To make key exchange protocols (providing perfect forward-secrecy) robust against quantum computers they need to rely on assumptions that are not susceptible to quantum attacks (post-quantum crypto) like hash based, lattice based or multivariate-quadratic-equations based.
Clearly, quantum key distribution is a candidate for key exchange with all the associated problems.
pqcrypto.org provides a quite updated reference to post-quantum research. It looks like the key exchange issue and in particular perfect forward-secrecy has not really seen much research so far.
At least the paper Smooth projective hashing and password-based authenticated key exchange from lattices provides a construction of an password-based authenticated key exchange for the post-quantum world.
Subsequently, I comment on the below cited part of the question and I only speak of protocols where we use hardness assumptions which do not longer hold in the advent of a quantum computer below.
...insuring that an intercept made at a date when the assumption of hardness underlying the asymmetric protocol holds, can't help recovering the ephemeral symmetric key established even after said assumption of hardness no longer holds.
We speak here of key-exchange/key-agreement protocols which allow to parties to compute a common secret over some
public channel where this common ephemeral secret may then be used to compute some common symmetric session key.
Perfect forward-secrecy in the key-exchange context typically means that both parties have some long term-key and on every session they
run some key-agreement to compute a common session key, but the compromise of long-term keys does not compromise the
secrecy of any past sessions.
In order to compute a session key there happens a key-agreement protocol (typically, the long term-keys will
be used to sign the respective ephemeral information of the key-agreement sent by the respective party), where the ephemeral information exchanged to compute the common
key do not deterministically depend on the long-term keys if perfect forward secrecy is provided. Anyways, there always
simply happens a key-agreement irrespective of the perfect
forward-secrecy feature or not.
So if the security of the key-agreement is based on some hardness assumptions $P_1,\ldots,P_n$, it does not make a difference if you consider
key-agreement protocols that provide perfect forward-secrecy or not. If the $P_i$'s do no longer hold, e.g., if we
have a fancy quantum computer, then the transcript of any key-agreement will allow you to recover the respective session key.
Perfect forward-secrecy is concerned with compromise of the long-term secrets (such as private keys for static DH keys), but not
with breaking the hardness assumptions when given the transcript of a key-agreement (clearly, because the key-agreement
only makes sense if the hardness assumption holds).