# Squashed entanglement

Squashed Entanglement, also called CMI Entanglement (CMI can be pronounced "see-me"), is an information theoretic measure of quantum entanglement for a bipartite quantum system. If $\varrho_{A, B}$ is the density matrix of a system (A, B) composed of two subsystems A and B, then the CMI Entanglement ECMI of system (A, B) is defined by

$Eq.(1)\qquad E_{CMI}(\varrho_{A, B}) = \frac{1}{2}\min_{\varrho_{A,B,\Lambda}\in K}S(A:B | \Lambda)$,

where K is the set of all density matrices $\varrho_{A, B, \Lambda}$ for a tripartite system (A, B, Λ) such that $\varrho_{A, B}=tr_\Lambda (\varrho_{A, B, \Lambda})$. Thus, CMI Entanglement is defined as an extremum of a functional S(A : B∣Λ) of $\varrho_{A, B, \Lambda}$. We define S(A : B∣Λ), the quantum Conditional Mutual Information (CMI), below. A more general version of Eq.(1) replaces the min" (minimum) in Eq.(1) by an inf" (infimum). When $\varrho_{A, B}$ is a pure state, $E_{CMI}(\varrho_{A, B})=S(\varrho_{A})=S(\varrho_{B})$, in agreement with the definition of Entanglement of Formation for pure states.

### Motivation for definition of CMI Entanglement

CMI Entanglement has its roots in Classical (non-quantum) Information Theory, as we explain next.

Given any two random variables A, B, Classical Information Theory defines the Mutual Information, a measure of correlations, as

Eq.(2)  H(A : B) = H(A) + H(B) − H(A, B) .

For three random variables A, B, C, it defines the CMI as $Eq.(3)\qquad\begin{matrix} H(A : B | \Lambda)&=& H(A|\Lambda)+ H(B | \Lambda)- H(A, B | \Lambda)\\ &=&H(A, \Lambda)+H(B, \Lambda)-H(\Lambda)-H(A, B, \Lambda) \end{matrix}$.

It can be shown that H(A : B∣Λ) ≥ 0.

Now suppose $\varrho_{A, B, \Lambda}$ is the density matrix for a tripartite system (A, B, Λ). We will represent the partial trace of $\varrho_{A, B, \Lambda}$ with respect to one or two of its subsystems by $\varrho_{A, B, \Lambda}$ with the symbol for the traced system erased. For example, $\varrho_{A, B}= trace_\Lambda(\varrho_{A, B, \Lambda})$. One can define a quantum analogue of Eq.(2) by

$Eq.(4)\qquad S(A:B) = S(\varrho_{A}) + S(\varrho_{B}) -S(\varrho_{A, B}) \,$,

and a quantum analogue of Eq.(3) by

$Eq.(5)\qquad S(A:B|\Lambda) = S(\varrho_{A, \Lambda}) +S(\varrho_{B, \Lambda}) -S(\varrho_\Lambda) -S(\varrho_{A, B, \Lambda}) \,$.

It can be shown that S(A : B∣Λ) ≥ 0. This inequality is often called the strong sub-additivity property of quantum entropy.

Consider three random variables A, B, Λ with probability distribution PA, B, Λ(a, b, λ), which we will abbreviate as P(a, b, λ). For those special P(a, b, λ) of the form

Eq.(6)  P(a, b, λ) = P(aλ)P(bλ)P(λ) ,

it can be shown that H(A : B∣Λ) = 0. Probability distributions of the form Eq.(6) are in fact described by the Bayesian Network shown in Fig.1.

One can define a classical CMI Entanglement by

Eq.(7)  ECMI(PA, B) = minPA, B, Λ ∈ KH(A : B∣Λ),

where K is the set of all probability distributions PA, B, Λ in three random variables A, B, Λ, such that ∑λPA, B, Λ(a, b, λ) = PA, B(a, b) for all a, b. Because, given a probability distribution PA, B, one can always extend it to a probability distribution PA, B, Λ that satisfies Eq.(6), it follows that the classical CMI Entanglement, ECMI(PA, B), is zero for all PA, B. The fact that ECMI(PA, B) always vanishes is an important motivation for the definition of $E_{CMI}( \varrho_{A,B})$. We want a measure of quantum entanglement that vanishes in the classical regime.

Suppose wλ for λ = 1, 2, ..., dim(Λ) is a set of non-negative numbers that add up to one, and ∣λ >  for λ = 1, 2, ..., dim(Λ) is an orthonormal basis for the Hilbert space associated with a quantum system Λ. Suppose $\varrho_A^\lambda$ and $\varrho_B^\lambda$, for λ = 1, 2, ..., dim(Λ) are density matrices for the systems A and B, respectively. It can be shown that the following density matrix

$Eq.(8)\qquad \varrho_{A, B, \Lambda}=\sum_\lambda \varrho_A^\lambda \varrho_B^\lambda w_\lambda |\lambda><\lambda| \,$

satisfies S(A : B∣Λ) = 0. Eq.(8) is the quantum counterpart of Eq.(6). Tracing the density matrix of Eq.(8) over Λ, we get $\varrho_{A,B} = \sum_\lambda \varrho_A^\lambda \varrho_B^\lambda w_\lambda \,$, which is a separable state. Therefore, $E_{CMI}(\varrho_{A, B})$ given by Eq.(1) vanishes for all separable states.

When $\varrho_{A, B}$ is a pure state, one gets $E_{CMI}(\varrho_{A, B})=S(\varrho_{A})=S(\varrho_{B})$. This agrees with the definition of Entanglement of Formation for pure states, as given in Ben96.

Next suppose ∣ψA, Bλ >  for λ = 1, 2, ..., dim(Λ) are some states in the Hilbert space associated with a quantum system (A, B). Let K be the set of density matrices defined previously for Eq.(1). Define Ko to be the set of all density matrices $\varrho_{A, B, \Lambda}$ that are elements of K and have the special form $\varrho_{A, B, \Lambda} = \sum_\lambda|\psi_{A,B}^\lambda><\psi_{A,B}^\lambda| w_\lambda |\lambda> <\lambda|\,$. It can be shown that if we replace in Eq.(1) the set K by its proper subset Ko, then Eq.(1) reduces to the definition of Entanglement of Formation for mixed states, as given in Ben96. K and Ko represent different degrees of knowledge as to how $\varrho_{A, B, \Lambda}$ was created. K represents total ignorance.

### History

Classical CMI, given by Eq.(3), first entered Information Theory lore, shortly after Shannon's seminal 1948 paper and at least as early as 1954 in McG54. The quantum CMI, given by Eq.(5), was first defined by Cerf and Adami in Cer96. However, it appears that Cerf and Adami did not realize the relation of CMI to entanglement or the possibility of obtaining a measure of quantum entanglement based on CMI; this can be inferred, for example, from a later paper, Cer97, where they try to use S(AB) instead of CMI to understand entanglement. The first paper to explicitly point out a connection between CMI and quantum entanglement appears to be Tuc99.

The final definition Eq.(1) of CMI entanglement was first given by Tucci in a series of 6 papers. (See, for example, Eq.(8) of Tuc02 and Eq.(42) of Tuc01a). In Tuc00b, he pointed out the classical probability motivation of Eq.(1), and its connection to the definitions of Entanglement of Formation for pure and mixed states. In Tuc01a, he presented an algorithm and computer program, based on the Arimoto-Blahut method of information theory, for calculating CMI entanglement numerically. In Tuc01b, he calculated CMI entanglement analytically, for a mixed state of two qubits.

In Hay03, Hayden, Jozsa, Petz and Winter explored the connection between quantum CMI and separability.

Since CMI Entanglement reduces to Entanglement of Formation if one minimizes over Ko instead of K, one expects that CMI Entanglement inherits many desirable properties from Entanglement of Formation. As first shown in Ben96, Entanglement of Formation does not increase under LOCC (Local Operations and Classical Communication). In Chr03, Christandl and Winter showed that CMI Entanglement also does not increase under LOCC, by adapting Ben96 arguments about Entanglement of Formation. In Chr03, they also proved many other interesting inequalities concerning CMI Entanglement, and explored its connection to other measures of entanglement. The name Squashed Entanglement first appeared in Chr03. In Chr05, Christandl and Winter calculated analytically the CMI Entanglement of some interesting states.

In Ali03, Alicki and Fannes proved the continuity of CMI Entanglement.

### References

• Ali03 R. Alicki, M. Fannes, Continuity of quantum mutual information", quant-ph/0312081
• Ben96 C.H. Bennett, D.P. DiVincenzo, J.A. Smolin, W.K. Wootters, Mixed State Entanglement and Quantum Error Correction", quant-ph/9604024
• Cer96 N. J. Cerf, C. Adami, Quantum Mechanics of Measurement", quant-ph/9605002
• Cer97 N.J. Cerf, C. Adami, R.M. Gingrich, Quantum conditional operator and a criterion for separability", quant-ph/9710001
• Chr03 M. Christandl, A. Winter, Squashed Entanglement - An Additive Entanglement Measure", quant-ph/0308088
• Chr05 M. Christandl, A. Winter, Uncertainty, Monogamy, and Locking of Quantum Correlations", quant-ph/0501090
• Chr06 M. Christandl, Ph.D. Thesis, quant-ph/0604183
• Hay03 P. Hayden, R. Jozsa, D. Petz, A. Winter, Structure of states which satisfy strong subadditivity of quantum entropy with equality" quant-ph/0304007
• McG54 W.J. McGill, Multivariate Information Transmission", IRE Trans. Info. Theory 4(1954) 93-111.
• Tuc99 R.R. Tucci, Quantum Entanglement and Conditional Information Transmission", quant-ph/9909041
• Tuc00a R.R. Tucci,Separability of Density Matrices and Conditional Information Transmission", quant-ph/0005119
• Tuc00b R.R. Tucci, Entanglement of Formation and Conditional Information Transmission", quant-ph/0010041
• Tuc01a R.R. Tucci, Relaxation Method For Calculating Quantum Entanglement", quant-ph/0101123
• Tuc01b R.R. Tucci, Entanglement of Bell Mixtures of Two Qubits", quant-ph/0103040
• Tuc02 R.R. Tucci, Entanglement of Distillation and Conditional Mutual Information", quant-ph/0202144

Category:quantum Information Theory