I have verified on Windows 7 64-bit with IE11 and UAC turned on, and the link given by @Ƭᴇcʜιᴇ007 is correct: Go in the IE menu Tools to Internet Options, Advanced tab, under Security, and check "Enable Enhanced Protected Mode". On newer version of Windows one also needs to check "Enable 64-bit processes for Enhanced Protected Mode". Re-launching IE was enough in my case, although the documentation advises to restart the computer.
I had no add-ons installed at all. The documentation says that 32-bit add-ons are disabled in 64-bit mode. Another note is that this also worked in Windows 10 with UAC disabled, so UAC does not seem important here.
This should do the job, but if for some reason it does not, there is another solution, which is in the Registry to create under the key CURRENT_USER\Software\Microsoft\Internet Explorer\Main a DWORD (32-bit) entry named TabProcGrowth with zero as its value. When this registry entry is set to 0, the tabs run in the same process as the manager process, which is always 64-bit on a 64-bit operating system. Documentation may be found here (for IE10, but was verified by me for IE11). Restarting IE seems enough for this registry setting to be enabled or disabled.