Exploiting a principle that Einstein derided as "spooky action at a distance," physicists on Wednesday reported they had devised a machine that generates genuinely random numbers and closes a loophole in cryptography.

The experiment helps open the way to next-generation ultra-secret communication, according to their paper, appearing in the British journal Nature.

Cryptographers use random numbers as a key in which to encode a message, thus thwarting code-busters looking for a pattern that can crack open its contents.

For practical purposes, cryptographers use algorithms known as pseudo-random number generators to provide these coveted digits.

But there are always twinges of uncertainty about using these tools.

One fear is that the programme may start to repeat itself at some point, churning out the same sequence of numbers and thus providing a telltale to spotters.

Another is that people who made the programme may have kept a copy of it, which of course would be a security weakness.

The experiment reported on Wednesday solves both dilemmas by providing what its authors call a certifiably random string of numbers, all generated in privacy.

The device hinges on a key principle of the sub-atomic world of quantum mechanics: a weird condition known as entanglement.

Particles of matter or light (photons) have a random state, such as the position of an electron or the polarisation of a photon.

Under entanglement, though, two particles can become so interdependent that even if they are far apart, their states are the same. Thus if you measure the property of one particle, you can also measure the property of its sister.

Quantum entanglement runs counter to our own tuition as well as classical physics, which is why Einstein argued against it.

In the experiment, the scientists held atoms in two separate traps, gave them a jolt of energy to excite them and then released a single photon from each trap at the same time.

The photons intersected at a prism called a beamsplitter, where they entangled, and then zipped onwards on separate paths to detectors.

Every time the detectors signalled entanglement, each atom was then rotated on its axis according to a random schedule, and the light it emitted was measured.

The value from each of the two atoms was then used to create a binary number -- the "1" or "0" that is the bedrock of computer code.

Over a month, the researchers carried out more than 3,000 consecutive entanglements, generating a string of 42 privately generated binary digits.

Such a speed would of course be useless for cryptographers who need random numbers quickly, but the study is only a "proof of concept" venture to show that it could be done.

"The random bit generation rate is extremely slow," said Chris Monroe, a professor of physics at the University of Maryland.

"But we expect speedups by orders of magnitude in coming years as we more efficiently entangle the atoms, perhaps by using atom-like quantum system embedded in a solid-state chip."