonian of standard quantum mechanics.

Energy levels of the 1D fractional quantum oscillator in semiclassical approximation[edit]

The energy levels of 1D fractional quantum oscillator with the Hamiltonian function were found in semiclassical approximation (see, Ref.[2]).

We set the total energy equal to E, so that

whence

At the turning points . Hence, the classical motion is possible in the range .

A routine use of the Bohr-Sommerfeld quantization rule yields

where the notation means the integral over one complete period of the classical motion and is the turning point of classical motion.

To evaluate the integral in the right hand we introduce a new variable . Then we have

The integral over dy can be expressed in terms of the Beta-function,

Therefore,

The above equation gives the energy levels of stationary states for the 1D fractional quantum oscillator (see, Ref.[2]),

This equation is generalization of the well-known energy levels equation of the standard quantum harmonic oscillator (see, Ref.[7]) and is transformed into it at α = 2 and β = 2. It follows from this equation that at the energy levels are equidistant. When and the equidistant energy levels can be for α = 2 and β = 2 only. It means that the only standard quantum harmonic oscillator has an equidistant energy spectrum.

Fractional quantum mechanics in solid state systems[edit]

The effective mass of states in solid state systems can depend on the wave vector k, i.e. formally one considers m=m(k). Polariton Bose-Einstein condensate modes are examples of states in solid state systems with mass sensitive to variations and locally in k fractional quantum mechanics is experimentally feasible [1].

Self-accelerating beams[edit]

Self-accelerating beams, such as the Airy beam, are known solutions of the conventional free Schrödinger equation (with and without a potential term). Equivalent solutions exist in the free fractional Schrödinger equation. The time-dependent fractional Schrödinger equation in momentum space (assuming and with one spatial coordinate) is:

.

In position space, an Airy beam is typically expressed using the special Airy function, although it possesses a more transparent expression in momentum space:

Here, the exponential function ensures the square-integrability of the wave function, i.e. that the beam possesses a finite energy, in order to be a physical solution. The parameter controls the exponential cut-off on the beam’s tail, while the parameter controls the width of the peaks in position space. The Airy beam solution for the fractional Schrödinger equation in momentum space is obtained from simple integration of the above equation and initial condition:

This solution self-accelerates at a rate proportional to .[13] When taking for the conventional Schrödinger equation, one recovers the original Airy beam solution with a parabolic acceleration ().

See also[edit]

References[edit]

  1. Laskin, Nikolai (2000). "Fractional quantum mechanics and Lévy path integrals". Physics Letters A. 268 (4–6): 298–305. arXiv:hep-ph/9910419. doi:10.1016/S0375-9601(00)00201-2.
  2. R. P. Feynman and A. R. Hibbs, Quantum Mechanics and Path Integrals ~McGraw-Hill, New York, 1965
  3. Laskin, Nick (1 August 2000). "Fractional quantum mechanics". Physical Review E. American Physical Society (APS). 62 (3): 3135–3145. arXiv:0811.1769. Bibcode:2000PhRvE..62.3135L. doi:10.1103/physreve.62.3135. ISSN 1063-651X.
  4. Laskin, Nick (18 November 2002). "Fractional Schrödinger equation". Physical Review E. American Physical Society (APS). 66 (5): 056108. arXiv:quant-ph/0206098. Bibcode:2002PhRvE..66e6108L. doi:10.1103/physreve.66.056108. ISSN 1063-651X. PMID 12513557.
  5. S. G. Samko, A. A. Kilbas, and O. I. Marichev, Fractional Integrals and Derivatives, Theory and Applications ~Gordon and Breach, Amsterdam, 1993
  6. Laskin, Nick (18 November 2002). "Fractional Schrödinger equation". Physical Review E. 66 (5): 056108. arXiv:quant-ph/0206098. doi:10.1103/physreve.66.056108. ISSN 1063-651X. PMID 12513557.
  7. S. G. Samko, A. A. Kilbas, and O. I. Marichev, Fractional Integrals and Derivatives, Theory and Applications ~Gordon and Breach, Amsterdam, 1993
  8. Laskin, Nick (1 August 2000). "Fractional quantum mechanics". Physical Review E. American Physical Society (APS). 62 (3): 3135–3145. arXiv:0811.1769. doi:10.1103/physreve.62.3135. ISSN 1063-651X. PMID 11088808.
  9. Tayurskii, D A; Lysogorskiy, Yu V (29 November 2012). "Superfluid hydrodynamic in fractal dimension space". Journal of Physics: Conference Series. IOP Publishing. 394: 012004. arXiv:1108.4666. doi:10.1088/1742-6596/394/1/012004. ISSN 1742-6588.
  10. Laskin, Nick (2000). "Fractals and quantum mechanics". Chaos: An Interdisciplinary Journal of Nonlinear Science. AIP Publishing. 10 (4): 780–790. doi:10.1063/1.1050284. ISSN 1054-1500. PMID 12779428.
  11. Bohr, N. (1913). "XXXVII. On the constitution of atoms and molecules". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. Informa UK Limited. 26 (153): 476–502. doi:10.1080/14786441308634993. ISSN 1941-5982.
  12. L.D. Landau and E.M. Lifshitz, Quantum mechanics (Non-relativistic Theory), Vol.3, Third Edition, Course of Theoretical Physics, Butterworth-Heinemann, Oxford, 2003
  13. Colas, David (2020). "Self-accelerating beam dynamics in the space fractional Schrödinger equation". Physical Review Research. 2: 033274. arXiv:2006.12743. doi:10.1103/PhysRevResearch.2.033274.

Further reading[edit]


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