# Gates¶

Note

In Strawberry Fields we use the convention $$\hbar=2$$ by default, but other conventions can also be chosen by setting the global variable sf.hbar at the beginning of a session. In this document we keep $$\hbar$$ explicit.

## Displacement¶

Definition

$D(\alpha) = \exp( \alpha \ad -\alpha^* \a) = \exp(r (e^{i\phi}\ad -e^{-i\phi}\a)), \quad D^\dagger(\alpha) \a D(\alpha)=\a +\alpha\I$

where $$\alpha=r e^{i \phi}$$ with $$r \geq 0$$ and $$\phi \in [0,2 \pi)$$.

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.Dgate

We obtain for the position and momentum operators

$\begin{split}D^\dagger(\alpha) \x D(\alpha) = \x +\sqrt{2 \hbar } \re(\alpha) \I,\\ D^\dagger(\alpha) \p D(\alpha) = \p +\sqrt{2 \hbar } \im(\alpha) \I.\end{split}$

Definition

The pure position and momentum displacement operators are defined as

$\begin{split}X(x) &= D\left( x/\sqrt{2 \hbar}\right) = \exp(-i x \p /\hbar), \quad X^\dagger(x) \x X(x) = \x +x\I,\\ Z(p) &= D\left(i p/\sqrt{2 \hbar}\right) = \exp(i p \x /\hbar ), \quad Z^\dagger(p) \p Z(p) = \p +p\I.\end{split}$

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.Xgate

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.Zgate

The matrix elements of the displacement operator in the Fock basis were derived by Cahill and Glauber [1]:

$\bra{m}\hat D(\alpha) \ket{n} = \sqrt{\frac{n!}{m!}} \alpha^{m-n} e^{-|\alpha|^2/2} L_n^{m-n}\left( |\alpha|^2 \right)$

where $$L_n^{m}(x)$$ is a generalized Laguerre polynomial [2].

## Squeezing¶

Definition

$\begin{split}& S(z) = \exp\left(\frac{1}{2}\left(z^* \a^2-z {\ad}^{2} \right) \right) = \exp\left(\frac{r}{2}\left(e^{-i\phi}\a^2 -e^{i\phi}{\ad}^{2} \right) \right)\\ & S^\dagger(z) \a S(z) = \a \cosh(r) -\ad e^{i \phi} \sinh (r)\\ & S^\dagger(z) \ad S(z) = \ad \cosh(r) -\a e^{-i \phi} \sinh(r)\end{split}$

where $$z=r e^{i \phi}$$ with $$r \geq 0$$ and $$\phi \in [0,2 \pi)$$.

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.Sgate

The squeeze gate affects the position and momentum operators as

$S^\dagger(z) \x_{\phi} S(z) = e^{-r}\x_{\phi}, ~~~ S^\dagger(z) \p_{\phi} S(z) = e^{r}\p_{\phi}$

The Fock basis decomposition of displacement and squeezing operations was analysed by Krall [3], and the following quantity was calculated,

$\begin{split}f_{n,m}(r,\phi,\beta)&=\bra{n}\exp\left(\frac{r}{2}\left(e^{i \phi} \a^2 -e^{-i \phi} \ad \right) \right) D(\beta) \ket{m} = \bra{n}S(z^*) D(\beta) \ket{m}\\ &=\sqrt{\frac{n!}{\mu m!}} e^{\frac{\beta ^2 \nu ^*}{2\mu }-\frac{\left| \beta \right| ^2}{2}} \sum_{i=0}^{\min(m,n)}\frac{\binom{m}{i} \left(\frac{1}{\mu \nu }\right)^{i/2}2^{\frac{i-m}{2}+\frac{i}{2}-\frac{n}{2}} \left(\frac{\nu }{\mu }\right)^{n/2} \left(-\frac{\nu ^*}{\mu }\right)^{\frac{m-i}{2}} H_{n-i}\left(\frac{\beta }{\sqrt{2} \sqrt{\mu \nu }}\right) H_{m-i}\left(-\frac{\alpha ^*}{\sqrt{2} \sqrt{-\mu \nu ^*}}\right)}{(n-i)!}\end{split}$

where $$\nu=e^{- i\phi} \sinh(r), \mu=\cosh(r), \alpha=\beta \mu - \beta^* \nu$$.

Two important special cases of the last formula are obtained when $$r \to 0$$ and when $$\beta \to 0$$:

• For $$r \to 0$$ we can take $$\nu \to 1, \mu \to r, \alpha \to \beta$$ and use the fact that for large $$x \gg 1$$ the leading order term of the Hermite polynomials is $$H_n(x) = 2^n x^n +O(x^{n-2})$$ to obtain

$f_{n,m}(0,\phi,\beta) = \bra{n}D(\beta) \ket{m}=\sqrt{\frac{n!}{ m!}} e^{-\frac{\left| \beta \right| ^2}{2}} \sum_{i=0}^{\min(m,n)} \frac{(-1)^{m-i}}{(n-i)!} \binom{m}{i} \beta^{n-i} (\beta^*)^{m-i}$
• On the other hand if we let $$\beta\to 0$$ we use the fact that

$\begin{split}H_n(0) =\begin{cases}0, & \mbox{if }n\mbox{ is odd} \$$-1)^{\tfrac{n}{2}} 2^{\tfrac{n}{2}} (n-1)!! , & \mbox{if }n\mbox{ is even} \end{cases}\end{split}$ to deduce that \(f_{n,m}(r,\phi,0)$$ is zero if $$n$$ is even and $$m$$ is odd or vice versa.

When writing the Bloch-Messiah reduction [4][5] of a Gaussian state in the Fock basis one often needs the following matrix element

$\bra{k} D(\alpha) R(\theta) S(r) \ket{l} = e^{i \theta l } \bra{k} D(\alpha) S(r e^{2i \theta}) \ket{l} = e^{i \theta l} f^*_{l,k}(-r,-2\theta,-\alpha)$

## Rotation¶

Note

We use the convention that a positive value of $$\phi$$ corresponds to an anticlockwise rotation in the phase space.

Definition

We write the phase space rotation operator as

$R(\phi) = \exp\left(i \phi \ad \a\right)=\exp\left(i \frac{\phi}{2} \left(\frac{\x^2+ \p^2}{\hbar}-\I\right)\right), \quad R^\dagger(\phi) \a R(\phi) = \a e^{i \phi}$

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.Rgate

It rotates the position and momentum quadratures to each other:

$\begin{split}R^\dagger(\phi)\x R(\phi) = \x \cos \phi -\p \sin \phi,\\ R^\dagger(\phi)\p R(\phi) = \p \cos \phi +\x \sin \phi.\end{split}$

Definition

A special case of the rotation operator is the case $$\phi=\pi/2$$; this corresponds to the Fourier gate,

$F = R(\pi/2) = e^{i (\pi/2) \ad \a},$

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.Fouriergate

The Fourier gate transforms the quadratures as follows:

$\begin{split}& F^\dagger\x F = -\p,\\ & F^\dagger\p F = \x.\end{split}$

## Quadratic phase¶

Definition

$P(s) = \exp\left(i \frac{s}{2 \hbar} \x^2\right), \quad P^\dagger(s) \a P(s) = \a +i\frac{s}{2}(\a +\ad)$

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.Pgate

It shears the phase space, preserving position:

$\begin{split}P^\dagger(s) \x P(s) &= \x,\\ P^\dagger(s) \p P(s) &= \p +s\x.\end{split}$

This gate can be decomposed as

$P(s) = R(\theta) S(r e^{i \phi})$

where $$\cosh(r) = \sqrt{1+(\frac{s}{2})^2}, \quad \tan(\theta) = \frac{s}{2}, \quad \phi = -\sign(s)\frac{\pi}{2} -\theta$$.

## Beamsplitter¶

Definition

For the annihilation and creation operators of two modes, denoted $$\a_1$$ and $$\a_2$$, the beamsplitter is defined by

$B(\theta,\phi) = \exp\left(\theta (e^{i \phi}\a_1 \ad_2 - e^{-i \phi} \ad_1 \a_2) \right)$

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.BSgate

Action on the creation and annihilation operators

They will transform the operators according to

$\begin{split}B^\dagger(\theta,\phi) \a_1 B(\theta,\phi) &= \a_1\cos \theta -\a_2 e^{-i \phi} \sin \theta = t \a_1 -r^* \a_2,\\ B^\dagger(\theta,\phi) \a_2 B(\theta,\phi) &= \a_2\cos \theta + \a_1 e^{i \phi} \sin \theta= t \a_2 +r \a_1.\end{split}$

where $$t = \cos \theta$$ and $$r = e^{i\phi} \sin \theta$$ are the transmittivity and reflectivity amplitudes of the beamsplitter respectively.

Therefore, the beamsplitter transforms two input coherent states to two output coherent states $$B(\theta, \phi) \ket{\alpha,\beta} = \ket{\alpha',\beta'}$$, where

$\begin{split}\alpha' &= \alpha\cos \theta-\beta e^{-i\phi}\sin\theta = t\alpha - r^*\beta\\ \beta' &= \beta\cos \theta+\alpha e^{i\phi}\sin\theta = t\beta + r\alpha\\\end{split}$

Action on the quadrature operators

By substituting in the definition of the creation and annihilation operators in terms of the position and momentum operators, it is possible to derive an expression for how the beamsplitter transforms the quadrature operators:

$\begin{split}&\begin{cases} B^\dagger(\theta,\phi) \x_1 B(\theta,\phi) = \x_1 \cos(\theta)-\sin(\theta) [\x_2\cos(\phi)+\p_2\sin(\phi)]\\ B^\dagger(\theta,\phi) \p_1 B(\theta,\phi) = \p_1 \cos(\theta)-\sin(\theta) [\p_2\cos(\phi)-\x_2\sin(\phi)]\\ \end{cases}\\[12pt] &\begin{cases} B^\dagger(\theta,\phi) \x_2 B(\theta,\phi) = \x_2 \cos(\theta)+\sin(\theta) [\x_1\cos(\phi)-\p_1\sin(\phi)]\\ B^\dagger(\theta,\phi) \p_2 B(\theta,\phi) = \p_2 \cos(\theta)+\sin(\theta) [\p_1\cos(\phi)+\x_1\sin(\phi)] \end{cases}\end{split}$

Action on the position and momentum eigenstates

A 50% or 50-50 beamsplitter has $$\theta=\pi/4$$ and $$\phi=0$$ or $$\phi=\pi$$; consequently $$|t|^2 = |r|^2 = \frac{1}{2}$$, and it acts as follows:

$\begin{split}& B(\pi/4,0)\xket{x_1}\xket{x_2} = \xket{\frac{1}{\sqrt{2}}(x_1-x_2)}\xket{\frac{1}{\sqrt{2}}(x_1+x_2)}\\ & B(\pi/4,0)\ket{p_1}_p\ket{p_2}_p = \xket{\frac{1}{\sqrt{2}}(p_1-p_2)}\xket{\frac{1}{\sqrt{2}}(p_1+p_2)}\end{split}$

and

$\begin{split}& B(\pi/4,\pi)\xket{x_1}\xket{x_2} = \xket{\frac{1}{\sqrt{2}}(x_1+x_2)}\xket{\frac{1}{\sqrt{2}}(x_2-x_1)}\\ & B(\pi/4,\pi)\ket{p_1}_p\ket{p_2}_p = \xket{\frac{1}{\sqrt{2}}(p_1+p_2)}\xket{\frac{1}{\sqrt{2}}(p_2-p_1)}\end{split}$

Alternatively, symmetric beamsplitter (one that does not distinguish between $$\a_1$$ and $$\a_2$$) is obtained by setting $$\phi=\pi/2$$.

## Two-mode squeezing¶

Definition

$S_2(z) = \exp\left(z \ad_1 \ad_2 - z^* \a_1\a_2 \right) = \exp\left(r (e^{i\phi} \ad_1\ad_2 -e^{-i\phi} \a_1 \a_2 \right)$

where $$z=r e^{i \phi}$$ with $$r \geq 0$$ and $$\phi \in [0,2 \pi)$$.

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.S2gate

It can be decomposed into two opposite local squeezers sandwiched between two 50% beamsplitters [6]:

$S_2(z) = B^\dagger(\pi/4,0) \: \left[ S(z) \otimes S(-z)\right] \: B(\pi/4,0)$

Two-mode squeezing will transform the operators according to

$\begin{split}S_2(z)^\dagger \a_1 S_2(z) &= \a_1 \cosh(r)-\ad_2 e^{i \phi} \sinh(r),\\ S_2(z)^\dagger \a_2 S_2(z) &= \a_2 \cosh(r) -\ad_1 e^{i \phi} \sinh(r),\\\end{split}$

where $$z=r e^{i \phi}$$ with $$r \geq 0$$ and $$\phi \in [0,2 \pi)$$.

## Controlled-X gate¶

Definition

The controlled-X gate, also known as the addition gate or the sum gate, is a controlled displacement in position. It is given by

$\text{CX}(s) = \int dx \xket{x}\xbra{x} \otimes D\left(\frac{s x}{\sqrt{2\hbar}}\right) = \exp\left({-i \frac{s}{\hbar} \: \x_1 \otimes \p_2}\right).$

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.CXgate

It is called addition because in the position basis $$\text{CX}(s) \xket{x_1, x_2} = \xket{x_1, x_2+s x_1}$$.

We can also write the action of the addition gate on the canonical operators:

$\begin{split}\text{CX}(s)^\dagger \x_1 \text{CX}(s) &= \x_1\\ \text{CX}(s)^\dagger \p_1 \text{CX}(s) &= \p_1- s \ \p_2\\ \text{CX}(s)^\dagger \x_2 \text{CX}(s) &= \x_2+ s \ \x_1\\ \text{CX}(s)^\dagger \p_2 \text{CX}(s) &= \p_2 \\ \text{CX}(s)^\dagger \hat{a}_1 \text{CX}(s) &= \a_1+ \frac{s}{2} (\ad_2 - \a_2)\\ \text{CX}(s)^\dagger \hat{a}_2 \text{CX}(s) &= \a_2+ \frac{s}{2} (\ad_1 + \a_1)\\\end{split}$

The addition gate can be decomposed in terms of single mode squeezers and beamsplitter as follows $$\text{CX}(s) = B(\frac{\pi}{2}+\theta,0) \left(S(r,0) \otimes S(-r,0) \right) B(\theta,0)$$ where $$\sin(2 \theta) = \frac{-1}{\cosh r}, \ \cos(2 \theta)=-\tanh(r), \ \sinh(r) = -\frac{ s}{2}$$

## Controlled-phase¶

Definition

$\text{CZ}(s) = \iint dx dy \: e^{i s x_1 x_2/\hbar } \xket{x_1,x_2}\xbra{x_1,x_2} = \exp\left({i s \: \hat{x_1} \otimes \hat{x_2} /\hbar}\right).$

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.CZgate

It is related to the addition gate by a phase space rotation in the second mode: $$\text{CZ}(s) = R_{(2)}(\pi/2) \: \text{CX}(s) \: R_{(2)}^\dagger(\pi/2)$$.

In the position basis $$\text{CZ}(s) \xket{x_1, x_2} = e^{i s x_1 x_2/\hbar} \xket{x_1, x_2}$$.

We can also write the action of the controlled-phase gate on the canonical operators:

$\begin{split}\text{CZ}(s)^\dagger \x_1 \text{CZ}(s) &= \x_1\\ \text{CZ}(s)^\dagger \p_1 \text{CZ}(s) &= \p_1+ s \ \x_2\\ \text{CZ}(s)^\dagger \x_2 \text{CZ}(s) &= \x_2\\ \text{CZ}(s)^\dagger \p_2 \text{CZ}(s) &= \p_2+ s \ \x_1 \\ \text{CZ}(s)^\dagger \hat{a}_1 \text{CZ}(s) &= \a_1+ i\frac{s}{2} (\ad_2 + \a_2)\\ \text{CZ}(s)^\dagger \hat{a}_2 \text{CZ}(s) &= \a_2+ i\frac{s}{2} (\ad_1 + \a_1)\\\end{split}$

## Cubic phase¶

Warning

The cubic phase gate is non-Gaussian, and thus can only be used in the Fock backends, not the Gaussian backend.

Warning

The cubic phase gate can suffer heavily from numerical inaccuracies due to finite-dimensional cutoffs in the Fock basis. The gate implementation in Strawberry Fields is unitary, but it does not implement an exact cubic phase gate. The Kerr gate provides an alternative non-Gaussian gate.

Definition

$V(\gamma) = \exp\left(i \frac{\gamma}{3 \hbar} \x^3\right), \quad V^\dagger(\gamma) \a V(\gamma) = \a +i\frac{\gamma}{2\sqrt{2/\hbar}} (\a +\ad)^2$

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.Vgate

It transforms the phase space as follows:

$\begin{split}V^\dagger(\gamma) \x V(\gamma) &= \x,\\ V^\dagger(\gamma) \p V(\gamma) &= \p +\gamma \x^2.\end{split}$

## Kerr interaction¶

Warning

The Kerr gate is non-Gaussian, and thus can only be used in the Fock backends, not the Gaussian backend.

Definition

The Kerr interaction is given by the Hamiltonian

$H = (\hat{a}^\dagger\hat{a})^2=\hat{n}^2$

which is non-Gaussian and diagonal in the Fock basis.

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.Kgate

We can therefore define the Kerr gate, with parameter $$\kappa$$ as

$K(\kappa) = \exp{(i\kappa\hat{n}^2)}.$

## Cross-Kerr interaction¶

Warning

The cross-Kerr gate is non-Gaussian, and thus can only be used in the Fock backends, not the Gaussian backend.

Definition

The cross-Kerr interaction is given by the Hamiltonian

$H = \hat{n}_1\hat{n_2}$

which is non-Gaussian and diagonal in the Fock basis.

Tip

Implemented in Strawberry Fields as a quantum gate by strawberryfields.ops.CKgate

We can therefore define the cross-Kerr gate, with parameter $$\kappa$$ as

$CK(\kappa) = \exp{(i\kappa\hat{n}_1\hat{n_2})}.$

## References¶

1

K. E. Cahill and R. J. Glauber. Ordered expansions in boson amplitude operators. Physical Review, 177:1857–1881, Jan 1969. doi:10.1103/PhysRev.177.1857.

2

F. W. J. Olver, A. B. Olde Daalhuis, D. W. Lozier, B. I. Schneider, R. F. Boisvert, C. W. Clark, B. R. Miller, and B. V. Saunders (eds.). NIST digital library of mathematical functions. Release 1.0.16 of 2017-09-18. [Online; accessed 2017-10-25]. URL: http://dlmf.nist.gov/.

3

P Král. Displaced and squeezed Fock states. Journal of Modern Optics, 37(5):889–917, 1990. doi:10.1080/09500349014550941.

4

Gianfranco Cariolaro and Gianfranco Pierobon. Bloch-Messiah reduction of gaussian unitaries by Takagi factorization. Physical Review A, 94:062109, Dec 2016. doi:10.1103/PhysRevA.94.062109.

5

Gianfranco Cariolaro and Gianfranco Pierobon. Reexamination of Bloch-Messiah reduction. Physical Review A, 93:062115, Jun 2016. doi:10.1103/PhysRevA.93.062115.

6

M. S. Kim, W. Son, V. Bužek, and P. L. Knight. Entanglement by a beam splitter: nonclassicality as a prerequisite for entanglement. Physical Review A, 65:032323, Feb 2002. arXiv:quant-ph/0106136, doi:10.1103/PhysRevA.65.032323.