The bicycle was parked at the checkpoint and its four-kilogram bomb was detonated by remote control, Quetta police chief Mir Zubair Mehmood told AFP.
"At least 12 people, including four policemen, were wounded," Mehmood said.
Asif Ghafoor, another senior police officer, said a 12-year-old boy who was among the wounded died later in hospital.
Mohammad Babel, a rescue official from the country's largest charity, also confirmed the boy's death.
Bomb attacks frequently target government security personnel in Quetta, the capital of Baluohistan province where insurgents in 2004 began fighting for autonomy and a greater share of the region's natural mineral wealth.
There was no immediate claim of responsibility for the attack.
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Who will unveil the terrorist............ We have to Pray & Duaa for peace......................
The condition in our Karachi and whole of Pakistan reminds me of a couplet by Mr.Abuzari, "hotey hain mere sheher mein har roz dhamakey, rehtee hai mere des mein shabra'at musalsil".