Quantum mechanics is so odd that even the visionary genius who described the secrets of the Universe seemed to dismiss certain aspects as eerie."Spooky action at a distance" is how Albert Einstein described quantum entanglement, a weird connection between particles that classical physics cannot explain. From our classical perspective, it seems to allow instantaneous communication, thereby breaking the speed of light and garnering Einstein's ire.For example, if we flipped two quantum-entangled coins and one landed heads up, we could know for sure whether the second coin would instantly land tails up – even if one were flipped on Earth and the other on Mars.That's because measuring one instantly reveals the state of the other, a correlation with no classical explanation.
This spooky synchronization is a basis for quantum computing, with untold potential for machine learning, pharmaceutical design, digital communications, and other essential applications.Now, physicists at the Institute of Science and Technology Austria (ISTA) and the Technical University of Munich in Germany have achieved a breakthrough by experimentally confirming a 20-year-old entanglement theory, providing a prototype for scaling up quantum computing processes.Not too dissimilar from hosing down an irascible chihuahua, the researchers used a 'quantum bath' of light particles to entangle isolated qubits (quantum bits), the functional units of quantum computers.An artist's rendition of two entangled particles, whose characteristics are linked. (Mark Garlick/Science Photo Library/Getty Images)Qubits are the quantum counterparts of bits (binary digits), the basic units of information that power conventional computers. Bits can represent two values, 1 or 0, which are akin to a switch being 'on' or 'off.'Qubits, however, can exist in a superposition of 1 and 0 states – like Schrödinger's famous box-trapped tabby, a metaphor for how such quantum particles can exist in multiple states at once.More traditional entanglement approaches include two methods. The first sends a single, actively controlled photon between two qubits. In the second, Nobel-prizing-winning protocol, each qubit releases a photon that must be matched. Yet this requires constant tinkering and doesn't always work.This quantum bath method offers a fully autonomous way to synchronize distant qubits by subjecting them both to low-energy microwaves, yielding a "stationary," applicable version of entanglement, rather than one that fluctuates in its properties like a pendulum.The "quantum bath" device used to deliver entangled microwave photons to the qubits. (ISTA)"In this work, we aimed to overcome this mismatch between the readily available and the practically useful forms of entanglement," explains Alejandro Andrés-Juanes, a physicist at ISTA and the study's first author."By stabilizing the entangled states remotely, our approach is fully autonomous and requires no active control or measurement."Importantly, this scheme can be applied over "arbitrary distances," ostensibly farther than the 50 centimeters (20 inches) of cable that separated the qubits from the entanglement-inducing photons in this experiment.Additionally, a single correlated (entangled) photon source can be manipulated to generate many entangled pairs.This research also holds promise for hybrid quantum systems, in which photons at different frequencies, such as optical light and microwaves, can "stabilize entanglement between qubits that are operated at vastly different energy scales," the researchers mention.Stable entanglement is equally crucial because it can be used on demand, as it were, rather than shorter-lived entanglements that must be opportunistically utilized:"This way, the entangled qubit state is stabilized, even beyond the qubits' own 'lifetime', and remains always available as a resource for further quantum processing," says Johannes Fink, a physicist at ISTA and the study's senior author. "This makes the approach conceptually significant."Finally, the researchers showed that they could verify that their qubits were indeed in sync. They did so by using incredibly short microwave pulses, of just billionths of a second, to measure the state of their qubits."Qubits can be in a superposition of states, but all these states collapse when we measure them, leaving us with a 0 or 1 state," notes Andrés-Juanes. In fact, this is one of the fundamental problems with quantum computing, as "decoherence" of qubits robs them of their quantum behavior and, therefore, their computational advantages.
