In a previous post entitled New Law of Nature I described a quantum operation (Quantum Wedding) which could be used to send signals faster-than-light. The new law of nature is this: the laws of quantum mechanics forbid Quantum Wedding--a pair of unknown quanta cannot be reliably tricked into the same state. A similar situation exists for Quantum Cloning. If you could reliably clone an unknown single quantum you could send signals FTL. But the laws of quantum mechanics (as shown by Wooters & Zurek in a well-known 1982 article in Nature) forbid Quantum Cloning.
Since quantum mechanics is reversible, one might reasonably conclude that the operations inverse to Quantum Cloning and Quantum Wedding would also be forbidden. Does nature therefore forbid Quantum Deletion (anti-cloning) and Quantum Divorce (anti-wedding)? I used to think so and said as much in the previous post.
In this I was mistaken.
Unbeknownst to me a group of Indians (Bhagwat, Khandekar, Menon, Puri and Sahni) from the Bhabha Atomic Research Center in Mumbai had shown in 2000 that nature does not forbid Quantum Deletion.
The reason? Cloning and Deletion are not precisely opposite operations. This loophole, the Indians then show, permits the deletion (uncloning) of a generic quantum state. Looking at this article from Bhabha I realized that Quantum Divorce is likewise not the precise opposite of Quantum Wedding. A short calculation shows that this same loophole also permits the unwedding of a generic quantum twosome.
Therefore this: Cloning and Wedding are forbidden but Anticloning and Divorce are not. Ain't quantum mechanics wonderful?