The molecular thread used is consist of a single long super dense compound with extremely fast angular momentum (referring to its quantum property) at nearly the speed of light. This filament less than 1 nm thick is shown to sever any molecular bonds upon contact with almost no resistance, how to effectively block against such weapon if possible using modern day innovation?
I would probably punch the guy who is wielding it.
We get a lot of questions on StackExchange in the form of "unstoppable forces" or "immovable objects," and then the request is to brainstorm solutions. Typically these solutions are given limitations well below the technology or magic used to create the unstoppable force or immovable object. In this case, no physics is provided for the monoblade, so it's all handwavium. This means there's no weaknesses in the implementation for us to explore.
A few potential solutions:
- Capture the filament with electrostatic forces.
- Burn the filament with plasma.
- If the filament is conductive, you might be able to use a tesla coil to zap the guy.
- Block the blade at the hilt, rather than at the pointy bit.
- Be elsewhere (not technically blocking, but not a bad idea any time someone comes at you with technology Clarke would call "magic")
- Set off a nuclear device near the monoblade (which has the convenient side effect of also ensuring the wielder stops wielding any blades)
Use your own mono-blade to parry your opponent's mono-blade. Of course you would, if your opponent has a mono-blade, then surely you do too.
The OP didn't specify how super-dense the mono-blade might be, so the chances are your opponent will be struggling to even lift his mono-blade because it is too heavy to lift. In that case, follow Cort Ammon's advice and punch him.
Alternatively, you run him through with a non-super-dense conventional blade or shoot Mr Mono-blade with an ordinary gun. If you're really lucky he will die of a heart attack trying to lift his too heavy, super-dense mono-blade.
Basically remarkably few modern innovations are needed to stop someone armed with a super-dense mono-blade. Conventional weapons are more than adequate at combating persons armed with swords. This is only sword, admittedly it does have the Rule of Cool on its side, but the sad truth is the Rule of Cool never saved anyone's life. A sword by any other name is still a sword.
I will take block to mean "stop him from hitting more people with his fancy sword."
Meaning kill the wielder with basic weapons
You can't block his sword and he can't block you either. You swing an axe at him he "blocks" it by cutting through it. The cut off segment keeps going and hits him. So he can always hit you and you can always hit him, just rush three guys at him kill him and take the sword.
Shoot him if he "blocks" the bullet / arrow it will hit him.
The monomolecular weapon may not even harm a human. IF it cuts a very sharp cut with a very thin blade (monomolecual) and moves very fast (the term used was infinite angular momentum which is impossible, but assume fast) the bonds may reseal on the other side with out the loss of blood of damage to the organs. It may be true he cuts you dozens of times to no effect at all.
How to counter an unstoppable sword? Why, with an invulnerable shield of course.
Do you have magnetic shields (aka 'force' fields) available? There're some ideas around already: http://www.popularmechanics.com/military/research/a14683/boeing-patent-plasma-shield/
Have you tried diplomacy? :-)
Just kidding. More seriously: what's the melting point of this monofilament? Perhaps it can be "parried" by a flame thrower or hyper-focused laser. Similarly, maybe extreme cold makes it brittle.
Alternate: What is its sliding friction coefficient? As it slices through objects, it is going to lose energy and slow down. There is some thickness that will stop it unless it is being actively propelled -- that thickness might be a planet or it might be a thick brick of uranium, depending upon the coefficient.