IT IS a meeting point, this vast expanse of near-nothingness hundreds of kilometres above our planet鈥檚 surface. Here, Earth鈥檚 gravity is too weak for atmospheric gases to linger, but the absolute emptiness of outer space has not yet quite begun. Human activity is ever-present. Satellites wink and blink, monitoring, directing, communicating.
These satellites too are a meeting point. Deep within their sensors and electronic circuits, electrons and photons dance to the tune of our most fundamental theory of nature鈥檚 workings: quantum theory. Its fuzzy uncertainties and instantaneous influences provide a peerless description of matter on the smallest scales. But to predict how a satellite itself will move 鈥� or any large body, from falling apples to stars, galaxies and the universe itself 鈥� we must call on a very different mathematical construction: the rigid, space-warping equations of Einstein鈥檚 theory of gravity, general relativity.
These two theories really don鈥檛 get along. In fact they are fundamentally, mutually antagonistic. So what happens when one trespasses on the other鈥檚 territory? To find out, there is only one way to go. We must take our quantum experiments into relativity鈥檚 realm 鈥� that marginal zone far above our heads.
Advertisement
If we have as yet failed to find a chink in the armour of quantum theory or relativity, it is not for want of trying. The space-warping quirks of relativity that lead to deviations from Newton鈥檚 earlier theory of gravity only become obvious on very large scales, but our passive observations of distant planets, stars and galaxies have yet to deliver anything incompatible with Einstein鈥檚 ideas. We can test quantum theory鈥檚 weirdness more directly here on Earth, with the same result 鈥� which was much to Einstein鈥檚 distaste.
Take experiments performed by of the University of Vienna, Austria, and his team over the past few years, beaming photons of light great distances in the Canary Islands. In quantum theory, particles such as photons exist as wave functions, probabilistic entities depicted as being in all their possible quantum states at once. When they are measured, however, they 鈥渃ollapse鈥� into a definite state. What鈥檚 more, the effect known as entanglement allows the fates of two or more particles to be intertwined. By measuring the state of one particle, you can collapse the wave function of another, seemingly instantaneously, however far away.
Einstein was dismissive of this 鈥渟pooky action at a distance鈥�, preferring to think that some hidden, physical influence connected the measurements. Yet working on moonless, still nights to beam one of an entangled pair of photons 144 kilometres between the islands of La Palma and Tenerife, Zeilinger and his team observed just the correlated collapse in the photons鈥� states that quantum theory dictated (New 杏吧原创, 26 February 2011, p 36).
Such trickery is already in practical use. An eavesdropper cannot listen in on information encoded in entangled quantum states without collapsing them, thus revealing the interception. Cryptographic keys written in photon polarisation states are now used to encode and decode messages in small-scale fibre-optic networks set up by government agencies, research labs and commercial companies worldwide.
This has its limits, though. Beyond a hundred kilometres or so, absorption within fibre-optic cables tends to disturb any quantum information transmitted along it, rendering it useless. Unlike for classical information, we do not have any reliable way to boost quantum signals along their route. If we want to be enjoying the fruits of secure, global quantum communication any time soon, we will have to beam the signals via satellite.
That gives a practical edge to a very fundamental question. 鈥淲e鈥檝e seen entanglement work over macroscopic distances, but is it going to work forever?鈥� asks Giovanni Amelino-Camelia, a theorist at the Sapienza University of Rome in Italy. And there is good reason to believe that the sky might indeed be the limit.
Conflicting clocks
Entanglement as we know it on Earth relies on quantum theory鈥檚 assumption that space and time form an impassive, unchanging background against which events such as measurements simply take place. Perform a measurement in one place, and observe a correlated effect in another, and you can be reasonably sure the one influenced the other. But in relativity鈥檚 realm, space and time are, well, relative. Time appears to tick more slowly and space to contract for objects moving at high speeds relative to one another. The closer an object is to a large source of gravity, the slower its clock will run.
Such effects are negligible on Earth鈥檚 surface, but GPS systems routinely correct for them as their signals bounce to and from satellites. And they could mean, for example, that the order in which things happen in quantum experiments is no longer so clear cut. How do relativistic effects change things such as entanglement, if at all?
It is not the first time we have posed that question, but previous proposals to answer it have been frustrated. Zeilinger has been in discussions with the European Space Agency since 2002 about so that he can fire them down to detectors on Earth. 鈥淭heir answer is not a no, but a not yet,鈥� he says. A proposal in 2010 for a quantum link-up to a microsatellite sent into low Earth orbit by the Canadian Space Agency was eventually passed over, although 鈥渢he possibility of doing fundamental science with the satellite remains鈥�, says one of the scientists involved, of the University of California, San Diego. Last year, NASA organised a , and there are 鈥渞umours鈥� of its willingness to fund an experiment, says Zeilinger 鈥� but nothing more than that.
Satellite state
Enter the new, muscular player in all things space: China. An erstwhile student of Zeilinger鈥檚, of the University of Science and Technology of China in Hefei, is now leading a team developing the Quantum Science Satellite, the world鈥檚 first dedicated quantum space probe. Due to launch in 2016, it will have a sun-synchronous orbit: it will pass over locations on Earth at the same time each day at an altitude of 600 kilometres.
Zeilinger is in on the project too. The first aim is to share a quantum cryptographic key via satellite between the Chinese Academy of Sciences in Beijing and Zeilinger鈥檚 institute in Vienna. This will be used to decrypt a secure intercontinental phone call between the two 鈥� if things pan out. 鈥淭he pressure鈥檚 going up, we have a lot of challenges to overcome,鈥� says of the Chinese team. Collisions with air molecules that disrupt the photon beams should be less of a problem than in the Canary Islands experiments: by going up and down, the photons travel less distance in the thickest layers of the atmosphere. Even so, only one of every million entangled particles transmitted is expected to make it to the detector.
The real problem is that the satellite will only be visible to the ground stations in China and Europe for a few minutes at a time as it shoots overhead at 8 kilometres per second. It will require a sure aim to lock the laser beams to the passing satellite. Pan and his team have been getting their eye in using a hot air balloon to simulate the vibrations, random movements and changes in altitude of a space platform, and a moving van with a rotating turntable to model the satellite鈥檚 rapid fly-by ().
The Chinese team is also investigating whether it is possible to make detectors that are sensitive enough to pick up entangled photons during the day 鈥� a decisive step to making a practical worldwide secure quantum communications network. 鈥淚 find it fascinating, having done fundamental research for so many years, that we are moving closer to applications now,鈥� says Zeilinger. 鈥淚t鈥檚 a completely new game.鈥�
One, perhaps, with completely unknown rules. 鈥淲e鈥檙e not expecting entanglement to break down,鈥� says Zeilinger. But at the very least we can expect some ticklish questions. Beam a pair of entangled photons up to two satellites moving at speed towards one another, and measure them concurrently from the point of view of Earth, then to each of the two satellites the other鈥檚 measurement appears to have happened first (see diagram). So which measurement caused the wave functions to collapse?
鈥淨uantum experiments in space are a completely new game 鈥� one, perhaps, with unknown rules鈥�
The hope is that by doing the measurements we can find out, and so bore more deeply into vexed questions of cause, effect and reality in the quantum world. Is a wave function a real object, and its collapse a process that happens in space and time? Or is it merely a mathematical shorthand for our state of knowledge of a quantum system, with the real action happening on a level we have not yet penetrated? Einstein favoured the second view, sparking a debate that continues to this day. 鈥淚t鈥檚 still very controversial, how to reconcile wave function collapse with our usual notions of reality and causality,鈥� says Rideout.
Others are not so sure that entanglement won鈥檛 just break down at some point. Calculations published in February by of the University of Queensland in Brisbane, Australia, and his colleagues suggest that the relativistic time-stretching of a few hundred femtoseconds involved in sending one of an entangled pair of photons from the ground to a satellite just 200 to 300 kilometres up could be enough to do just that. But the calculated effect only really kicks in for short, well-defined photon pulses that are themselves less than about 100 femtoseconds long. That should not be a problem for the initial Chinese experiments, which will use a continuous beam, but it is something to watch out for ().
For a dedicated test, , a quantum theorist based at the University of Nottingham in the UK, and her colleagues propose analysing the entanglement between Bose-Einstein condensates. These large collections of atoms, chilled to near absolute zero, behave as one quantum system. The idea is to hold entangled condensates in two separate satellites, and then kick one into a different orbit. Calculations indicate that the acceleration needed for a change in orbital radius of just 400 metres would disturb a condensate enough to noticeably degrade its entanglement. A typical satellite manoeuvre changes orbital radius by as much as 60 kilometres 鈥� enough to completely disrupt the transmission of information in a future quantum network ().
鈥淔rom a theoretical point of view, we鈥檝e shown that there are effects, but we need to confirm this with experiment,鈥� says Fuentes. And to test effects in space, we need to go to space: on Earth, we are limited to experiments in 鈥渄rop towers鈥� where condensates fall a maximum of 100 metres or so.
Entanglement is not the only quantum phenomenon we need to examine carefully in relativity鈥檚 realm, either. Interferometer experiments on Earth, in which single photons are pinged along two paths of equal length, then made to combine at the other end, have seemingly confirmed that quantum objects can be in two places at once. In space, the idea would be to repeat the experiment along two different paths, one straight up to a satellite and along to a second, and one along close to Earth鈥檚 surface and then up to the second satellite. The two photons will have passed along paths identical in everything but the average strength of gravity along them. What happens then?
Long-sought prize
We don鈥檛 know, but that is exactly the point, says Ralph. 鈥淪omething different might happen. It鈥檚 very important to do these sorts of experiments to see if they really behave the way we expect them to.鈥� He has been discussing how such an experiment might be made reality with researchers from the European and Canadian space agencies, among others.
Quite apart from their relevance to any future satellite quantum-information network, deviations from what quantum theory or relativity predict could deliver a long-sought prize: the first whiff of a greater theory that might allow them to set aside their differences and unite. Theorists are not short of suggestions in the form of grand constructs such as string theory, but testing them was long believed beyond our technological capabilities. 鈥淨uantum gravity research was a free-for-all, the best theories were decided by a show of hands,鈥� says Amelino-Camelia.
In the late 1990s, he was among the first to point out an observable effect that a very large quantum experiment in space might just catch. Over distances of less than about 10-35 metres, at the Planck scale, most theories of quantum gravity predict that space-time no longer acts like a smoothly flowing continuum, as Einstein鈥檚 relativity would have it. This bumpiness could have a slow cumulative effect on the polarisation of photons, randomly knocking them slightly out of kilter.
If the most optimistic models turn out to be right, we could see an effect by beaming polarised photons from the ground to a satellite in a low Earth orbit. At the worst, though, we would need to put a detector billions of light years away. 鈥淲e have to be prudently pessimistic: we are just opening a window; it will only work if we are lucky,鈥� says Amelino-Camelia. Yet even a no could be a significant result, allowing us to start ruling out those theories that predict larger, albeit still small, effects.
But the possibility of one of these experiments presenting evidence of anomalous effects provides all the motivation he needs to take quantum tests into relativity鈥檚 realm high above us. 鈥淚f we get a yes, it will resound in the history of physics forever.鈥�
This article appeared in print under the headline 鈥淲here worlds collide鈥�
