We study the free probabilistic analog of optimal couplings for the quadratic cost, where classical probability spaces are replaced by tracial von Neumann algebras, and probability measures on $${\mathbb {R}}^m$$
R
m
are replaced by non-commutative laws of m-tuples. We prove an analog of the Monge–Kantorovich duality which characterizes optimal couplings of non-commutative laws with respect to Biane and Voiculescu’s non-commutative $$L^2$$
L
2
-Wasserstein distance using a new type of convex functions. As a consequence, we show that if (X, Y) is a pair of optimally coupled m-tuples of non-commutative random variables in a tracial $$\mathrm {W}^*$$
W
∗
-algebra $$\mathcal {A}$$
A
, then $$\mathrm {W}^*((1 - t)X + tY) = \mathrm {W}^*(X,Y)$$
W
∗
(
(
1
-
t
)
X
+
t
Y
)
=
W
∗
(
X
,
Y
)
for all $$t \in (0,1)$$
t
∈
(
0
,
1
)
. Finally, we illustrate the subtleties of non-commutative optimal couplings through connections with results in quantum information theory and operator algebras. For instance, two non-commutative laws that can be realized in finite-dimensional algebras may still require an infinite-dimensional algebra to optimally couple. Moreover, the space of non-commutative laws of m-tuples is not separable with respect to the Wasserstein distance for $$m > 1$$
m
>
1
.