RESEARCH NEWS & VIEWS
50 Years Ago On August 20, 1964, one of us … while trapping for small mammals near Listowel, County Kerry, caught an unusual ‘mouse’. On subsequent examination it proved to be a member of the family Cricetidae, the bank vole, Clethrionomys glareolus Schreber — a family of mammals hitherto unknown from Ireland … Comparisons of cranial characters were made with series of British mainland and continental specimens … The only detectable difference is that the nasals are on average shorter and the condyle width greater than the British forms, but even here there is considerable overlap … Investigations now being carried out are aimed at establishing the present distribution of this species in Ireland. From Nature 27 February 1965
100 Years Ago The Medical Committee of the British Science Guild has done a good work by its resolution condemning a notorious antivivisection advertisement. The object of the advertisement was to prevent our soldiers from being protected against typhoid fever. If it be asked why any one of the many anti-vivisection societies should behave in this way, we can only say with Dr. Watts that “Satan finds some mischief still for idle hands to do.” … Few of us are wanting to hear Pasteur called a charlatan; few of us are wanting anti-vivisection lectures and shops. Everybody is sure, who is capable of clear thinking, that our men of science are neither cruel nor stupid. But anti-vivisection cannot rest. It must find something to attack, something to abuse … We hope that it will be many years before anti-vivisection emerges out of the public disgrace which it has brought upon itself. From Nature 25 February 1915
why ST-HSCs can contribute to long-term haematopoiesis in unperturbed conditions, but contribute only transiently to posttransplantation haematopoiesis. Whether this reflects an effect of the post-transplant environment on these cells, or whether it is related to the increased proliferation in ST-HSCs during repopulation, is not clear. Finally, as Busch et al. mention, if these results extend to humans, efforts to capture the potential of STHSCs for clinical transplants could be valuable. These areas of uncertainty cannot be addressed without the development of experimental techniques to mark prospective ST- and LT-HSCs in vivo, to quantitatively analyse the resulting lineages and model the data statistically7. Only then will we be able to fully interpret this complex and dynamic process. ■
Sidhartha Goyal is in the Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada. Peter W. Zandstra is at the Institute of Biomaterials and Biomedical Engineering, University of Toronto, and at the Donnelly Centre for Cellular and Biomolecular Research, University of Toronto. e-mail: [email protected] 1. Busch, K. et al. Nature 518, 542–546 (2015). 2. McCulloch, E. A., Siminovitch, L. & Till, J. E. Science 144, 844–846 (1964). 3. Qiao, W. et al. Mol. Syst. Biol. 10, 741 (2014). 4. Sun, J. et al. Nature 514, 322–327 (2014). 5. Harrison, D. E., Lerner, C., Hoppe, P. C., Carlson, G. A. & Alling, D. Blood 69, 773–777 (1987). 6. Dykstra, B. et al. Cell Stem Cell 1, 218–229 (2007). 7. Blanpain, C. & Simons, B. D. Nature Rev. Mol. Cell Biol. 14, 489–502 (2013). This article was published online on 11 February 2015.
C O SM O LO GY
A giant in the young Universe Astronomers have discovered an extremely massive black hole from a time when the Universe was less than 900 million years old. The result provides insight into the growth of black holes and galaxies in the young Universe. See Letter p.512 BRAM VENEMANS
I
t is commonly believed that every massive galaxy in the Universe harbours a supermassive black hole at its centre. These black holes are thought to have formed in the young Universe with initial masses of between 100 and 100,000 times the mass of the Sun1. Over time, some of them have grown to be up to billions of solar masses by pulling (accreting) inter stellar material from their surroundings and/or through merging with other black holes. The most massive black holes that have been found in the nearby Universe have masses of more than 10 billion solar masses2,3. For comparison, our own Galaxy harbours a black hole with a mass of between 4 million and 5 million solar masses4. On page 512 of this issue, Wu et al.5 report the discovery of a supermassive black hole with a mass of a remarkable 12 billion solar masses, from a time when the Universe was only 875 million years old — that is, about 6% of its current age of 13.8 billion years. Wu and colleagues identified this monster in optical and near-infrared imaging data because it was accreting gas at a high rate. This gas is pulled towards the black hole by gravity and can efficiently radiate away part of its potential energy. Accreting supermassive black holes can therefore be very bright, and can be seen across the Universe as luminous sources
4 9 0 | 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
termed quasars. Because the light coming from a very distant quasar takes billions of years to reach Earth, astronomers can observe such accreting black holes as they were when the Universe was young. Theoretically, it is not implausible to find a black hole of more than 10 billion solar masses within 1 billion years after the Big Bang. But it is still surprising to uncover such a massive black hole in the early Universe. It must have been accreting gas at close to the maximum rate for most of its existence; the maximum rate is set by the pressure of the radiation emitted by the in-falling material. The prolonged period of almost maximum accretion is puzzling, because the strong radiation emitted by a quasar is generally assumed to be capable of halting accretion, limiting its existence to 10 million to 100 million years. The fact that the supermassive black hole has grown to 12 billion solar masses in less than a billion years implies that the radiation did not inhibit the high accretion. In general, studies of supermassive black holes at the centres of nearby galaxies have revealed a tight correlation between the mass of the black hole and the total mass in stars of the galaxy hosting it6. Typically, the mass of a black hole is higher when it resides in a more massive galaxy, with the ratio of the black-hole mass to galaxy mass6,7 being about 0.14–0.5%.
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
50 Years Ago On August 20, 1964, one of us … while trapping for small mammals near Listowel, County Kerry, caught an unusual ‘mouse’. On subsequent examination it proved to be a member of the family Cricetidae, the bank vole, Clethrionomys glareolus Schreber — a family of mammals hitherto unknown from Ireland … Comparisons of cranial characters were made with series of British mainland and continental specimens … The only detectable difference is that the nasals are on average shorter and the condyle width greater than the British forms, but even here there is considerable overlap … Investigations now being carried out are aimed at establishing the present distribution of this species in Ireland. From Nature 27 February 1965
100 Years Ago The Medical Committee of the British Science Guild has done a good work by its resolution condemning a notorious antivivisection advertisement. The object of the advertisement was to prevent our soldiers from being protected against typhoid fever. If it be asked why any one of the many anti-vivisection societies should behave in this way, we can only say with Dr. Watts that “Satan finds some mischief still for idle hands to do.” … Few of us are wanting to hear Pasteur called a charlatan; few of us are wanting anti-vivisection lectures and shops. Everybody is sure, who is capable of clear thinking, that our men of science are neither cruel nor stupid. But anti-vivisection cannot rest. It must find something to attack, something to abuse … We hope that it will be many years before anti-vivisection emerges out of the public disgrace which it has brought upon itself. From Nature 25 February 1915
why ST-HSCs can contribute to long-term haematopoiesis in unperturbed conditions, but contribute only transiently to posttransplantation haematopoiesis. Whether this reflects an effect of the post-transplant environment on these cells, or whether it is related to the increased proliferation in ST-HSCs during repopulation, is not clear. Finally, as Busch et al. mention, if these results extend to humans, efforts to capture the potential of STHSCs for clinical transplants could be valuable. These areas of uncertainty cannot be addressed without the development of experimental techniques to mark prospective ST- and LT-HSCs in vivo, to quantitatively analyse the resulting lineages and model the data statistically7. Only then will we be able to fully interpret this complex and dynamic process. ■
Sidhartha Goyal is in the Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada. Peter W. Zandstra is at the Institute of Biomaterials and Biomedical Engineering, University of Toronto, and at the Donnelly Centre for Cellular and Biomolecular Research, University of Toronto. e-mail: [email protected] 1. Busch, K. et al. Nature 518, 542–546 (2015). 2. McCulloch, E. A., Siminovitch, L. & Till, J. E. Science 144, 844–846 (1964). 3. Qiao, W. et al. Mol. Syst. Biol. 10, 741 (2014). 4. Sun, J. et al. Nature 514, 322–327 (2014). 5. Harrison, D. E., Lerner, C., Hoppe, P. C., Carlson, G. A. & Alling, D. Blood 69, 773–777 (1987). 6. Dykstra, B. et al. Cell Stem Cell 1, 218–229 (2007). 7. Blanpain, C. & Simons, B. D. Nature Rev. Mol. Cell Biol. 14, 489–502 (2013). This article was published online on 11 February 2015.
C O SM O LO GY
A giant in the young Universe Astronomers have discovered an extremely massive black hole from a time when the Universe was less than 900 million years old. The result provides insight into the growth of black holes and galaxies in the young Universe. See Letter p.512 BRAM VENEMANS
I
t is commonly believed that every massive galaxy in the Universe harbours a supermassive black hole at its centre. These black holes are thought to have formed in the young Universe with initial masses of between 100 and 100,000 times the mass of the Sun1. Over time, some of them have grown to be up to billions of solar masses by pulling (accreting) inter stellar material from their surroundings and/or through merging with other black holes. The most massive black holes that have been found in the nearby Universe have masses of more than 10 billion solar masses2,3. For comparison, our own Galaxy harbours a black hole with a mass of between 4 million and 5 million solar masses4. On page 512 of this issue, Wu et al.5 report the discovery of a supermassive black hole with a mass of a remarkable 12 billion solar masses, from a time when the Universe was only 875 million years old — that is, about 6% of its current age of 13.8 billion years. Wu and colleagues identified this monster in optical and near-infrared imaging data because it was accreting gas at a high rate. This gas is pulled towards the black hole by gravity and can efficiently radiate away part of its potential energy. Accreting supermassive black holes can therefore be very bright, and can be seen across the Universe as luminous sources
4 9 0 | 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
termed quasars. Because the light coming from a very distant quasar takes billions of years to reach Earth, astronomers can observe such accreting black holes as they were when the Universe was young. Theoretically, it is not implausible to find a black hole of more than 10 billion solar masses within 1 billion years after the Big Bang. But it is still surprising to uncover such a massive black hole in the early Universe. It must have been accreting gas at close to the maximum rate for most of its existence; the maximum rate is set by the pressure of the radiation emitted by the in-falling material. The prolonged period of almost maximum accretion is puzzling, because the strong radiation emitted by a quasar is generally assumed to be capable of halting accretion, limiting its existence to 10 million to 100 million years. The fact that the supermassive black hole has grown to 12 billion solar masses in less than a billion years implies that the radiation did not inhibit the high accretion. In general, studies of supermassive black holes at the centres of nearby galaxies have revealed a tight correlation between the mass of the black hole and the total mass in stars of the galaxy hosting it6. Typically, the mass of a black hole is higher when it resides in a more massive galaxy, with the ratio of the black-hole mass to galaxy mass6,7 being about 0.14–0.5%.
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
