Physicists bang the drum for quantum memory

Physicists in the US say they are the first to store – and then retrieve – quantum information in a mechanical oscillator. Their device consists of an extremely thin disc of aluminium that is connected to a microwave circuit. Quantum information encoded in a microwave signal is transferred to the disc, which vibrates much like a drumhead. This information can then be retrieved by converting the mechanical oscillations back into microwaves. Built by Konrad Lehnert and colleagues at the National Institute of Standards and Technology in Boulder and the neighbouring University of Colorado, the team says the device can store quantum information for long enough to make it a potential candidate for a future quantum computer. Such machines could, in principle, outperform conventional computers at certain tasks, but require quantum bits (or qubits) that can store and transfer quantum information without it being destroyed by interacting with the outside world. Cool stuff Like many attempts to make "quantum memory", the qubits built by Lehnert and colleagues are "mesoscopic" objects – small enough to behave as quantum systems, yet large enough to fabricate on a chip and connect to other qubits. Weighing just 48 pg, their drumhead is a circular sheet of aluminium 15 μm in diameter and 100 nm thick. The entire device was chilled to 25 mK and the drum itself is put into its lowest-energy vibrational state (its ground state) using a microwave cooling technique. Quantum information is first encoded in the amplitude and phase of a microwave pulse. This pulse is then sent along a waveguide that sits next to a spiral-like resonant circuit that includes the drum (see figure; click to enlarge). The circuit is designed such that the pulse is completely absorbed by the circuit and the microwave energy is converted to vibrational energy stored in the drum. "At the end of the process, the mechanical oscillator quivers with a particular amplitude and phase, which is the amplitude and phase that the microwave signal had before it was absorbed," explains Lehnert. Towards improved efficiency This absorption process is controlled using a second microwave signal, with the presence or absence of this "transfer field" determining whether the pulse can move from the waveguide to the resonant circuit and vice versa. To read out the quantum information, the transfer field is adjusted so that the vibration of the drum is converted back to a microwave pulse, which can then jump into the waveguide and be measured by Lehnert and colleagues. In a typical experiment the team was able to store quantum information for about 25 μs without it degrading significantly. The team was also able to successfully store and retrieve the pulses in about 65% of attempts. According to Lehnert, this inefficiency arises because the microwave circuit is not perfect and some of the microwave signal is lost. But he is confident that smaller circuits will improve the performance. "We believe the prospects for improving this number are quite good, as other researchers have recently demonstrated microwave circuits with loss about 100 times smaller than we achieved," says Lehnert. Lehnert told physicsworld.com that his team's drums could be used in conjunction with superconducting qubits, which also operate at low temperatures and microwave frequencies. While mechanical oscillators cannot easily be connected with each other to create logic devices, they could be used to store quantum information that is then processed in superconducting devices. In particular, Lehnert points out that the linear nature of the device means that it could be used to store more than one qubit at the same time. Optical boost Another possible application in quantum information is using mechanical oscillators as quantum transducers, which convert quantum information from one form to another. "We are working on using mechanical oscillators to convert quantum information encoded in microwave electrical signals into information encoded as an optical field [light]," Lehnert says. Converting qubits from microwave to optical signals could play an important role in large-scale quantum information systems of the future. Although microwaves are very well suited for transferring quantum information in small ultracold devices, they cannot carry information over large distances. Optical signals, on the other hand, can travel tens or even hundreds of kilometres without losing their quantum nature.

Quantum simulation targets particle physics

A major challenge for physics in the last century was connecting two of its leading theories – quantum physics and special relativity. Researchers are still grappling with the relativistic quantum theories that resulted. Writing in Physical Review Letters, CQT researchers propose a solution to one outstanding problem: how to make predictions from the Thirring Model. The Thirring Model, which dates back to the 1950s, describes the interactions of relativistic fermions. Such particles are moving at speeds close enough to the speed of light to significantly experience time slowing and length contracting, as predicted by special relativity. Weird effects occur when the particles interact, such as the 'dressing' or changing of the particle's mass. Fermions are particles having the quantum property of half-integer spin. But researchers only know exactly what the Thirring Model predicts for fermions that have no mass. In this case, an exact solution to the equation of motion exists. However, when the model needs a mass term – for example, to describe the interactions of electrons or quarks – the mathematical solutions are cumbersome. Predictions for the behaviour of matter at these regimes are then hard or impossible to make. In their new paper, CQT's Dimitris Angelakis and his collaborators suggest a way to make predictions by quantum simulation. Over the past few years, Dimitris and his team have pioneered the idea of simulating quantum systems using photons. This approach was initially surprising because photons do not easily interact, making them apparently poor candidates to simulate interacting particles. The proposed simulations use techniques from nonlinear optics to create the effect of interactions between the photons. The team has previously shown that such photon-based quantum simulators could recreate phenomena such as the 'Luttinger liquid', an elusive state predicted in condensed matter physics, and the Cooper pairing of fermions in a superconductor. A collaboration with mathematical physicist Vladimir Korepin of the State University of New York at Stony Brook, US, who visited CQT in 2012, has now helped Dimitris and his collaborators show that the same kind of simulator system can mimic the Thirring Model, offering a way to tackled the unsolved cases. "He spoke maths and we spoke physics, but we managed to get something that makes sense from our collaboration," jokes Dimitris. The team proposes that polarised photons are confined in a one-dimensional system such as an optical fibre or waveguide, together with atoms that act as a nonlinear medium. Dimitris' student MingXia Huo, a co-author on the paper, performed calculations and simulations to show how the effective interactions could be controlled by outside lasers to make the photons behave like fermions. A photon with vertical polarisation simulates a fermion with spin 'up' and one with horizontal polarisation a fermion with spin 'down'. The proposal could point the way to simulations for other exotic fields theories, such as those describing the interaction of quarks in quantum chromodynamics. The photon-based simulator would be implementable in a table-top set-up, and Dimitris says some groups are close to having the kind of system required to test the simulation proposals. Handily, the fact that the Thirring model has already been solved for massless particles provides a way to test the simulator's output. The unknown solutions for massive particles can then be probed by tuning optical parameters such as the photon-atom interactions which give mass to the particles.. Dimitris is a Principal Investigator at CQT and faculty at the Science Department, Technical University of Crete in Greece. He, MingXia and Vladimir carried out the work with CQT PI Leong Chuan Kwek and Darrick Chang from the Institut de Ciencies Fotoniques (ICFO) near Barcelona, Spain. MingXia is a final-year PhD student co-supervised by Dimitris and Kwek. For more details, see the research article "Mimicking interacting relativistic theories with stationary pulses of light", Phys. Rev. Lett. 110, 100502 (2013); arXiv: 1207.727

Quantum darwinism, The reality of reality?

An electron, atom, or other quantum system can be many places at once. It collapses into a single location when an observer pinpoints it, but what happens when an object is not being measured? Can it choose a location on its own, or must it wait for an observer? SFI External Professor Wojciech Zurek might have an answer. His Theory of Quantum Darwinism applies natural selection to our quantum universe. It builds on the theory of decoherence, which shows that a quantum particle can no longer be in many places at once not only at the moment it’s measured, but also when it interacts with its environment, which can be a de facto observer. (For a particle, common environments are photons and air molecules. Scaling up to a more tangible level, our world persists in its classical state only because large objects can never be fully isolated from their surroundings.) Quantum Darwinism goes beyond decoherence by studying imprints left by the system on the environment, he says. Drawing from natural selection, where the fittest survive and proliferate, Quantum Darwinism shows that certain states are selected because they survive better than others in a given environment. Surviving states then proliferate by spreading information about themselves, and we discover them indirectly by intercepting tiny fractions of the environment. This is how consensus about the fittest states -- the “objective classical reality” we take for granted -- arises in our quantum universe, Zurek says, and how effectively classical states that exist objectively, immune to measurement, emerge from a quantum substrate. Recent studies have offered evidence in support of Zurek’s theory. Now, with his newly awarded grant from the John Templeton Foundation, he will examine how quantum information propagates, and how the classical world of our everyday experience emerges from the quantum ingredients.

Timing glitches dog neutrino claim

BY EUGENIE SAMUEL REICH

Is it an epic blunder or a textbook demonstration of how science should work? To some physicists, the OPERA (Oscillation Project with Emulsion-tracking Apparatus) collaboration deserves credit for disclosing possible errors in its paradigm-challenging measurement of neutrinos travelling faster than light. “I think we did the right thing to continue to investigate,” says Dario Autiero of the Institute of Nuclear Physics of Lyons in France, who presented the original results and notes that the collaboration had spent six months checking its result before its announcement last September. To others, the revelation shows that the OPERA team went public too soon with its claim that neutrinos from CERN, the European particle-physics laboratory near Geneva in Switzerland, were flouting Albert Einstein’s absolute limit on the speed of light as they travelled the 730 kilometres to the OPERA detector at the underground Gran Sasso National Laboratory near L’Aquila, Italy. “I find it embarrassing,” says Luca Stanco of the National Institute of Nuclear Physics in Padova, Italy, an OPERA member who initially refused to sign a paper about the result. “Maybe we should have been more cautious and done more checks.”
On 23 February, OPERA team members reported two possible sources of error in the experiment. The initial result suggested that the neutrinos were reaching the detector 60 nanoseconds faster than the speed of light would allow. Both potential errors would affect the neutrinos’ arrival time, as measured by OPERA’s master clock (see ‘Timing trouble’). The first is a faulty connection at the point at which the light from a fibre-optic cable brings a synchronizing Global Positioning System (GPS) signal into the master clock. The fault could have delayed the GPS signal, causing the master clock to run slow and thus causing the neutrinos’ travel time to appear shorter than it actually was.
“It’s a subtle effect,” says Autiero, and one that was evident only when the team examined many measurements of signals passing through the connection. Tests of the timing system turned up a second, opposing effect: an oscillator within the master clock that keeps time between the arrivals of synchronization signals was running fast. That would have made the neutrinos’ travel time seem longer.
The collaboration says that it has not yet worked out the magnitude of these effects. Autiero says that because of the high profile of the result and the possibility of rumours and leaks, the collaboration wanted to disclose the potential errors promptly. The OPERA team plans to correct the faults and repeat the experiment after CERN’s neutrino beam is switched on again in March, following a winter break. Two independent checks of the measurement are also being considered. One, at Japan’s Tokai to Kamioka (T2K) neutrino experiment, would still be valuable despite the doubt cast on the OPERA data, but may now prove harder to fund, says international co-spokesman Chang Kee Jung, a physicist at Stony Brook University in New York. But another, the Main Injector Neutrino Oscillation Search (MINOS) experiment, which fires neutrinos from Fermilab in Batavia, Illinois, to an underground detector in northern Minnesota, will proceed, at a cost of about US$500,000. “It’s never a bad idea to have multiple measurements,” says MINOS
co-spokesman Rob Plunkett.
Jorge Páramos, a physicist at the Higher Technical Institute in Lisbon, says that the admissions by OPERA point to an honest mistake, albeit one that should have been avoided. “The putative origin of the systematic error reflects the innards of the experiment — something that should have been checked exhaustively before any public announcement,” he says.

Source: Nature Physics
By Shabir Barzanjeh.

10 Tops topics which will shake our scientific notion

The two physics stories that dominated the news in 2011 were questions rather than solid scientific results, namely "Do neutrinos travel faster than light?" and "Has the Higgs boson been found?". However, there have also been some fantastic bona fide research discoveries over the last 12 months, which made it difficult to decide on the Physics World 2011 Breakthrough of the Year. But after much debate among the Physics World editorial team, this year's honour goes to Aephraim Steinberg and colleagues from the University of Toronto in Canada for their experimental work on the fundamentals of quantum mechanics. Using an emerging technique called "weak measurement", the team is the first to track the average paths of single photons passing through a Young's double-slit experiment – something that Steinberg says physicists had been "brainwashed" into thinking is impossible. We have also awarded nine runners-up (see below). The choice between first and second place was particularly close this year because the number-two finding also involves weak measurement – this time to map the wavefunction of a bunch of photons. But we felt that Steinberg's finding edged it. Other breakthroughs in the list include the first "space–time" cloak, a laser made from a living cell and a new way to measure cosmic distances.

1st place: Shifting the morals of quantum measurement
Steinberg's work stood out because it challenges the widely held notion that quantum mechanics forbids us any knowledge of the paths taken by individual photons as they travel through two closely spaced slits to create an interference pattern. This interference is exactly what one would expect if we think of light as an electromagnetic wave. But quantum mechanics also allows us to think of the light as photons – although with the weird consequence that if we determine which slit individual photons travel through, then the interference pattern vanishes. By using weak measurements Steinberg and his team have been able to gain some information about the paths taken by the photons without destroying the pattern. In the experiment, the double slit is replaced by a beamsplitter and a pair of optical fibres. A single photon strikes the beamsplitter and travels along either the right or the left fibre. After emerging from the closely spaced ends of the parallel fibres, it creates an interference pattern on a detector screen. The weak measurement is performed by passing the emerging photons through a piece of calcite, which imparts a tiny rotation in the polarization of the photon. The amount of rotation depends on the direction of travel of the photon – in other words, its momentum. The photons are then "post-selected" according to where they strike the screen, which allows the researchers to determine the average direction of travel of photons that arrive there. The experiment reveals, for example, that a photon detected on the right-hand side of the diffraction pattern is more likely to have emerged from the optical fibre on the right than from the optical fibre on the left. While this knowledge is not forbidden by quantum mechanics, Steinberg says that physicists have been taught that "asking where a photon is before it is detected is somehow immoral". "Little by little, people are asking forbidden questions," says Steinberg, who adds that his team's experiment will "push [physicists] to change how they think about things".

2nd place: Measuring the wavefunction
Second place goes to another group that has asked a "forbidden question". Led by Jeff Lundeen at the National Research Council of Canada in Ottawa – a former colleague of Steinberg – a team has used weak measurement to map out the wavefunction of an ensemble of identical photons without actually destroying any of them. Quantum tomography, in contrast, maps out the wavefunction at the expense of destroying the state. As well as boosting our understanding of the fundamentals of quantum mechanics, the technique could prove useful in cases where tomography cannot be used.

3rd place: Cloaking in space and time
Coming in at third place are two teams – one at Cornell University in the US led by Alexander Gaeta, and the other at Imperial College London headed by Martin McCall. In early 2011 McCall's team published a theoretical analysis of how an event in space and time could be cloaked, which he later described in a special Physics World feature. A few months later, Gaeta and colleagues built a device that uses two "split time lenses" to do just that. As well as changing our ideas about what can and cannot be cloaked, space–time cloaking could also be used in the perfect bank heist – at least in theory.

4th place: Measuring the universe using black holes
Fourth spot on the list goes to Darach Watson and colleagues at the University of Copenhagen, Denmark, and the University of Queensland, Australia, who have worked out a way of using supermassive black holes – which power active galactic nuclei (AGNs) – as "standard candles" for making accurate measurements of cosmic distances. The work is important because AGNs can be found just about everywhere in the universe, and unlike the supernovae currently used as standard candles, the light from AGNs endures for long periods of time.

5th place: Turning darkness into light
Christopher Wilson and colleagues of Chalmers University of Technology in Sweden together with physicists in Japan, Australia and the US have bagged fifth place because they are the first to see the dynamical Casimir effect in the lab. The effect arises when a mirror is moving so quickly through a vacuum that pairs of virtual photons – which are always appearing and then annihilating – are pulled apart to create real photons that can then be detected. As well as shedding new light on the Casimir effect, the team's use of a superconducting quantum interference device (SQUID) as the mirror make this an extremely clever experiment.

6th place: Taking the temperature of the early universe
Just after the Big Bang, the universe was a complicated soup of free quarks and gluons that eventually condensed to form the protons and neutrons we see today. Sixth place in our top 10 goes to a team of physicists in the US, India and China that has made the best calculation yet of this condensation temperature: two trillion degrees Kelvin. As well as providing important insights into the early universe, the work also advances our understanding of quantum chromodynamics, which describes the properties of neutrons, protons and other hadrons.

7th place: Catching the flavour of a neutrino oscillation
Seventh place is awarded to the international team of physicists working on the Tokai-to-Kamioka (T2K) experiment in Japan. The researchers fired a beam of muon neutrinos 300 km underground to a detector, where they found that six neutrinos had changed, or "oscillated", into electron neutrinos. While the measurement is not good enough to claim the discovery of the muon-to-electron neutrino oscillation, it is the best evidence yet that one "flavour" of neutrino can oscillate into another.

8th place: Living laser brought to life
In a fascinating bit of biophysics, Malte Gather and Seok Hyun Yun at Harvard Medical School in the US share eighth place for being the first to make a laser from a living biological cell. By shining intense blue light onto green fluorescent protein molecules inside an embryonic kidney cell, the molecules generate light that is intense, monochromatic and directional. The cells survive the ordeal and this amazing phenomenon could potentially be used to distinguish cancerous cells from healthy ones.

9th place: Complete quantum computer made on a single chip
Ninth place goes to Matteo Mariantoni and colleagues at the University of California, Santa Barbara for being the first to implement a quantum version of the "Von Neumann" architecture found in PCs. Based on superconducting circuits and integrated on a single chip, the new device has been used to perform two important quantum-computing algorithms. Its development moves us closer to the creation of practical quantum computers that solve real-life problems.

10th place: Seeing pure relics from the Big Bang
Michele Fumagalli and Xavier Prochaska of the University of California, Santa Cruz and John O'Meara of Saint Michael's College in Vermont take 10th spot for being the first to catch sight of clouds of gas that are pure relics of the Big Bang. Unlike other clouds in the distant universe – which appear to contain elements created by stars – these clouds contain just the hydrogen, helium and lithium created by the Big Bang. As well as confirming predictions of the Big Bang theory, the clouds provide a unique insight into the materials from which the first stars and galaxies were born.

Source:physicsworld
Shabir. Barzanjeh

Another Step Back for Wave-Particle Duality

Published December 2, 2011 | Physics 4, 102 (2011) | DOI: 10.1103/Physics.4.102

A new thought experiment makes it clearer than ever that photons aren’t simply particles or waves.
Quantum physics tells us that a photon isn’t strictly a particle or strictly a wave. And yet most of us will revert back—whenever we can—to familiar concepts of billiard balls or vibrating strings when picturing photons in our heads. A new thought experiment, proposed in Physical Review Letters, hopes to break us of these old habits. The authors imagine a type of quantum switch that controls whether a simple optical measurement tests for particlelike or wavelike behavior in a single photon. This slight reworking of a famous experiment demonstrates with logical precision the futility of trying to label the photon as a particle or a wave.

The wave-particle duality is often illustrated by splitting a light beam so that it travels along two separate paths that later merge to form an interference pattern from the combined beams. For a dim beam delivering photons one-at-a-time, this interference suggests that each photon is a wave that travels down both paths simultaneously. But if the paths are observed individually, then the photon will behave like a particle, traveling down only one path or the other and generating no interference. The fact that no experiment can measure both the wave and the particle behaviors simultaneously is called the principle of complementarity.

Someone unhappy with this indeterminacy could imagine that the photon somehow knows what measuring devices lie ahead and thereby decides in advance to go down both paths or one path, accordingly. To deny us this conceit, John Wheeler, at the University of Texas, proposed a thought experiment 30 years ago in which a “wave detector” suddenly switches to a “particle detector” during the photon’s travels, or vice versa.

This so-called delayed-choice experiment was performed in 2007 using an interferometer [1]. In the normal setup, a beam splitter creates two separate light beams that later recombine in a second beam splitter. Detectors placed at the two outputs of this beam splitter both register an interference pattern. However, this wave detector can be turned into a particle detector by removing the second beam splitter, so that the two paths no longer interfere. In the experiment, the choice to add or remove the second beam splitter was made after an individual photon had already passed through the first beam splitter. The data showed that particle and wave behavior were unaffected by the delayed choice, as expected from standard quantum mechanics.

Radu Ionicioiu, now at the Institute for Quantum Computing in Waterloo, Canada, and Daniel Terno of Macquarie University in Sydney, Australia, wanted to see what happens in the thought experiment if the delayed choice is made through quantum means. They imagined that the interferometer contains a quantum device—perhaps an atom in a cavity or a micro-mirror placed on a cantilever—that can exist in two possible states. One state selects the particle experiment, and the other selects the wave experiment. This quantum control element can be placed in a combination, or superposition, of its two states, making the whole experiment participate in the wave-particle duality.

“We show you can do both wave and particle experiments at once,” Ionicioiu says. This means the choice of wave vs particle can be delayed indefinitely. The photon can be observed at one of the detectors and still not “know” if it is supposed to be a wave or a particle. It’s only when the observer decides to measure the state of the quantum control that the photon’s behavior can be identified as wavelike or particlelike.

The idea of having measurements depend on some secondary quantum effect was explored previously in the so-called quantum eraser [2]. But this new, simpler thought experiment allows a direct analysis of the information flows between experimental elements, Ionicioiu says. Within this simplified framework, the theorists evaluated certain alternative theories to standard quantum mechanics known as “hidden variable” theories and showed that they would have to be absurdly complicated to reproduce the results of their quantum delayed-choice experiment.

This proof doesn’t come as a surprise. “We know very well that quantum phenomena cannot be described by hidden-variable theories, unless they have utterly implausible and unacceptable properties,” says Berthold-Georg Englert of the National University of Singapore. Marco Genovese of the Italian Metrological Institute (INRIM) in Turin agrees that the basic concepts here are not new. But he still very much likes this simple thought experiment for the way it points out the paradoxes that emerge from treating the photon as a classical particle or wave.

-Shabir Barzanjeh.

References

V. Jacques, E Wu, F. Grosshans, F. Treussart, P. Grangier, A. Aspect, and J.-F. Roch, “Experimental Realization of Wheeler’s Delayed-Choice Gedanken Experiment,” Science 315, 966 (2007).
Y-H. Kim, R. Yu, S. P. Kulik, Y. Shih, and M. O. Scully, “Delayed ‘Choice’ Quantum Eraser,” Phys. Rev. Lett. 84, 1 (2000).

Coherent Schrödinger's cat still confounds

The famous paradox of Schrödinger's cat starts from principles of quantum physics and ends with the bizarre conclusion that a cat can be simultaneously in two physical states – one in which the cat is alive and the other in which it is dead. In real life, however, large objects such as cats clearly don't exist in a superposition of two or more states and this paradox is usually resolved in terms of quantum decoherence. But now physicists in Canada and Switzerland argue that even if decoherence could be prevented, the difficulty of making perfect measurements would stop us from confirming the cat's superposition.

Erwin Schrödinger, one of the fathers of quantum theory, formulated his paradox in 1935 to highlight the apparent absurdity of the quantum principle of superposition – that an unobserved quantum object is simultaneously in multiple states. He envisaged a black box containing a radioactive nucleus, a Geiger counter, a vial of poison gas and a cat. The Geiger counter is primed to release the poison gas, killing the cat, if it detects any radiation from a nuclear decay. The grisly game is played out according to the rules of quantum mechanics because nuclear decay is a quantum process.

If the apparatus is left for a period of time and then observed, you may find either that the nucleus has decayed or that it has not decayed, and therefore that the poison has or has not been released, and that the cat has or has not been killed. However, quantum mechanics tells us that, before the observation has been made, the system is in a superposition of both states – the nucleus has both decayed and not decayed, the poison has both been released and not been released, and the cat is both alive and dead.
Mixing micro and macro

Schrödinger's cat is an example of "micro-macro entanglement", whereby quantum mechanics allows (in principle) a microscopic object such as an atomic nucleus and a macroscopic object such as a cat to have a much closer relationship than permitted by classical physics. However, it is clear to any observer that microscopic objects obey quantum physics, while macroscopic things obey the classical physics rules that we experience in our everyday lives. But if the two are entangled it is impossible that each can be governed by different physical rules.

The most common way to avoid this problem is to appeal to quantum decoherence, whereby multiple interactions between an object and its surroundings destroy the coherence of superposition and entanglement. The result is that the object appears to obey classical physics, even though it is actually following the rules of quantum mechanics. It is impossible for a large system such as a cat to remain completely isolated from its surroundings, and therefore we do not perceive it as a quantum object.

While not disputing this explanation, Christoph Simon and a colleague at the University of Calgary, and another at the University of Geneva, have asked what would happen if decoherence did not affect the cat. In a thought experiment backed up by computer simulations, the physicists consider pairs of photons (A and B) generated from the same source with equal and opposite polarizations, travelling in opposite directions. For each pair, photon A is sent directly to a detector, but photon B is duplicated many times by an amplifier to make a macroscopic light beam that stands in for the cat. The polarizations of the photons in this light beam are then measured.
Two types of amplifier

They consider two different types of amplifier. The first measures the state of photon B, which has the effect of destroying the entanglement with A, before producing more photons with whatever polarization it measures photon B to have. This is rather like the purely classical process of observing the Geiger counter to see whether it has detected any radiation, and then using the information to decide whether or not to kill the cat. The second amplifier copies photon B without measuring its state, thus preserving the entanglement with A.

The researchers ask how the measured polarizations of the photons in the light beam will differ depending on which amplifier is used. They find that, if perfect resolution can be achieved, the results look quite different. However, with currently available experimental techniques, the differences cannot be seen. "If you have a big system and you want to see quantum features like entanglement in it, you have to make sure that your precision is extremely good," explains Simon. "You have to be able to distinguish a million photons from a million plus one photons, and there is no current technology that would allow you to do that."

Quantum-information theorist Renato Renner of ETH Zurich is impressed: "Even if there was no decoherence, this paper would explain why we do not see quantum effects and why the world appears classical to us, which is a very fundamental question of course." But, he cautions, "The paper raises a very fundamental question and gives us an answer in an interesting special case, but whether it is general remains to be seen."

The research will be published in Physical Review Letters.