Nature: Quantum effects brought to light

[bystander]

Quantum effects brought to light
Nature News | Zeeya Merali | 2011 Apr 28

Results of entanglement made visible to human eyes.

By linking a single photon to a field of thousands,
researchers have made the invisible visible.
(Credit: Equinox Imagery / Alamy)

It's an eye test with a quantum twist: physicists have used humans to detect the results of a quantum phenomenon for the first time.

Nicolas Gisin, a physicist at the University of Geneva in Switzerland, devised a new test to see if the human eye could pick out signs of 'entanglement'.¹ This weird quantum effect inextricably links two or more objects in such a way that measurements carried out on one immediately change the properties of its partners, no matter how far apart they are. Quantum effects, such as entanglement, are usually confined to the invisible microscopic world and are detected only indirectly using precision instruments.

Gisin and his colleagues were inspired by an experiment carried out in 2008 by Fabio Sciarrino and his team at La Sapienza University in Rome, Italy.² Usually, physicists working with entangled photons only deal with a small number at a time. In the Rome experiment, the physicists entangled a pair of photons and then 'amplified' one of them to create a shower of thousands of photons with the same quantum state. In this way, one 'microscopic' photon seemingly became entangled with thousands of others in a 'macroscopic' light field. "I immediately realized that the human eye could see that many photons," says Gisin.

Using a similar set-up to that of Sciarrino, Gisin and his team entangled two photons. One was sent to a standard photon detector, while the other was amplified using a machine that generated a shower of photons with the same polarization, thereby, in theory, generating a micro–macro entangled state.

But Gisin replaced the photon detector Sciarrino used for the light field with a human. The beam of light produced by the amplifier could appear in one of two positions, and the location of the beam reflected the polarization state of the photons in the field. Gisin and his team sat in the dark for hours, marking the position of the light spot over repeated runs of the experiment, for the first time seeing the effects of quantum entanglement with the naked eye.

Like Sciarrino's group, Gisin and his team used the standard test for entanglement, known as a Bell test, to compare how well the polarization of the single photon and the light field matched up. This test sets a level for the correlation of states between the two objects above which their entanglement is confirmed. And, like the Rome group, Gisin's team got a positive result.

The results of the human 'detectors' were checked with photon detectors, and they matched — although the latter were "faster and more reliable than humans and didn't complain of tiredness", Gisin says

More than meets the eye

But there was a hitch. What Gisin's team saw was not micro-macro entanglement. Gisin had a nagging suspicion that the Bell test may not be valid for macroscopic objects, so he deliberately set up the experiment so that the state of the second photon was measured before it was amplified. According to the rules of quantum mechanics, this act of measurement would break the entanglement, meaning that the first photon and the light field could not be in an entangled state. The system should not have passed the Bell test.

"We set up the worst kind of amplifier precisely to see what result the standard Bell test would give, and it gave the wrong — positive — answer," says Gisin.

The reason for the false positive result is that no detector — human or mechanical — is perfect, says Gisin. Some photons will always be lost during the experiment — an effect known as the detection loophole. Normally, this does not affect the Bell test, but Gisin says that as more photons come into play, the loophole hugely distorts the results. So regardless of whether or not Sciarrino and his team did create micro–macro entanglement in their 2008 experiment, the Bell test would always give a positive result, says Gisin.

"This is brilliant work showing that if we do not control everything in the experiment, we can be fooled into thinking we have seen a macroscopic quantum effect, when we haven't," says Magdalena Stobińska, a quantum physicist at the Max Planck Institute for the Science of Light in Erlangen, Germany.

Sciarrino is not surprised by the results. He and his team had already realized that they could not trust their Bell test, and are working on a new way to verify micro–macro entanglement. They plan to use a laser to boost the light signal from the photon field to improve its chances of detection.³ "Unfortunately, we cannot do this experiment with humans as detectors because the laser would burn out their eyes," Sciarrino says.

Even though Gisin's team did not see micro-macro entanglement, he notes that the positive Bell test does show that the original pair of single photons was entangled before amplification, confirming micro-micro entanglement existed at the start of the experiment.

"The experiment is lovely because in this sense you can 'see' entanglement," says Sciarrino. "It brings quantumness closer to human experience."

1. Experimental amplification of an entangled photon: what if the detection loophole is ignored? - E Pomarico et al
   • arXiv.org > quant-ph > arXiv:1104.2212 > 12 Apr 2011
2. Entanglement Test on a Microscopic-Macroscopic System - F De Martini, F Sciarrino, C Vitelli
   • Physical Review Letters 100 253601 (27 Jun 2008) DOI: 10.1103/PhysRevLett.100.253601
     arXiv.org > quant-ph > arXiv:0806.2218 > 13 Jun 2008
3. Hybrid non-locality test in a microscopic-macroscopic system - N Spagnolo et al
   • arXiv.org > quant-ph > arXiv:1012.3660 > 16 Dec 2010

[neufer]

Scientists Link Diamonds in Strange Quantum Entanglement
By Clara Moskowitz, December 02, 2011

The vibrational states of two spatially separated, millimeter-sized diamonds are entangled at room temperature by beaming laser light at them (green). The researchers verified this entanglement by studying the subsequent laser pulses beamed through the system.

---

<<Scientists have linked two diamonds in a mysterious process called entanglement that is normally only seen on the quantum scale.

Entanglement is so weird that Einstein dubbed it "spooky action at a distance." It's a strange effect where one object gets connected to another so that even if they are separated by large distances, an action performed on one will affect the other. Entanglement usually occurs with subatomic particles, and was predicted by the theory of quantum mechanics, which governs the realm of the very small.

But now physicists have succeeded in entangling two macroscopic diamonds, demonstrating that quantum mechanical effects are not limited to the microscopic scale. "I think it's an important step into a new regime of thinking about quantum phenomena," physicist Ian Walmsley of England's University of Oxford said."That is, in this regime of the bigger world, room temperatures, ambient conditions. Although the phenomenon was expected to exist, actually being able to observe it in such a system we think is quite exciting."

Another study recently used quantum entanglement to teleport bits of light from one place to another. And other researchers have succeeded in entangling macroscopic objects before, but they have generally been under special circumstances, prepared in special ways, and cooled to cryogenic temperatures. In the new achievement, the diamonds were large and not prepared in any special way, the researchers said.

"It's big enough you can see it," Walmsley told LiveScience of the diamonds."They're sitting on the table, out in plain view. The laboratory isn't particularly cold or particularly hot, it's just your everyday room." Walmsley, along with a team of physicists led by Oxford graduate student Ka Chung Lee, accomplished this feat by entangling the vibration of two diamond crystals. To do so, the researchers set up an apparatus to send a laser pulse at both diamonds simultaneously. Sometimes, the laser light changed color, to a lower frequency, after hitting the diamonds. That told the scientists it had lost a bit of energy.

Because energy must be conserved in closed systems (where there's no input of outside energy), the researchers knew that the "lost" energy had been used in some way. In fact, the energy had been converted into vibrational motion for one of the diamonds (albeit motion that is too small to observe visually). However, the scientists had no way of knowing which diamond was vibrating. Then, the researchers sent a second pulse of laser light through the now-vibrating system. This time, if the light emerged with a color of higher frequency, it meant it had gained the energy back by absorbing it from the diamond, stopping its vibration.

The scientists had set up two separate detectors to measure the laser light — one for each diamond. If the two diamonds weren't entangled, the researchers would expect each detector to register a changed laser beam about 50 percent of the time. It's similar to tossing a coin, where random chance would lead to heads about half the time and tails the other half the time on average. Instead, because the two diamonds were linked, they found that one detector measured the change every time, and the other detector never fired. The two diamonds, it seemed, were so connected they reacted as a single entity, rather than two individual objects.

The scientists report their results in the Dec. 2 issue of the journal Science. "Recent advances in quantum control techniques have allowed entanglement to be observed for physical systems with increasing complexity and separation distance," University of Michigan physicist Luming Duan, who was not involved in the study, wrote in an accompanying essay in the same issue of Science."Lee et al. take an important step in this direction by demonstrating entanglement between oscillation patterns of atoms—phonon modes—of two diamond samples of millimeter size at room temperature, separated by a macroscopic distance of about 15 cm."

In addition to furthering scientists' understanding of entanglement, the research could help develop faster computers called photonic processors, relying on quantum effects, said Oxford physicist Michael Sprague, another team member on the project. "The long-term goal is that if you can harness the power of quantum phenomena, you can potentially do things more efficiently than is currently possible," Sprague said.>>