†Corresponding author. E-mail: email@example.com
*Project supported by the National Natural Science Foundation of China (Grant No. 11175113), the Fundamental Research Funds for the Central Universities of China (Grant No. WK2060140013), and the Natural Science Foundation of Jiangsu Higher Education Institution of China (Grant No. 14KJD140001).
By virtue of the operator-Hermite-polynomial method, we derive some new generating function formulas of the product of two bivariate Hermite polynomials. Their applications in studying quantum optical states are presented.
In quantum mechanics and mathematical physics, the special function, the Hermite polynomial, is of fundamental importance. Physically, it is the eigenfunction of the harmonic oscillator. In addition, by introducing a convenient complex form of the α -th two-dimensional fractional Fourier transform operation, it is found that the bivariate Hermite polynomial is a new eigenmode that propagates in quadratic graded index media. Mathematically, the single-variable Hermite polynomial Hn(x) can be defined in terms of its generating function,
In Ref. , by combining the operator-Hermite-polynomials (OHP) method and the technique of integration within an ordered product (IWOP) of operators,  we derived the generating functions of even and odd Hermite polynomials
when l = 0, 1. Since H0(x) = 1, H1(x) = 2x, we have
and these formulas are not only accumulated in mathematical handbooks but can also be useful in constructing some optical quantum states. Furthermore, by introducing the Hermite-polynomial-operator Hn(X), where X is the coordinate operator (or the quadrature operator in quantum optics theory), and combining the IWOP technique, some new operator identities about quantum squeezing are derived, which are useful for studying the squeezed number state. On the other hand, by replacing the arguments of the special function by quantum mechanical operators, a binomial theorem involving Hermite polynomials and a negative-binomial theorem involving Laguerre polynomials have been derived. These two theorems will have essential applications in quantum optics calculations, and the method is concise and helpful in deducing many operator identities, which may become a new branch in mathematical physics theory. Using the entangled state representation, a two-mode squeezed number state can be converted into a Hermite polynomial excited squeezed vacuum state. The generalized photon-added coherent state is obtained by repeatedly acting the combination of Bose creation and annihilation operations on the coherent state. It is found that it can be regarded as a Hermite excited coherent state due to its normalization factor related to Hermite polynomials.
For the bivariate Hermite polynomials Hm, n(x, y) (note that Hm, n is not a direct product of two independent single-variable Hermite polynomials)
with the generating function in the form of
The bivariate Hermite polynomials Hm, n have their own applications in studying quantum optics. For example, in reference to the Weyl ordering
where X and P are coordinate and momentum operator, respectively. In Ref. , we examined the operators’ parameterized ordering and its classical correspondence, and found the fundamental function-operator correspondence
and its complementary relation
where the symbol : : denotes normal ordering.
In Ref.  we derived
These two equations are the generating functions of Hn, 2m (x, y) and the more complicated one is
where A = 1 – 4st.
When k = l = 0, we have
In this work we shall derive some new generating function formula of the product of bivariate Hermite polynomials (PBHP), and we shall employ the OHP method to realize our goal. By the OHP method in this work, we aim to replace the classical Hermite polynomials Hm, n(x, y) by the operator ⋮ Hm, n(a† , a)⋮ first, where a† and a are the Bose creation and annihilation operator, respectively, obeying [a, a† ] = 1, ⋮ ⋮ denotes anti-normal ordering. By using
we can derive some operator identities in definite operator ordering (proof is given in Appendix A). Then we convert them back to the classical case. We also present some applications of these new generating function formulas of PBHP in quantum optics. Next, we shall briefly review the OHP method and exhibit its convenience and simplicity.
One of the fundamental operator identities in the OHP method is
Again by using
Converting X back to x, we obtain
Thus, the merit of using the OHP method is fully displayed.
Now we prove the following one-index-summation of the product of two bivariate Hermite polynomials:
In this case, the OHP method we use is to replace Hm, n(x′ , y′ ) by ⋮ Hm, n(a† , a)⋮ .
Proof Rewrite Eq. (5) as
By comparing the same power term of tn/n! on the two sides we conclude
Then we employ the OHP method to consider the summation
Then we need to change the right-hand side of Eq. (23) into anti-normal ordering for comparing with the left-hand side. Using the identity which is responsible for converting an operator ρ (a, a† ) into its anti-normally ordered form
where | β 〉 = exp(− | β | 2/2 + β a† ) | 0〉 is a coherent state, 〈 − | β 〉 = e– 2| β | 2, we try to put (y – sa† )n (a – sx)n into its anti-normal ordering
where in the last step we have used the integration formula
Based on the relation to the Laguerre polynomials
we can obtain
Similarly, we can prove
By using the generating function of Laguerre polynomials
we can further make summation
This formula can be used to derive
On the other hand, using Eq. (13) it is equal to
we obtain a new operator identity which links normal ordering to anti-normal ordering
In particular, when x = y = 0,
Next we examine the summation
To proceed, using the OHP method and Eq. (13) we instead consider
Using the binomial theorem regarding the bivariate Hemite polynomials
Thus, equation (29) becomes
Since the both sides are in anti-normal ordering, we have
This is another type of generating function of bivariate Hermite polynomials.
As an application of Eq. (30), we consider the two-mode photon-subtracted squeezed vacuum state ambmS2(λ )| 00〉 , where S2(λ ) = exp [λ (a† b† – ab)] is the two-mode squeezing operator, with λ being a real squeezing parameter. The two-mode squeezed state is
its derivation is shown in Appendix A. From Eq. (30), we obtain
Thus, the two-mode photon-subtracted squeezed vacuum state can be expressed as
Therefore, | λ 〉 n is equivalent to Laguerre polynomial excitation on the two-mode squeezed vacuum state. In Ref. , we calculated its normalization factor
where Pm(x) is the Legendre polynomial
which is a new integration formula.
As an application of Eq. (33), we obtain
which changes the anti-normally ordered form esab eta† b† into its normal ordering, or
Then we turn to an application of Eq. (42) in quantum optics. We consider the following state vector which is generated by operating Hl, k (ξ – b† , ξ * – a† ) on the entangled state | ξ 〉 , where
which takes the bivariate Hermite polynomials Hm, n(ξ , ξ * ) as its expansion function in the two-mode Fock space
| ξ 〉 obeys
In comparison with Eq. (42) we see
It follows the Fock space wave function of Hl, k (ξ – b† , ξ * – a† ) | ξ 〉 that
In summary, by virtue of the OHP method and the IWOP technique, we have derived some new generating function formulas about the bivariate Hermite polynomials. Their applications in studying quantum optics are presented as well.