NEWS & VIEWS RESEARCH Therefore, it has been suggested that the growth of both the black hole and the host galaxy are causally connected. If the relation between black-hole mass and host-galaxy mass were to hold true even in the distant Universe, we would expect the galaxy harbouring the 12-billion-solar-mass black hole to contain a whopping 4 trillion to 9 trillion solar masses in stars, which is the same as the most massive galaxies seen in the current Universe. Studying this host galaxy will give us a glimpse of how massive galaxies formed in the early Universe, and of the interplay between the formation of stars in the galaxy and the accretion onto its central black hole. Intriguingly, the black hole discovered by Wu and collaborators is not only the most massive of its kind known in the early Universe, it is also, owing to the high accretion rate, by far the most luminous object detected at that cosmic epoch. The quasar can therefore be used as a means of learning about the distant cosmos. As the quasar’s light travels towards observers on Earth, it passes through the gas of the inter galactic medium. This medium contains hydrogen, helium and various metals (elements heavier than helium that are produced inside stars), which leave an imprint on the spectrum of the quasar by absorbing a small amount of the quasar’s light at specific wavelengths. The brighter the quasar, the more comprehensive the investigation of the intervening gas can be. Thus, the extreme brightness of the newly discovered quasar will allow the abundance of metals in the intergalactic medium of the early Universe to be measured in unprecedented detail. Such measurements will provide information about the star-formation processes at work shortly after the Big Bang, which produced these metals. Finally, quasars as bright as the one reported here could easily be seen at larger distances from Earth than that of this quasar, and hence in an even younger Universe. Although accreting supermassive black holes become increasingly rare at earlier cosmic times8, current and future wide-field near-infrared imaging surveys should be able to uncover such objects. These giants of the Universe will provide the ideal targets from which to learn about the Universe during the first few hundred million years after the Big Bang. ■ Bram Venemans is at the Max Planck Institute for Astronomy, 69117 Heidelberg, Germany. e-mail: [email protected] 1. Volonteri, M. Astron. Astrophys. Rev. 18, 279–315 (2010). 2. McConnell, N. J. et al. Astrophys. J. 756, 179 (2012). 3. van den Bosch, R. C. E. et al. Nature 491, 729–731 (2012). 4. Meyer, L. et al. Science 338, 84–87 (2012). 5. Wu, X.-B. et al. Nature 518, 512–515 (2015). 6. Kormendy, J. & Ho, L. C. Annu. Rev. Astron. Astrophys. 51, 511–653 (2013). 7. Häring, N. & Rix, H.-W. Astrophys. J. 604, L89–L92 (2004). 8. Willott, C. J. et al. Astron. J. 139, 906–918 (2010).
QUA N TUM P H YS I CS
Teleportation for two The ‘no-cloning’ theorem of quantum mechanics forbids the perfect copying of properties of photons or electrons. But quantum teleportation allows their flawless transfer — now even for two properties simultaneously. See Letter p.516 WOLFGANG TITTEL
S
uppose you see a beautiful table in a museum and you would like to have the same one at home. What could you do? One strategy is to accurately measure all its properties — its form (length, height and width) and its appearance (material and colour) — and then reproduce an identical copy for your living room. But this ‘measureand-reproduce’ strategy would fail if the table were a quantum particle, such as a photon or an electron orbiting an atomic nucleus. The no-cloning theorem1 of quantum mechanics tells us that it is impossible to copy such a particle perfectly. On page 516 of this issue, Wang et al.2 show how to get around this apparent limitation of quantum physics. In a beautiful extension of previous experiments, they demonstrate how to transfer the values of two properties of a photon — the spin angular momentum
(the direction of the photon’s electric field, generally referred to as polarization) and the orbital angular momentum (which depends on the field distribution) — through quantum teleportation onto another photon. Quantum teleportation was proposed3 in 1993 and first demonstrated4 in 1997 for a single property of a photon (the polarization). It allows the flawless transfer of the unknown properties of an object onto a second object without contradicting the no-cloning theorem: the first object loses all its properties at the same time, that is, the properties are not ‘copied’ during quantum teleportation, they are transferred. However, the properties of the second object after this transfer remain unknown — all that is known is that they have been made identical to those of the first object before teleportation. What is more, the transfer does not happen instantaneously, a common mistake in the non-scientific literature.
C Joint measurements
Result OAM rotation
CM-OAM
Non-destructive photon-number measurement Path 1
Path 2 Result
Polarization rotation
CM-P
A
B
C
Entangled photon pair
Figure 1 | Teleportation of photon polarization and orbital angular momentum. Photon A, whose polarization and orbital angular momentum are shown with a small arrow and an ellipse, respectively, is measured jointly with photon B, which is quantum-mechanically entangled with photon C. This act consists of: a comparative measurement of the polarizations of photons A and B (CM-P); a non-destructive verification that exactly one photon exits this measurement in path 1, and hence exactly one photon exits in path 2, given that two photons entered CM-P; and a comparative measurement of the orbital angular momenta of photons A and B (CM-OAM). The measurements result in the teleportation (that is, the transfer) of photon A’s properties onto photon C. The transfer may require rotations of photon C’s (unknown) polarization and orbital angular momentum, as determined by the outcomes of the comparative measurements. Wang et al.2 have implemented all but the rotation steps in this transfer scheme. Teleporting the polarization alone does not require the non-destructive measurement, the CM-OAM, nor the rotation of photon C’s orbital angular momentum. 2 6 F E B R UA RY 2 0 1 5 | VO L 5 1 8 | N AT U R E | 4 9 1
© 2015 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS In addition to the object (A) that carries the property to be teleported, quantum teleportation requires two more objects (B and C; Fig. 1). Objects B and C have to be entangled, which means that their properties are strongly correlated. For instance, the two photons B and C should have the same polarization, but the actual direction of their individual electric fields is not defined. Sounds weird? Just think, for example, that they are either both horizontally polarized or both vertically polarized, or both polarized at 45°. Photon A, whose polarization, say, will be teleported onto photon C, is measured jointly with photon B in a way that reveals, loosely speaking, the difference in the electric fields’ directions without revealing the individual directions. What would we learn from getting, for instance, zero as the result? From the outcome of this comparative measurement, we know that the polarization of photon A equals that of photon B. Furthermore, from the entanglement of photons B and C, we know that the polarization of photon B equals that of photon C. Hence, we find that the electric field of photon C must now point in the same direction as that of photon A before the measurement. Note that the outcome of the joint measure ment could also have been different: for example, A and B are orthogonally polarized. Similar reasoning to that used before would lead to the conclusion that photon C’s electric field is rotated by 90° with respect to that of photon A. Therefore, rotating it back would allow one to perfectly recover the original polarization encoded in photon A. In short, the joint measurement, possibly followed by a well-defined rotation of the (unknown) polarization of photon C, has allowed the teleporting (transferring) of the polarization property from photon A to photon C without error. To demonstrate the teleportation of two properties, Wang and colleagues started with a single photon (photon A in Fig. 1) prepared in a combination of polarization and orbital angular momentum. Using high-intensity laser pulses that pass through a crystal, they also created a photon pair (photons B and C in Fig. 1) in a ‘hyper-entangled’ state, in which the photons are simultaneously entangled in the two properties to be teleported. Making two joint measurements (one per property) that compared the polarizations and the orbital angular momenta of photon A and photon B then led to the teleportation of photon A’s properties onto photon C. The biggest challenge for the researchers was the concatenation of the two joint measurements. It required, as an intermediate step, the verification that exactly one photon exited the first measurement (that of polarization) in each of the two possible paths leading to the second measurement (that of angular orbital momenta), without destroying the photons. The non-destructive detection of, say, a photon in path 1 can be implemented by teleporting
its orbital angular momentum onto another photon, which then enters the second joint measurement. This is because teleportation not only transfers a property from one photon to another, but also indicates that a photon existed. And, given that two photons entered (and hence left) the first comparative measurement, the non-destructive detection of a photon in path 1 also indicates that one photon was present in path 2 — exactly the requirement for the verification step. This step needed another pair of photons This is an (not shown in important step in Fig. 1) entanunderstanding, and gled in their showcasing, one of orbital angular the most profound momenta. and puzzling An interestpredictions of i ng qu e st i on quantum physics. is whether the demonstra ted method for the teleportation of two properties can be generalized to more properties. The authors affirm that this is possible in principle. However, the probability of the required joint measurements leading to a useful outcome becomes smaller and smaller as the number of properties (and thus of joint measurements) increases. Although the probability is half in the case of standard (single-property) teleportation, it is 1/32 for two properties, as shown for the first time by Wang and co-workers. Furthermore, it decreases to 1/4,096 when
teleporting an object that is described by three properties. Adding photons and photon detectors may increase the efficiency5, but this adds even more complexity to an already difficult measurement. Even without these additional photons, the joint measurement becomes increasingly challenging as the number of properties increases: in the teleportation of two properties, a ‘one-property teleporter’ is used, and in the teleportation of three properties, a ‘twoproperty teleporter’ and a ‘one-property tele porter’ would be needed. You can guess what is required for the teleportation of N properties. Yet, Wang and colleagues’ demonstration is an important step in understanding, and showcasing, one of the most profound and puzzling predictions of quantum physics. It may serve as a powerful building block for future quantum networks, which generally require teleportation units for the transmission of quantum data. ■ Wolfgang Tittel is in the Institute for Quantum Science and Technology, and the Department of Physics and Astronomy, University of Calgary, Calgary T2N 1N4, Canada. e-mail: [email protected] 1. Wootters, W. K. & Zurek, W. H. Nature 299, 802–803 (1982). 2. Wang, X.-L. et al. Nature 518, 516–519 (2015). 3. Bennett, C. H. et al. Phys. Rev. Lett. 70, 1895–1899 (1993). 4. Bouwmeester, D. et al. Nature 390, 575–579 (1997). 5. Grice, W. P. Phys. Rev. A 84, 042331 (2011).
M O L EC U L A R B I OLOGY
RNA modification does a regulatory two-step The m6A structural modification of RNA regulates gene expression. It has now been found to mediate an unusual control mechanism: by altering the structure of RNA, m6A allows a regulatory protein to bind to that RNA. See Letter p.560 D O M I N I K T H E L E R & F R É D É R I C H . -T. A L L A I N
O
ne of the most abundant modifications of messenger RNA is thought to be N6-methyladenosine (m6A), in which a methyl group is attached to the N6 position of adenine, an RNA base. The m6A modification has a role in regulating gene expression, and perturbations of this regulatory machinery are associated with human disease. But little is known about the mechanism by which the single methyl group of m6A exerts its effect. On page 560 of this issue, Liu et al.1 report that m6A alters the secondary structure of RNA, allowing an RNA-binding protein to access the RNA sequence opposite the modification
4 9 2 | N AT U R E | VO L 5 1 8 | 2 6 F E B R UA RY 2 0 1 5
© 2015 Macmillan Publishers Limited. All rights reserved
and therefore to regulate expression. Most experimental evidence for the mechanism and role of RNA modifications has been gathered in non-protein-coding RNAs. The m6A modification of mRNA was first described in 1974 (refs 2, 3), and subsequent studies quickly identified the methylase protein complex as the machinery that ‘writes’ m6A into mRNA (for reviews of m6A, see refs 4–6). Impairment of this complex leads to developmental arrest in several organisms. After those early discoveries, not much was learnt about the role of m6A until the start of this decade, when m6A demethylase enzymes were identified as ‘erasers’ of this modification4–6. These findings hinted at the dynamic
QUA N TUM P H YS I CS
Teleportation for two The ‘no-cloning’ theorem of quantum mechanics forbids the perfect copying of properties of photons or electrons. But quantum teleportation allows their flawless transfer — now even for two properties simultaneously. See Letter p.516 WOLFGANG TITTEL
S
uppose you see a beautiful table in a museum and you would like to have the same one at home. What could you do? One strategy is to accurately measure all its properties — its form (length, height and width) and its appearance (material and colour) — and then reproduce an identical copy for your living room. But this ‘measureand-reproduce’ strategy would fail if the table were a quantum particle, such as a photon or an electron orbiting an atomic nucleus. The no-cloning theorem1 of quantum mechanics tells us that it is impossible to copy such a particle perfectly. On page 516 of this issue, Wang et al.2 show how to get around this apparent limitation of quantum physics. In a beautiful extension of previous experiments, they demonstrate how to transfer the values of two properties of a photon — the spin angular momentum
(the direction of the photon’s electric field, generally referred to as polarization) and the orbital angular momentum (which depends on the field distribution) — through quantum teleportation onto another photon. Quantum teleportation was proposed3 in 1993 and first demonstrated4 in 1997 for a single property of a photon (the polarization). It allows the flawless transfer of the unknown properties of an object onto a second object without contradicting the no-cloning theorem: the first object loses all its properties at the same time, that is, the properties are not ‘copied’ during quantum teleportation, they are transferred. However, the properties of the second object after this transfer remain unknown — all that is known is that they have been made identical to those of the first object before teleportation. What is more, the transfer does not happen instantaneously, a common mistake in the non-scientific literature.
C Joint measurements
Result OAM rotation
CM-OAM
Non-destructive photon-number measurement Path 1
Path 2 Result
Polarization rotation
CM-P
A
B
C
Entangled photon pair
Figure 1 | Teleportation of photon polarization and orbital angular momentum. Photon A, whose polarization and orbital angular momentum are shown with a small arrow and an ellipse, respectively, is measured jointly with photon B, which is quantum-mechanically entangled with photon C. This act consists of: a comparative measurement of the polarizations of photons A and B (CM-P); a non-destructive verification that exactly one photon exits this measurement in path 1, and hence exactly one photon exits in path 2, given that two photons entered CM-P; and a comparative measurement of the orbital angular momenta of photons A and B (CM-OAM). The measurements result in the teleportation (that is, the transfer) of photon A’s properties onto photon C. The transfer may require rotations of photon C’s (unknown) polarization and orbital angular momentum, as determined by the outcomes of the comparative measurements. Wang et al.2 have implemented all but the rotation steps in this transfer scheme. Teleporting the polarization alone does not require the non-destructive measurement, the CM-OAM, nor the rotation of photon C’s orbital angular momentum. 2 6 F E B R UA RY 2 0 1 5 | VO L 5 1 8 | N AT U R E | 4 9 1
© 2015 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS In addition to the object (A) that carries the property to be teleported, quantum teleportation requires two more objects (B and C; Fig. 1). Objects B and C have to be entangled, which means that their properties are strongly correlated. For instance, the two photons B and C should have the same polarization, but the actual direction of their individual electric fields is not defined. Sounds weird? Just think, for example, that they are either both horizontally polarized or both vertically polarized, or both polarized at 45°. Photon A, whose polarization, say, will be teleported onto photon C, is measured jointly with photon B in a way that reveals, loosely speaking, the difference in the electric fields’ directions without revealing the individual directions. What would we learn from getting, for instance, zero as the result? From the outcome of this comparative measurement, we know that the polarization of photon A equals that of photon B. Furthermore, from the entanglement of photons B and C, we know that the polarization of photon B equals that of photon C. Hence, we find that the electric field of photon C must now point in the same direction as that of photon A before the measurement. Note that the outcome of the joint measure ment could also have been different: for example, A and B are orthogonally polarized. Similar reasoning to that used before would lead to the conclusion that photon C’s electric field is rotated by 90° with respect to that of photon A. Therefore, rotating it back would allow one to perfectly recover the original polarization encoded in photon A. In short, the joint measurement, possibly followed by a well-defined rotation of the (unknown) polarization of photon C, has allowed the teleporting (transferring) of the polarization property from photon A to photon C without error. To demonstrate the teleportation of two properties, Wang and colleagues started with a single photon (photon A in Fig. 1) prepared in a combination of polarization and orbital angular momentum. Using high-intensity laser pulses that pass through a crystal, they also created a photon pair (photons B and C in Fig. 1) in a ‘hyper-entangled’ state, in which the photons are simultaneously entangled in the two properties to be teleported. Making two joint measurements (one per property) that compared the polarizations and the orbital angular momenta of photon A and photon B then led to the teleportation of photon A’s properties onto photon C. The biggest challenge for the researchers was the concatenation of the two joint measurements. It required, as an intermediate step, the verification that exactly one photon exited the first measurement (that of polarization) in each of the two possible paths leading to the second measurement (that of angular orbital momenta), without destroying the photons. The non-destructive detection of, say, a photon in path 1 can be implemented by teleporting
its orbital angular momentum onto another photon, which then enters the second joint measurement. This is because teleportation not only transfers a property from one photon to another, but also indicates that a photon existed. And, given that two photons entered (and hence left) the first comparative measurement, the non-destructive detection of a photon in path 1 also indicates that one photon was present in path 2 — exactly the requirement for the verification step. This step needed another pair of photons This is an (not shown in important step in Fig. 1) entanunderstanding, and gled in their showcasing, one of orbital angular the most profound momenta. and puzzling An interestpredictions of i ng qu e st i on quantum physics. is whether the demonstra ted method for the teleportation of two properties can be generalized to more properties. The authors affirm that this is possible in principle. However, the probability of the required joint measurements leading to a useful outcome becomes smaller and smaller as the number of properties (and thus of joint measurements) increases. Although the probability is half in the case of standard (single-property) teleportation, it is 1/32 for two properties, as shown for the first time by Wang and co-workers. Furthermore, it decreases to 1/4,096 when
teleporting an object that is described by three properties. Adding photons and photon detectors may increase the efficiency5, but this adds even more complexity to an already difficult measurement. Even without these additional photons, the joint measurement becomes increasingly challenging as the number of properties increases: in the teleportation of two properties, a ‘one-property teleporter’ is used, and in the teleportation of three properties, a ‘twoproperty teleporter’ and a ‘one-property tele porter’ would be needed. You can guess what is required for the teleportation of N properties. Yet, Wang and colleagues’ demonstration is an important step in understanding, and showcasing, one of the most profound and puzzling predictions of quantum physics. It may serve as a powerful building block for future quantum networks, which generally require teleportation units for the transmission of quantum data. ■ Wolfgang Tittel is in the Institute for Quantum Science and Technology, and the Department of Physics and Astronomy, University of Calgary, Calgary T2N 1N4, Canada. e-mail: [email protected] 1. Wootters, W. K. & Zurek, W. H. Nature 299, 802–803 (1982). 2. Wang, X.-L. et al. Nature 518, 516–519 (2015). 3. Bennett, C. H. et al. Phys. Rev. Lett. 70, 1895–1899 (1993). 4. Bouwmeester, D. et al. Nature 390, 575–579 (1997). 5. Grice, W. P. Phys. Rev. A 84, 042331 (2011).
M O L EC U L A R B I OLOGY
RNA modification does a regulatory two-step The m6A structural modification of RNA regulates gene expression. It has now been found to mediate an unusual control mechanism: by altering the structure of RNA, m6A allows a regulatory protein to bind to that RNA. See Letter p.560 D O M I N I K T H E L E R & F R É D É R I C H . -T. A L L A I N
O
ne of the most abundant modifications of messenger RNA is thought to be N6-methyladenosine (m6A), in which a methyl group is attached to the N6 position of adenine, an RNA base. The m6A modification has a role in regulating gene expression, and perturbations of this regulatory machinery are associated with human disease. But little is known about the mechanism by which the single methyl group of m6A exerts its effect. On page 560 of this issue, Liu et al.1 report that m6A alters the secondary structure of RNA, allowing an RNA-binding protein to access the RNA sequence opposite the modification
4 9 2 | N AT U R E | VO L 5 1 8 | 2 6 F E B R UA RY 2 0 1 5
© 2015 Macmillan Publishers Limited. All rights reserved
and therefore to regulate expression. Most experimental evidence for the mechanism and role of RNA modifications has been gathered in non-protein-coding RNAs. The m6A modification of mRNA was first described in 1974 (refs 2, 3), and subsequent studies quickly identified the methylase protein complex as the machinery that ‘writes’ m6A into mRNA (for reviews of m6A, see refs 4–6). Impairment of this complex leads to developmental arrest in several organisms. After those early discoveries, not much was learnt about the role of m6A until the start of this decade, when m6A demethylase enzymes were identified as ‘erasers’ of this modification4–6. These findings hinted at the dynamic
