Why does CKKS encryptor need modulus switching?

[kbj1213]

I have same wonder with the post (https://github.com/microsoft/SEAL/issues/402#issue-1022757442).

Let's suppose that CKKS use the primes for crt with {60, 40, 40, 60} bitwidth.

Let's call them q0, q1, q2, q3 respectively.

I checked that the encrypted data are in modulus q0, q1, q2 (crt format) after encryption from the encoder's plaintext.

But I saw void Encryptor::encrypt_zero_internal function includes "Modulus Switching". (using q3)

I wanna understand why the modulus switching process is needed.

[fionser]

This is undocumented trick to decrease the noise from asymmetric encryption