Staves count as both an implement and as a weapon (a “weapliment” in common parlance). When used as a weapon, they are treated as quarterstaves, a two-handed weapon, so undoubtedly when used to make weapon attacks it requires two hands.
But what about when used as an implement? Here, I’m seeing a lack-of-a-rule that suggests one-handed usage is just fine for this, which is also fitting for typical fantasy tropes (not to mention the wizard art itself!). Furthermore, if its status as a two-handed weapon transferred to its usage as an implement, that might imply that small characters cannot use a staff as an implement at all, since they cannot use two-handed weapons—if this were the case, I would expect this disadvantage to be noted!
Relevant rules I’ve found:
Using an Implement
Implements are used with powers that have the implement keyword. A creature must be holding an implement to use it, unless otherwise stated.
An implement cannot be used to make a weapon attack. Some types of implements, such as staffs, expressly break this rule.
Here we see the requirement is merely “holding” rather than “wielding” or what have you, and staves’ exception to the rule about weapon attacks with implements is explicitly addressed without mentioning any requirement to use two hands.
Staff: A staff is a shaft, usually of wood, that is typically as tall or slightly taller than its wielder. Staffs are sometimes crowned with decorative crystals or other symbols of magical power.
This implement also counts as a quarterstaff. Even a creature who doesn’t have proficiency with the staff as an implement can use it as a weapon, but if the staff is magical, the creature cannot use its properties or powers, only its enhancement bonus and critical hit effect.
And this is where the staff exceptions are really spelled out, and again, no mention of requiring two hands to use it as an implement (in fact, it requiring two hands, even for weapon attacks, is completely unmentioned).
I have not been able to find any more information in the books about this, and I am not familiar with any rulings offered by the developers. Citing some authoritative source, that is a book or a developer, can a staff be used as an implement with one hand, or does it require two hands even in this case?