Q U A N T U M I N F O R M AT I O N

Entanglement on iceThe ability to store entangled photons in a solid-state memory, and to retrieve them while preserving the entanglement, is a required step on the way to practical quantum communication. This step has now been taken. See Letters p.508 & p.512

J E V O N L O N G d E L L

Communication over long distances needs repeaters — for classical commu-nication these are devices that receive

input data and retransmit them. In an impor-tant move towards long-distance communi-cation of quantum information, two groups1,2 (pages 508 and 512 of this issue) have demon-strated the basic building block of a solid-state quantum repeater. The authors have managed to put entangled photons ‘on ice’. That is, they were able to show that they could keep a pair of photons entangled, even after storing and retrieving one of the photons in a cryogenically cooled crystal.

Two quantum-mechanical systems are entangled when it is impossible to properly describe the quantum-mechanical state of one of them in isolation. The concept was intro-duced by Einstein, Podolsky and Rosen in their famous 1935 paper3. Quantum mechanics pre-dicts completely random, but perfectly corre-lated, results for some measurements of the two entangled systems. It also states that the results of these measurements are not deter-mined until one of the systems is measured. However, the fact that these correlations still existed when the systems were too far apart for any signalling between them to be possi-ble, led Einstein and colleagues3 to conclude that quantum mechanics was not a complete description of entangled systems. They argued that the measurement results must have been

determined by some ‘hidden variable’ when the entangled pair was created, and not at the time of measurement.

Since 1935, entangled states have been produced, and the strange predictions that quantum mechanics makes about them have been validated. Furthermore, Bell showed4 that all hidden-variable theories will give predictions that, in certain circumstances, differ from those of quantum mechanics. It was therefore possible to do experiments that supported quantum mechanics over hidden-variable theories. Today, entanglement is at the heart of quantum information. The challenge in achieving secure, long-distance communi-cation of quantum information is entangling systems that are spatially far apart.

In many ways, photons are ideal carriers of quantum information: they are easy to manipulate with optics, can be sent long dis-tances either through optical fibres or in free space, and good single-photon detectors are also available. There are downsides, how-ever. Unless speeding through a vacuum, photons have very short lives. This problem is exacerbated by the fact that most methods for creating, and many methods for manipu-lating, entangled photons are probabilistic. And this is why the ability to store entangled photons in a memory and then recall them, while preserving the entanglement, which has now been demonstrated by Clausen et al.1 and Saglamyurek et al.2, is so important. It enables much more sophisticated operations

through synchronization of these probabilistic processes.

An example of these more sophisticated operations is long-distance quantum com-munication. Entanglement, and therefore secure communication, protected by quantum mechanics5, can be achieved over moderate distances (tens of kilometres) by simple prop-agation of entangled pairs of photons. But at larger distances, photon loss quickly becomes a significant problem, preventing the distri-bution of entanglement. One way to solve this problem is to break the link between the two parties into a small number of sub-links joined together with quantum repeaters6. Entangled photons would then be transmitted between the repeaters. The memories in the repeaters can store entangled photon pairs associated with successful transmission along part of the link until other parts of the link are also suc-cessful. In this way, quantum repeaters prevent the effective bit rates for transferring informa-tion between the two parties from dropping off exponentially (or worse) with distance. This is because, with repeaters, entanglement can be achieved without one photon having to travel along the whole link, which for long links can be very unlikely.

When implementing quantum repeaters, it has been the memory aspect that has been the most troublesome. But now Clausen et al.1 and Saglamyurek et al.2 have given us proof-of-principle demonstrations. They have produced entangled photons by means of spontaneous parametric downconversion. In this process, an entangled pair of two lower-energy photons is created from a higher-energy photon in an optically nonlinear crystal. The wavelength of the ‘pump’ lasers that produce the higher-energy photon was chosen by both groups1,2 such that one member of the entangled photon pair had a suitable wavelength for being stored in a quantum memory based on a cryogenic crystal doped with rare-earth ions; and, as required by quantum repeaters, the wavelength

a mere first — albeit promising — step on a fresh path in the emerging field of cognitive- enhancement research.

These outstanding questions notwithstand-ing, the current study2 has yet another inter-esting twist. The genes encoding IGF-II and IGF-IIR are known to be imprinted (they dis-play a dichotomous effect on growth in most tissues studied8). The IGF-II gene — like many other genes for which only the paternal copy is expressed — favours growth. Conversely, the IGF-IIR gene — like most maternally expressed genes — suppresses growth. Chen and co-workers’ data are not only the first to attribute a clear-cut function to these genes in the brain, but also to show that, at least in the rat hippocampus, their protein products coop-erate to enhance memory. But, given that the spatial and developmental patterns of IGF-II

and IGF-IIR expression in the brain are highly complex9, more work is needed to decipher their exact regulation.

One process by which the expression of maternal and paternal copies of IGF-II and IGF-IIR is regulated is DNA methylation8 — an epigenetic mechanism that is dynamically altered after learning10. So it would be interest-ing to know whether learning per se can also trigger epi genetic changes and so the subse-quent alterations in the expression of IGF-II and IGF-IIR. More over, ‘natural cognition enhanc-ers’ such as environmental enrichment, which are known to act through epigenetic mecha-nisms11, might also epigenetically regulate IGF-II and IGF-IIR. If so, for those in search of a memory boost, natural enhancers would be an attractive alternative to purely pharmacological agents. ■

Johannes Gräff and Li-Huei Tsai are at the Picower Institute for Learning and Memory, Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. e-mails: jgraeff@mit.edu; lhtsai@mit.edu

1. Maher, B. Nature 452, 674–675 (2008).2. Chen, D. Y. et al. Nature 469, 491–497 (2011).3. Russo, V. C. et al. Endocr. Rev. 26, 916–943 (2005). 4. Sonntag, W. E. et al. Neuroscience 88, 269–279

(1999). 5. Dudai, Y. Annu. Rev. Psychol. 55, 51–86 (2004). 6. Milekic, M. H. & Alberini, C. M. Neuron 36, 521–525

(2002). 7. Feinberg, A. P. Nature 447, 433–440 (2007). 8. Reik, W. & Walter, J. Nature Rev. Genet. 2, 21–32

(2001).9. Gregg, C. et al. Science 329, 643–648 (2010). 10. Day, J. J. & Sweatt, J. D. Nature Neurosci. 13,

1319–1323 (2010).11. Fischer, A., Sananbenesi, F., Wang, X., Dobbin, M. &

Tsai, L.-H. Nature 447, 178–182 (2007).

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of the other member of the pair was suitable for long-distance transmission in optical fibre.

Mapping of photonic entanglement into and out of a quantum memory has been demon-strated already with trapped-atom systems7. However, the new work1,2 is the first to achieve it using a solid-state memory. The use of the solid state offers certain practical advantages. For example, Saglamyurek and colleagues2 formed the memory in an optical waveguide, which could enable integrated devices to be built. More significantly though, the authors’ approach1,2 to quantum memories using cryo-genic rare-earth-ion-doped crystals is rapidly developing and has already surpassed the storage bandwidths2, capacities8, efficiencies9 and storage times10 of other approaches.

Such cryogenic rare-earth-ion-doped systems have already been studied for classi-cal optical signal processing because of these systems’ large ratio of inhomogeneous to homo geneous broadening11 for their optical absorption lines. That is, the optical absorp-tion linewidth of each dopant atom is very narrow, whereas the linewidth of the ensemble

of dopants can be very large. This makes them very suitable systems for photon echoes, which is where the dopants emit a pulse of light (echo) in response to earlier applied light pulses. In particular, photon echoes allow sig-nal processing with simultaneous large band-width, determined by ensemble linewidth, and high resolution, determined by single-dopant linewidth. The rapid advances of rare-earth quantum memories have been, in large part, due to the development of photon-echo techniques, which are suitable for preserving quantum states of light.

Although the storage of entanglement in a solid is a significant step, the efficiencies and storage times in the entanglement-storage experiments of Clausen et al.1 and Saglamyu-rek et al.2 need to be improved; they are cur-rently inferior to those that can be achieved in a small spool of optical fibre. And whereas good efficiency, storage time and bandwidth have all been demonstrated by others, in separate demonstrations, the next challenge awaiting researchers is to achieve all these performance metrics for the same memory. This should

open up new capabilities and technologies that will stretch quantum mechanics in a way that we have not yet been able to. Who knows, it might break. ■

Jevon Longdell is at the Jack Dodd Centre for Quantum Technology, Department of Physics, University of Otago, Dunedin 9054, New Zealand.e-mail: jevon@physics.otago.ac.nz

1. Clausen, C. et al. Nature 469, 508–511 (2011).2. Saglamyurek, E. et al. Nature 469, 512–515 (2011).3. Einstein, A., Podolsky, B. & Rosen, N. Phys. Rev. 47,

777–780 (1935).4. Bell, J. S. Rev. Mod. Phys. 38, 447–452 (1966).5. Ekert, A. K. Phys. Rev. Lett. 67, 661–663 (1991).6. Briegel, H.-J., Dür, W., Cirac, J. I. & Zoller, P. Phys. Rev.

Lett. 81, 5932–5935 (1998).7. Choi, K. S., Deng, H., Laurat, J. & Kimble, H. J. Nature

452, 67–71 (2008).8. Usmani, I., Afzelius, M., de Riedmatten, H. & Gisin,

N. Nature Commun. 1, 12 (2010).9. Hedges, M. P., Longdell, J. J., Li, Y. & Sellars, M. J.

Nature 465, 1052–1056 (2010).10. Longdell, J. J., Fraval, E., Sellars, M. J. & Manson, N. B.

Phys. Rev. Lett. 95, 063601 (2005).11. Barber, Z. W. et al. J. Lumin. 130, 1614–1618

(2010).

C I R C A d I A N R H Y T H M S

Redox reduxOscillations in gene transcription that occur in response to biological daily clocks coordinate the physiological workings of living organisms. But turnover in cellular energy may be sufficient to make the clock tick. See Article p.498 & Letter p.554

J O S E P H B A S S & J O S E P H S . T A K A H A S H I

Last spring, a visitor at the biennial meeting of the Society for Research on Biological Rhythms in Florida

approached the geneticist Sydney Brenner inquiring as to what it was that scientists studying circadian rhythms actually do. With a glimmer in his eye, Brenner responded that the meeting concerned “those things that only happen once each day”. Indeed, all forms of life undergo circadian (roughly 24-hour) fluctuations in energy availability that are tied to alternating cycles of light and darkness. Biological clocks organize such internal ener-getic cycles through transcription–translation feedback loops. But two papers1,2 in this issue show that, in both humans and green algae, rhythmic cycles in the activity of peroxi-redoxin enzymes can occur independently of transcription.

Biological circadian oscillators have long been recognized as a self-sustained phenom-enon, their 24-hour length being both invari-ant over a wide range of temperatures and responsive to light. Early indications that genes underlie the clocks came3 from the isolation of mutant fruitflies carrying altered, and yet

heritable, circadian rhythms. This and sub-sequent work4,5 established that endogenous molecular clocks consist of a transcription–translation feedback loop that oscillates every 24 hours in cyanobacteria, plants, fungi and animals.

Although the specific clock genes are not evolutionarily conserved across distinct phyla, their architecture is similar. The forward limb of the clock involves a set of transcriptional activators that induce the transcription of a set of repressors. The latter comprise the negative limb, which feeds back to inhibit the forward limb. This cycle repeats itself every 24 hours (Fig. 1).

Energetic cycles are one type of physiologi-cal process that shows transcription-depend-ent circadian periodicity6,7; such cycles include the alternating oxygenic and nitrogen-fixing phases of photosynthesis, and the glycolytic and oxidative cycles in eukaryotes (organisms with nucleated cells). The idea that biochemical flux per se may couple circadian and energetic cycles was first suggested by McKnight and colleagues8, who showed that varying the redox state of the metabolic cofactor NAD(P) affects the activity of two clock proteins, and it gained further support from subsequent studies9–14.

But exactly how transcriptional and non- transcriptional cycles may be interrelated was still not fully understood.

To address this relationship, O’Neill and Reddy1 (page 498) examined the rhythmic properties of human red blood cells (RBCs). In their mature form, these cells lack both a nucleus and most other organelles, including energy-producing mitochondria. They func-tion mainly as oxygen shuttles, utilizing the protein haemoglobin as the delivery vehicle.

Some of the most abundant proteins in mature RBCs are the evolutionarily conserved enzymes of the peroxiredoxin family, which can inactivate reactive oxygen species (ROS). Class-2 peroxiredoxins contain a cysteine amino-acid residue in their active site that undergoes oxidation when ROS accumulate. This results in the enzyme’s transition from a monomeric to a dimeric state. Excess ROS accumulation induces the formation of even higher-order oligomers. Peroxiredoxin func-tion is essential for RBC survival, as defects in the expression or activity of these enzymes lead to the breakdown of the cells.

A previous survey15 searching for proteins that show circadian rhythms of expression in liver identified peroxiredoxins. In their study, O’Neill and Reddy1 monitored the monomer–dimer transition of these proteins in RBCs from three humans. They observed two main circadian features in these enucleated cells. First, the oligomerization pattern was self- sustained over several cycles within an approx-imate 24-hour period and was not affected by temperature. Second, peroxiredoxin oxidation cycles were synchronized in response to tem-perature cycles, a property called entrainment that is a hallmark of circadian oscillators.

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