VISITORS to Lene Hau’s lab at Harvard University leave their shoes at the door. She doesn’t want even a speck of dust trodden into the lab: it could fly up and get in the way of the laser beam she uses. Hau’s apparatus is just about as delicate as it gets, the optics tables glittering with tiny components painstakingly laid out to slice up the stream of photons from the laser. A recent addition is a curtain of soft plastic fringes that hang down to below the level of the tabletop. Are they to protect the researcher’s eyes from the laser glare or yet another layer of protection against dust? No: they are there to stop people getting too close. “When a German television crew came,” she says, “I left them alone for a couple of minutes. When I came back they’d switched on a smoke box to create ‘atmosphere'”.
When you are dealing with the most fundamental particles of energy, smoke machines and “atmosphere” tend not to help. Hau is a pioneer in what is now an explosive area of research. She is trying to preserve and process information carried in the quantum states of individual photons by slowing and even stopping them. She does this using something thoroughly mysterious: the “dark state”.
Physicists know what light is. They know what matter is. But when the two interact in a particular way, the result – a dark state – defies description. Ask them to describe what a dark state is, and researchers often give an answer that tells you what a dark state is not. “It is neither light nor matter, nor a simple combination of non-interacting light and matter,” says Ron Walsworth, who works on dark states at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. “Rather, it is a profoundly different phenomenon of interacting light and matter.”
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This enigmatic substance could prove a precious commodity. For centuries we have struggled to exploit light, and though we have succeeded to some degree, it has always been slippery and elusive, a fast-moving sprite. The dark state allows us to slow light down, even bring it to a halt. We can change its properties and then send it on its way again, so tasks we could never otherwise achieve can be tackled. In a paper to be published in Physical Review Letters later this year, for instance, Jeff Kimble and his colleagues at the California Institute of Technology in Pasadena have shown how the dark state provides an unprecedented means to control and use the quantum properties of light. Thanks to the dark state, light could become our omnipotent genie in the lamp.
Researchers have been creating dark states since the 1960s, but perhaps the first demonstration of their true potential came in 1999, when Hau and colleagues announced that they had slowed down a stream of photons to the point where Hau could pass it by on her bicycle (鶹ý, 20 February 1999, p 10). By January 2001, she had succeeded in bringing it to a halt in a cloud of cold sodium atoms. The same month, a group led by Mikhail Lukin, now also at Harvard, and Walsworth also reported stopping light in a warm gas of rubidium atoms.
Of course, many ordinary materials slow light down. Although it travels at around 3 × 108 metres per second in a vacuum, light loses a quarter of that speed when travelling through water, for example. But only dark states can slow light down to cycling pace, or stop it altogether.
Hau forms her dark state in a cigar-shaped cloud of sodium atoms just a few tenths of a millimetre long. She cools the atoms until they are in their lowest energy state and then illuminates them with two laser beams. The “coupling” beam has an energy that matches the energy difference between two of the many higher energy levels of the atom. The signal beam has the energy equivalent to the jump from the ground state into the upper of the two chosen higher levels.
If these two beams are allowed to interfere, and have the right conditions of amplitude and phase, they will produce a “destructive interference”. In effect, this means the atom cloud sees neither beam, so no absorption occurs. However, the photons of the signal beam still interact with the atoms – the photons and the atoms become entangled, meaning their quantum states become linked together.
The result is something that is neither light nor matter but somewhere in between. And this is what slows – and stops – the light. Setting up the entanglement with the atoms takes energy from the photons, which slows them down as they enter the atom cloud. The back of the pulse continues to travel at normal speed, and so piles into the front end: the whole pulse becomes bunched up. “A pulse that was maybe 1 or 2 kilometres long becomes just a few micrometres long,” Hau says. It is so compressed that the whole light pulse fits inside the quantum structure of the dark state. When Hau timed the pulse, she showed that its passage through the dark state had slowed down to 17 metres per second – less than the cruising speed of a car.
Once this strange set-up has been achieved, the researchers can play a further astonishing trick – one that they themselves find extraordinary. When they dim the coupling laser beam, this affects the entanglement with the atoms. The result is that even more energy gets extracted from the photons. Keep dimming down the coupling beam and the signal beam moves ever more slowly in the atom cloud until – and this is the really strange bit – when the coupling beam power is zero, so is the signal beam’s speed. Although they can’t slow the light without a coupling beam, the light will grind to a halt if the coupling beam is turned off in a carefully controlled manner. Eventually, as the coupling beam switches off, the light is stored entirely in the dark state.
As well as showing that it is possible to stop a light beam, this achievement also opens up the possibility of processing the information carried by photons. Some quantum states, such as the spin of a photon, have only two possible values. That means these states provide a way to encode information: a clockwise spin state, for example, might encode a binary 0, an anticlockwise spin a 1. But with quantum information, these states can be mixed in a superposition. And if you have a 0 and a 1 at the same time, this provides a way to speed up computations enormously.
Researchers have long known that photons would provide a great way to transfer information around a quantum computer or even – since you can fire them at enormous speeds over long distances – a network of quantum computers. The problem comes when the photons reach their destination. Normally, when photons hit atoms their energy is used to promote the atom to one of its higher energy levels, and when this happens, any information encoded in the spin state of the photon is lost. In quantum information processing, researchers want to preserve and hand over these quantum states, not be constantly destroying them and having to recreate the information they carried. So, for quantum information processing, researchers need to be able to convert the spin state of an individual photon into a spin state of a particular atom.
And that is a facility that the dark state seems to provide: there is a strong correlation between the state of each photon and the state of the dark state atom that it engages with. How do we know? Well, for one thing, the photons need not be trapped in the dark state for ever. By restoring the coupling beam, it is possible to release photons from the dark state and send them on their way again. In 1999, Hau had already shown that almost all the light that is captured can be retrieved in this way. And last year, Lukin and Walsworth studied extra photons – called Stokes photons – that carry away the extra energy every time a quantum state carried on a light pulse is stored into an atomic state. By counting Stokes photons, the researchers were able to count the atomic states stored in their rubidium vapour.
Kimble and his colleagues at Caltech have topped even this. Using a cloud of cold caesium atoms, they managed to match individual photons being stored with the corresponding photon being read out. This made it much easier for them to separate the storage process from any noise and show that the quantum state of each photon coming out is linked, or correlated, to the state of the photon that went in. “We have shown a vastly improved degree of quantum correlation,” he says.
The facility to read and write quantum states cleanly from photons to atoms and back suggests that photons could be used as the wiring in a quantum network. Just as the copper wires in conventional computers transport electric charge faithfully from place to place, photons can transport quantum bits (qubits) of information. “Ultimately you need on the order of 1 million physical qubits,” says Kimble. “Arrays of trapped ions are extremely promising, but the ions are going to have to interact to talk to each other.” For that, you need wires – or at least photon wires.
How far could such links stretch? With the dark state now succumbing to control, a very long way indeed. Today, quantum states that might be used for computing can only be shared over a few hundred kilometres, because the build-up of noise in optical fibres eventually disrupts them: the fidelity of the quantum signal fades exponentially fast. But a team led by Luming Duan at the University of Michigan, Ann Arbor, has shown that the dark state can be used as an amplifier to renew the quantum signal, by storing the quantum state from a photon into a dark state, and then extracting it, leaving behind the noise (Nature, vol 414, p 413).
Dark logic
It might even be possible to make a quantum computer from a cloud of dark-state atoms. If the quantum information is stored intact in the dark state, it might be possible to process qubits in logic gates made up of dark-state atoms. Last December, Lukin and colleagues showed hints of how this might be done. They sent a signal pulse into a cloud of rubidium atoms as in the first light-stopping experiments (Nature, vol 426, p 638). But instead of shining the coupling beam over the whole cloud, they sent in two coupling beams to produce bands of constructive interference (light) and destructive interference (darkness) inside the cloud.
In the dark regions, the photons of the signal beam remained completely engaged with the atomic state, and in these spaces the speed of the signal beam is zero. In the regions of constructive interference, the signal beam photons had been renewed, but because they were flanked by regions where light has no speed this rekindled pulse had no way out, so it was held there, a standing wave of light. “They stored the light pulse, trapped it and held it,” says Marlan Scully, who works on light-stopping at Texas A&M University in College Station. “This really is stopped light.”
Walsworth says a standing wave of light like this can in turn influence the energy levels of other, nearby atoms, producing new superpositions. Send in yet another stream of photons, in a second signal beam, and they too will enter the dark state. This has important implications for quantum computing. In the dark state, the two trains of photons will directly interact in a way that would be impossible if the trapped photons passed by each other while flying through empty space. You could get their quantum states to interact and interfere destructively or constructively, producing new states and thus processing the information they encode. Lukin hopes this could make it possible to change the phase or the intensity of a standing light pulse before setting it free again – in other words, change the information it encodes. This would truly be processing light.
Hau believes processing light in the dark state could even have benefits before quantum computers are built. Today, the data that is routinely sent along optical fibres is encoded in the shape and frequency of light pulses, and Hau reckons she may have found a new way to change those properties. Researchers have found that they cannot control the state of the photons trapped in the dark state for longer than a few nanoseconds before the interactions between light and matter change the read-out when the photons are regenerated. But that apparent limitation could prove useful, Hau says. She has saved light pulses into atom clouds and then waited several milliseconds before recovering the light pulse by switching the coupling beam back on. When she did, the light pulse had a different shape: instead of being a smooth hump shape, ripples were sculpted onto it – a record of the interactions that had taken place with the dark state.
By using a strong coupling laser, Hau has managed to control the shape of the ripples. When she left the atom cloud to oscillate long enough, waves of atomic spins developed and travelled back and forth across the atom cloud to the other side as they were reflected at the edges. When she then retrieved the frozen pulse, it had moving waves written onto it.
Hau is now also investigating ways in which the dark state could be used in quantum computing. The idea is to get two atom clouds that bear the imprint of different photon pulses to interact, so that when the photons are later recovered, their spin state will be changed. Of course, it will be difficult to build a quantum processor if it has to be made entirely out of atom clouds, but that may change soon. Since 2001 Hau, Walsworth and Lukin’s tricks of slowing and stopping light dead have been repeated in solid crystals, which raises the possibility of manufacturable components.
The dark state might be a weird phenomenon that defies clear description. But no one is letting that stop them dreaming up ways of putting it to work.