Fusion we can do now is an "Orion" nuclear-pulse drive using fusion bombs. Means the ship is HUGE to deflect megaton blasts.

"Mars in a week" is a bit vague. When Mars is closest to Earth - just 56 million km at Opposition - doesn't happen too often. If we use that figure then one week means we need to fly at ~93 km/s, not taking gravity drag from the Sun into account. Call it 100 km/s. That's about the performance for a deuterium fusion pulse-drive that Friedwardt Winterberg recently described.

But it's not really very practical to fly to Mars in a radial line because both planets orbit the Sun perpendicular to a radial line pointing out from the Sun. Thus the total delta-vee to accelerate to 100 km/s and then deccelerate into Martian orbit (24 km/s around the Sun) is ~207 km/s. Winterberg's fusion-pulse drive can do it easily, but it's currently well beyond our proven technologies.

What if we take our time? A Hohmann transfer orbit takes 258 days on average - the range is 228-288 days depending on where the planets are. A bit more speed brings that down to 130-180 days, which is what NASA usually plans for. The fuel spent speeding up is saved by cutting down on consumables needed for the crew.

But faster trips are possible for non-chemical rockets and propulsors. A gas-core nuclear rocket can cut the trip times down to 90 days. A Mag-beam can get it down to ~35 days outbound and 51 days return. Likewise similar trip times with a Bussard QED fusion rocket. Much faster and it gets really hard to stop except for extreme rockets like pulse-drives.

Nuclear energy could take us anywhere in the solar system in under a fortnight, theoretically, but the problem is managing the energy of the exhaust-stream. The faster it goes, the more energy that must be handled. Standard nuclear rockets either vent the excess heat as exhaust - but it can't be too hot or the core goes with it. Or they power plasma/ion engines at much lower power-levels but higher exhaust speeds. Thus taking years to travel anywhere.

Perfection, especially in rockets, is really hard to achieve.