Antonio Acín, a physicist at the Institute of Photonic Sciences in Spain and an author of a paper describing the approach in the April 15 issue of Nature, says that true randomness is elusive. "If you go to a casino and play roulette, or you flip a coin, if you had access to the initial position and speed of the ball or coin, you could predict the result with certainty," he says. "The randomness that we have in our world is because of lack of knowledge."
The researchers utilized a pair of ytterbium ions as quantum bits, or qubits, each confined to a private vacuum chamber about a meter apart in an experimental system at the Joint Quantum Institute of the University of Maryland and the National Institute of Standards and Technology. Depending on the state of the ions, a resonant laser pulse will either cause them to emit a photon, representing a binary 1, or remain dark, representing a zero. Each atom's state cannot be known with certainty until it is measured with the laser pulse—that is, it is probabilistic rather than deterministic—so the measurements can be used to generate an intrinsically random string of binary digits.
Acín's group used statistical tests to show that the output from the new device indeed stems from quantum uncertainty rather than from residual deterministic—and hence predictable—effects. Using so-called Bell inequalities, the researchers demonstrated that the two qubits shared a quantum-mechanical link knows as entanglement, meaning that the measurement of one qubit's quantum-mechanical state instantaneously affects that of the other qubit. Bell inequalities, named for Irish physicist John Bell, mark how much correlation a purely deterministic, non-entangled system should have. (In other words, they dictate how the qubits should behave if measurement of one has no effect on the other.) If those inequalities are violated, some unseen and instantaneous link must be in play that allows distant systems to influence each other. Entanglement is not possible in classical physics, so the nature of the system must be governed by quantum randomness.
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