A two-year-old child knows that if a toy is hidden behind a screen, the toy still exists and its form hasn't changed.
This is called object permanence, and it's a concept that underpins the way we humans understand the world around us.
You might assume that the fundamental building blocks of the universe, things such as electrons and photons, must behave in the same predictable way.
Bizarrely, they don't. And it's this apparent contradiction that attracted Defence scientist Ben Harris to the extraordinary field of quantum physics.
"What I love most about quantum science is its very counterintuitive nature," Mr Harris said.
Before you measure them, quantum particles can be in multiple places all at once, similar to how a coin spinning through the air is not heads or tails but a combination of both outcomes until it lands.
"Different phenomena like that always intrigued me and always pushed me to want to learn more," he said.
New scientific discoveries are revealing more about the rules that govern the universe. At the same time, advances in quantum physics are unlocking powerful technologies that are likely to affect the way military forces operate.
Mr Harris explained that quantum sensors might be used to detect what would otherwise be hidden, such as an underground tunnel.
Quantum computers could potentially do calculations beyond what even the fastest classical supercomputers can achieve today.
'The idea of solving a virtually impossible problem was always very, very enticing to me.'
"What interests me most is its problem-solving potential," he said. "The idea of solving a virtually impossible problem was always very, very enticing to me."
Mr Harris plays an active role in quantum research at Defence, allowing him to make a contribution to the defence of Australia.
Quantum computing is still in its infancy, but breakthroughs are happening all the time.
Working in the quantum-computing team at Defence Science and Technology Group involves keeping track of these rapid developments and thinking about how they might have an impact on Australia's national security.
"A large part of what Defence does is monitors and supports all these advancements, as well as testing what the newest and latest devices are capable of and how far we can push them," Mr Harris said.
"When quantum computing becomes more and more widespread, it'll be fascinating to see the different ideas and applications that can be dreamt up and where that moves the field as a whole."
Quantum is one of the six priorities outlined in Defence's innovation, science and technology strategy.
In an increasingly challenging and contested global security environment, asymmetric technologies such as these are delivering next-generation capabilities, providing a competitive advantage to our sailors, soldiers and aviators.
