Quote from: PrimeNumber7 on February 17, 2020, 07:36:48 PM

A drone needs to communicate with someone on the ground to receive instructions on how to operate. If this communications channel is not encrypted, anyone could send instructions to the drone to tell it what to do.

Agreed. Encryption is necessary.

Quote from: PrimeNumber7 on February 17, 2020, 07:36:48 PM

A QC could possibly crack whatever encryption is being used to communicate with the drone, then another computer could use the now found decryption key to communicate with the drone, and give it instructions to shut off its motor, or whatever.

Disagree. QCs can break public key cryptography, but symmetric cryptography is not vulnerable. If good post-quantum cryptographic encryption is in place, there is no threat - or at least no threat beyond what there already is without the QC.

Quote from: PrimeNumber7 on February 17, 2020, 07:36:48 PM

So any military using QC in the battlefield would need to use QC, plus additional technology. I believe the additional technology is already widely available.

Disagree that you need a QC to defend against quantum attack. You can do so using classical methods, with no need for a quantum computer - use symmetric key post-quantum cryptography such as AES256. An example:

For standard asymmetric cryptography, a QC running Shor's algorithm absolutely obliterates the difficulty. It takes 2¹²⁸ classical operations to break ECDSA and derive a bitcoin private key from the public key, but only 128³ for a QC running Shor.
For symmetric cryptography, Grover's algorithm is the best attack. But this only square-roots the difficulty, so for something that takes 2¹²⁸ classical operations, a QC running Grover still takes a huge 2⁶⁴.