Synopsis: Gauging Randomness for Loophole-free Quantum Tests

A fast random number generator based on laser dynamics helps test the foundations of quantum physics.
Synopsis figure
Tomas Charles/ICFO

Quantum physics says that measuring an entangled particle instantaneously affects the state of its distant partner. To observe this nonlocality, a so-called Bell test experiment asks unpredictable questions of the particles and looks for agreement, or “correlation,” in their responses. However, such correlations might be replicated by some sort of pre-measurement conspiring among the experimental elements. To close this sort of “loophole,” the questions must be chosen unpredictably and faster than any conspiring signals could connect the different parts of the experiment. Researchers have now developed a random number generator (RNG) both fast enough and random enough for this task, allowing three new Bell tests to go “loophole-free” (see 16 December 2015 Viewpoint).

A typical Bell test experiment uses entangled photons, and the “questions” are measurements of the photon polarizations. The “locality loophole” refers to the possibility that the polarizer settings could have an effect on the distant particles, while the “freedom of choice” loophole imagines the particles could influence an RNG used to set the polarizers. Morgan Mitchell and his colleagues from the Institute of Photonic Sciences have devised an RNG that virtually eliminates these possibilities. Their setup uses a laser to produce pulses with random optical phase. The pulses pass through an interferometer before reaching a detector, whose output is converted to a binary bit. The randomness of the bit is guaranteed by the laser amplification being 100 times stronger than any external influence that might carry a conspiring signal. The RNG combines several of these “raw” bits to obtain a single “extracted” bit, within 0.001% of perfect unpredictability (a 50-50 chance for 0 or 1). The whole process takes less time than light would take to travel 20 meters, which is far less than the distance between stations in the new Bell tests.

This research is published in Physical Review Letters.

–Michael Schirber


Features

More Features »

Announcements

More Announcements »

Subject Areas

Quantum Physics

Previous Synopsis

Graphene

Graphene Majoranas

Read More »

Next Synopsis

Statistical Physics

Fractal London

Read More »

Related Articles

Viewpoint: Squeezed Environment Boosts Engine Performance
Nanophysics

Viewpoint: Squeezed Environment Boosts Engine Performance

A tiny engine can surpass the Carnot limit of efficiency when researchers engineer the thermal properties of the environment. Read More »

Viewpoint: The Thermodynamic Cost of Measuring Time
Quantum Physics

Viewpoint: The Thermodynamic Cost of Measuring Time

A simple model of an autonomous quantum clock yields a quantitative connection between the clock’s thermodynamic cost and its accuracy and resolution. Read More »

Synopsis: Quantum Sensing of Magnetic Fields
Quantum Physics

Synopsis: Quantum Sensing of Magnetic Fields

A new design for an atomic magnetometer utilizes so-called quantum nondemolition measurements to detect very weak magnetic-field signals. Read More »

More Articles