The Advantages of Extra Entanglement Artur Widera Science 344, 160 (2014); DOI: 10.1126/science.1251472
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
The following resources related to this article are available online at www.sciencemag.org (this information is current as of April 11, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6180/160.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6180/160.full.html#related This article cites 8 articles, 1 of which can be accessed free: http://www.sciencemag.org/content/344/6180/160.full.html#ref-list-1 This article appears in the following subject collections: Physics http://www.sciencemag.org/cgi/collection/physics
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on April 11, 2014
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
PERSPECTIVES to corotate with the spinning pulsar would be forced to travel at relativistic speeds. Beyond the light cylinder, the magnetosphere opens up to a transition zone through which the relativistic particles and magnetic fields flow in an energetic pulsar wind. This picture implies that the gamma-ray beams should be big, sweeping a different, and substantially larger, region of the sky than the radio beacons, and should be bright, carrying a substantial fraction of the pulsar’s spindown power. The Large Area Telescope (LAT) on NASA’s Fermi Gamma-Ray Space Observatory has now taken the crucial next step, detecting nearly 150 spin-powered pulsars, including large numbers of Geminga-like pulsars visible only via their gamma-ray beams (3, 4). Even after 5 years, gamma-ray pulsar discoveries continue, with increasing numbers of millisecond period recycled pulsars, for which the magnetospheres extend only a few times the neutron star radius (5). The consensus view now is that gamma-ray emission is a
high-altitude phenomenon and that the wide gamma-ray beams located toward the spin equator (6) are generally well separated from the radio axis (see the figure). The gamma-ray power of these outer magnetosphere beams appears to require as much as a third of the spin-down energy budget. This suggests that the gamma-ray–producing particle accelerator imposes large torques at the light cylinder and that its energization accounts for much of the spin-down luminosity. Detailed numerical models incorporating such torques and the currents that generate them are being pursued (7, 8). In these models, the accelerator/emission zone can in fact extend somewhat beyond the light cylinder radius, into the wind zone. The host of Fermi pulsar discoveries can be attributed to these big, bright gamma-ray beams, and the detailed study of the Fermi pulse shapes and phase-resolved spectral variations provides new hope for reverse-engineering the pulsar machine to reveal the electrodynamics of these extreme particle accelerators. What of the energetically unimportant,
but extremely useful, radio-pulse sideshow? Here, too, gamma-ray studies can help. Production of the radio pair plasma should be accompanied by gamma-ray photons. Efforts are under way to improve the LAT sensitivity, especially at low energy (9). This enhancement would allow us to search for small, gamma-ray faint pair production regions from the near-surface polar cap zone. The hope is that the radio-producing cap accelerators can then join their big, bright outer magnetosphere cousins in the gamma-ray sky. References 1. J. P. Halpern, S. S. Holt, Nature 357, 222 (1992). 2. R. W. Romani, I. A. Yadigaroglu, Astrophys. J. 438, 314 (1995). 3. A. A. Abdo et al., Science 325, 840 (2009). 4. A. A. Abdo et al., Astrophys. J. Suppl. Ser. 208, 17 (2013). 5. P. A. Caraveo, http://xxx.lanl.gov/abs/1312.2913 (2013). 6. K. P. Watters, R. W. Romani, Astrophys. J. 727, 123 (2011). 7. X.-N. Bai, A. Spitkovsky, Astrophys. J. 715, 1282 (2010). 8. C. Kalapotharakos et al., http://xxx.lanl.gov/abs/1310. 3545 (2013). 9. W. Atwood et al., http://xxx.lanl.gov/abs/1303.3514 (2013). 10.1126/science.1251943
PHYSICS
The Advantages of Extra Entanglement
An ensemble of 40 ultracold atoms forms an entangled state when just one of the atoms is excited.
Artur Widera
T
he development of quantum physics has led to a revolution in modern technology and lies at the heart of applications in communication, computation, medicine, and navigation. Fragile quantum correlations, known from Einstein’s infamous wording as “spooky action at a distance,” point to novel ways to compute even faster or communicate more securely. Although the original two-particle entangled states following the seminal paper by Einstein, Podolsky, and Rosen (1) are well understood and can be routinely prepared in experiments, manyparticle entangled states are in general hard to describe theoretically or to produce experimentally. The increasing fragility of quantum states with increasing numbers of atoms makes macroscopic entanglement rare. On page 180 of this issue, Haas et al. (2) have demonstrated the creation of a robust entangled state of more than 40 ultracold atoms within a single operation. Their method relies Physics Department and Research Center OPTIMAS, Kaiserslautern University, Erwin-Schrödinger-Str., 67663 Kaiserslautern, Germany. E-mail: [email protected]
160
on a simultaneous, coherent interaction with the light field of a high-finesse optical cavity to reveal the presence of only a single atom with a different internal state. Approaches to engineering large-scale entangled quantum states in atomic systems typically fall into two different categories. A bottom-up approach uses single, highly controllable particles, such as cold trapped ions. Quantum states can be assembled atom by atom, and entanglement of a steadily increasing number of ions has been achieved (3) up to a number of 14 (4). Current efforts aim to increase the number of entangled particles while maintaining exceptionally good control over the individual ion. In a top-down approach, quantum many-body systems such as Bose-Einstein condensates form the starting point. Here, thousands of atoms share the same quantum state. Specifically engineered interactions can drive a time evolution to entangled states of large numbers of particles (5–7), but examples of control over individual particles have been scarce. Haas et al. have chosen a combination of both approaches to realize many-particle
entanglement that is in principle scalable to larger numbers of particles. The atomic system is in a many-body state and there is hardly any control over individual atoms, but they have exceptionally good detection capability of atomic excitations in their cavity. Even a single quantum of excitation, corresponding to a single excited atom, can be detected. In this system, quantum entanglement comes naturally into play. For N indistinguishable particles all sharing the same quantum state, a single excitation—adding one quantum of energy resonant to an atomic transition—would lead to an apparent ambiguity: The excitation is quantized and cannot be distributed in smaller parts over all particles of the system, but all particles are indistinguishable, and there is no reason why one in particular should be excited. The quantum system’s solution is entanglement, a coherent superposition state |ψ冔 of all possible combinations that place one quantized excitation in the excited state 1
|ψ冔⫽ _ (|100…00冔⫹|010…00冔⫹…⫹|000…01冔) √N
11 APRIL 2014 VOL 344 SCIENCE www.sciencemag.org Published by AAAS
PERSPECTIVES A
Fiber ends form cavity mirrors Fiber
Fiber
High rate count at detector
Laser in
Expanded view
B
Fiber Laser in
general not available from a single measurement. Rather, a whole series of measurements must be performed to extract the so-called density Low rate count matrix that contains all state at detector populations as well as coherence and correlation properties of the quantum state. For an increasing number of atoms, this task becomes harder because the number of values to be determined scales as 2N × 2N, which becomes a large number even for small numbers of particles (8). Instead, Haas et al. considered only a relevant subpart of the full density matrix spanned by symmetric Dicke-type states with varying numbers of excitations shared. By first manipulating the prepared quantum state via microwave fields and subsequently checking for the presence of excitations, they determined the overlap of the state prepared with one of the Dicke states.
N atoms between mirrors in state |0典 ( ) or |1典 ( )
Weak microwave excitation Fiber ends form cavity mirrors
How darkness sheds light on entanglement. (A) An ensemble of N atoms in their internal ground state (blue) is transparent to a light field resonant with the cavity formed by the ends of two fibers (green). As a result, the photon count rate detected through one of the fibers is “high.” (B) Upon irradiation with a weak microwave field, exciting atoms to another hyperfine state (red), a single excitation in the ensemble renders the resonator opaque. The corresponding photon count rate is “low” and indicates an entangled atomic state in the resonator.
1/√N + + +
Expanded view
where |0冔 denotes a particle in the ground state and |1冔 denotes the excited particle. This state, called the W state in quantum information processing and the Dicke state in quantum optics, is rather robust against decoherence relative to other entangled states, such as Schrödinger’s-cat states or GreenbergerHorne-Zeilinger (GHZ) states, the manyparticle generalization of Einstein-PodolskyRosen pairs. Preparation of indistinguishable particles in a Bose-Einstein condensate is routinely performed in laboratories worldwide, and excitations of the atoms’ hyperfine states can be achieved with microwave fields. The key question, then, is “When has a single excitation entered the system?” Haas et al. tackled this problem using a high-finesse optical cavity formed by two opposing mirrors at the ends of optical fibers. Photons typically bounce between these mirrors more than 10,000 times. Precisely tuning the resonator length to the wavelength of a weak impinging laser beam leads to light transmission with a small line width. Such a resonator is extremely amenable to even the slightest change of refractive index of its contents. In fact, a single atom in an excited state changes the effective optical path length for the light field sufficiently to render the resonator opaque, while atoms in other internal states are transparent for the light field. Thus, even for many atoms in one internal state, the laser beam is fully transmitted, but if an external microwave field injects just a single excitation, the transmission vanishes, heralding the presence of the entangled W state (see the figure). Full information about a quantum state— even about a single quantum system—is in
Research into larger and other entangled states not only drives the development of emerging quantum applications; it elucidates the fundamental question, “Why does quantum physics explain perfectly everything we know about the microscopic world but is never observed in our everyday macroscopic life?” Only with experiments creating and analyzing larger and larger entangled states will we be able to track, and perhaps steer, the quantum-to-classical transition. References 1. A. Einstein, B. Podolsky, N. Rosen, Phys. Rev. 47, 777 (1935). 2. F. Haas et al., Science 343, 180 (2014); 10.1126/ science.1248905. 3. R. Blatt, D. Wineland, Nature 453, 1008 (2008). 4. T. Monz et al., Phys. Rev. Lett. 106, 130506 (2011). 5. O. Mandel et al., Nature 425, 937 (2003). 6. J. Estève, C. Gross, A. Weller, S. Giovanazzi, M. K. Oberthaler, Nature 455, 1216 (2008). 7. M. F. Riedel et al., Nature 464, 1170 (2010). 8. H. Häffner et al., Nature 438, 643 (2005). 10.1126/science.1251472
MATERIALS SCIENCE
Materials both Tough and Soft Jian Ping Gong Tough elastomers are created by adapting an approach previously used for hydrogels.
H
ydrogels and elastomers are soft materials that have similar network structures but very different affinities to water. Consisting mostly of water, hydrogels resemble biological soft tissues and have great potential for use in biomedical applications; they tend to be very brittle, like fragile jellies. Elastomers are formed of nonhydrated polymer networks and are widely used as load-dispersing and shock-absorbing materials. They are stretchable but break easily along a notch. On page 186 of this issue, Ducrot et al. (1) show that the toughness of elastomers can be improved substantially by Faculty of Advanced Life Science, Hokkaido University, Sapporo, 060-0810, Japan. E-mail: [email protected]. ac.jp
combining two different network materials, an approach previously applied to hydrogels. Double-network hydrogels contain 80 to 90 weight percent (wt %) of water, yet are both hard and strong, with mechanical properties comparable to that of rubbers and cartilages (2, 3). The gels consist of two interpenetrating polymer networks with contrasting mechanical properties. The first network is highly stretched and densely cross-linked, making it stiff and brittle. The second network is flexible and sparsely cross-linked, making it soft and stretchable. The toughness of a material is its ability to absorb mechanical energy and deform without fracturing. One definition of material toughness is the fracture energy, which is the energy per unit area required to make
www.sciencemag.org SCIENCE VOL 344 11 APRIL 2014 Published by AAAS
161
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
The following resources related to this article are available online at www.sciencemag.org (this information is current as of April 11, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6180/160.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6180/160.full.html#related This article cites 8 articles, 1 of which can be accessed free: http://www.sciencemag.org/content/344/6180/160.full.html#ref-list-1 This article appears in the following subject collections: Physics http://www.sciencemag.org/cgi/collection/physics
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on April 11, 2014
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
PERSPECTIVES to corotate with the spinning pulsar would be forced to travel at relativistic speeds. Beyond the light cylinder, the magnetosphere opens up to a transition zone through which the relativistic particles and magnetic fields flow in an energetic pulsar wind. This picture implies that the gamma-ray beams should be big, sweeping a different, and substantially larger, region of the sky than the radio beacons, and should be bright, carrying a substantial fraction of the pulsar’s spindown power. The Large Area Telescope (LAT) on NASA’s Fermi Gamma-Ray Space Observatory has now taken the crucial next step, detecting nearly 150 spin-powered pulsars, including large numbers of Geminga-like pulsars visible only via their gamma-ray beams (3, 4). Even after 5 years, gamma-ray pulsar discoveries continue, with increasing numbers of millisecond period recycled pulsars, for which the magnetospheres extend only a few times the neutron star radius (5). The consensus view now is that gamma-ray emission is a
high-altitude phenomenon and that the wide gamma-ray beams located toward the spin equator (6) are generally well separated from the radio axis (see the figure). The gamma-ray power of these outer magnetosphere beams appears to require as much as a third of the spin-down energy budget. This suggests that the gamma-ray–producing particle accelerator imposes large torques at the light cylinder and that its energization accounts for much of the spin-down luminosity. Detailed numerical models incorporating such torques and the currents that generate them are being pursued (7, 8). In these models, the accelerator/emission zone can in fact extend somewhat beyond the light cylinder radius, into the wind zone. The host of Fermi pulsar discoveries can be attributed to these big, bright gamma-ray beams, and the detailed study of the Fermi pulse shapes and phase-resolved spectral variations provides new hope for reverse-engineering the pulsar machine to reveal the electrodynamics of these extreme particle accelerators. What of the energetically unimportant,
but extremely useful, radio-pulse sideshow? Here, too, gamma-ray studies can help. Production of the radio pair plasma should be accompanied by gamma-ray photons. Efforts are under way to improve the LAT sensitivity, especially at low energy (9). This enhancement would allow us to search for small, gamma-ray faint pair production regions from the near-surface polar cap zone. The hope is that the radio-producing cap accelerators can then join their big, bright outer magnetosphere cousins in the gamma-ray sky. References 1. J. P. Halpern, S. S. Holt, Nature 357, 222 (1992). 2. R. W. Romani, I. A. Yadigaroglu, Astrophys. J. 438, 314 (1995). 3. A. A. Abdo et al., Science 325, 840 (2009). 4. A. A. Abdo et al., Astrophys. J. Suppl. Ser. 208, 17 (2013). 5. P. A. Caraveo, http://xxx.lanl.gov/abs/1312.2913 (2013). 6. K. P. Watters, R. W. Romani, Astrophys. J. 727, 123 (2011). 7. X.-N. Bai, A. Spitkovsky, Astrophys. J. 715, 1282 (2010). 8. C. Kalapotharakos et al., http://xxx.lanl.gov/abs/1310. 3545 (2013). 9. W. Atwood et al., http://xxx.lanl.gov/abs/1303.3514 (2013). 10.1126/science.1251943
PHYSICS
The Advantages of Extra Entanglement
An ensemble of 40 ultracold atoms forms an entangled state when just one of the atoms is excited.
Artur Widera
T
he development of quantum physics has led to a revolution in modern technology and lies at the heart of applications in communication, computation, medicine, and navigation. Fragile quantum correlations, known from Einstein’s infamous wording as “spooky action at a distance,” point to novel ways to compute even faster or communicate more securely. Although the original two-particle entangled states following the seminal paper by Einstein, Podolsky, and Rosen (1) are well understood and can be routinely prepared in experiments, manyparticle entangled states are in general hard to describe theoretically or to produce experimentally. The increasing fragility of quantum states with increasing numbers of atoms makes macroscopic entanglement rare. On page 180 of this issue, Haas et al. (2) have demonstrated the creation of a robust entangled state of more than 40 ultracold atoms within a single operation. Their method relies Physics Department and Research Center OPTIMAS, Kaiserslautern University, Erwin-Schrödinger-Str., 67663 Kaiserslautern, Germany. E-mail: [email protected]
160
on a simultaneous, coherent interaction with the light field of a high-finesse optical cavity to reveal the presence of only a single atom with a different internal state. Approaches to engineering large-scale entangled quantum states in atomic systems typically fall into two different categories. A bottom-up approach uses single, highly controllable particles, such as cold trapped ions. Quantum states can be assembled atom by atom, and entanglement of a steadily increasing number of ions has been achieved (3) up to a number of 14 (4). Current efforts aim to increase the number of entangled particles while maintaining exceptionally good control over the individual ion. In a top-down approach, quantum many-body systems such as Bose-Einstein condensates form the starting point. Here, thousands of atoms share the same quantum state. Specifically engineered interactions can drive a time evolution to entangled states of large numbers of particles (5–7), but examples of control over individual particles have been scarce. Haas et al. have chosen a combination of both approaches to realize many-particle
entanglement that is in principle scalable to larger numbers of particles. The atomic system is in a many-body state and there is hardly any control over individual atoms, but they have exceptionally good detection capability of atomic excitations in their cavity. Even a single quantum of excitation, corresponding to a single excited atom, can be detected. In this system, quantum entanglement comes naturally into play. For N indistinguishable particles all sharing the same quantum state, a single excitation—adding one quantum of energy resonant to an atomic transition—would lead to an apparent ambiguity: The excitation is quantized and cannot be distributed in smaller parts over all particles of the system, but all particles are indistinguishable, and there is no reason why one in particular should be excited. The quantum system’s solution is entanglement, a coherent superposition state |ψ冔 of all possible combinations that place one quantized excitation in the excited state 1
|ψ冔⫽ _ (|100…00冔⫹|010…00冔⫹…⫹|000…01冔) √N
11 APRIL 2014 VOL 344 SCIENCE www.sciencemag.org Published by AAAS
PERSPECTIVES A
Fiber ends form cavity mirrors Fiber
Fiber
High rate count at detector
Laser in
Expanded view
B
Fiber Laser in
general not available from a single measurement. Rather, a whole series of measurements must be performed to extract the so-called density Low rate count matrix that contains all state at detector populations as well as coherence and correlation properties of the quantum state. For an increasing number of atoms, this task becomes harder because the number of values to be determined scales as 2N × 2N, which becomes a large number even for small numbers of particles (8). Instead, Haas et al. considered only a relevant subpart of the full density matrix spanned by symmetric Dicke-type states with varying numbers of excitations shared. By first manipulating the prepared quantum state via microwave fields and subsequently checking for the presence of excitations, they determined the overlap of the state prepared with one of the Dicke states.
N atoms between mirrors in state |0典 ( ) or |1典 ( )
Weak microwave excitation Fiber ends form cavity mirrors
How darkness sheds light on entanglement. (A) An ensemble of N atoms in their internal ground state (blue) is transparent to a light field resonant with the cavity formed by the ends of two fibers (green). As a result, the photon count rate detected through one of the fibers is “high.” (B) Upon irradiation with a weak microwave field, exciting atoms to another hyperfine state (red), a single excitation in the ensemble renders the resonator opaque. The corresponding photon count rate is “low” and indicates an entangled atomic state in the resonator.
1/√N + + +
Expanded view
where |0冔 denotes a particle in the ground state and |1冔 denotes the excited particle. This state, called the W state in quantum information processing and the Dicke state in quantum optics, is rather robust against decoherence relative to other entangled states, such as Schrödinger’s-cat states or GreenbergerHorne-Zeilinger (GHZ) states, the manyparticle generalization of Einstein-PodolskyRosen pairs. Preparation of indistinguishable particles in a Bose-Einstein condensate is routinely performed in laboratories worldwide, and excitations of the atoms’ hyperfine states can be achieved with microwave fields. The key question, then, is “When has a single excitation entered the system?” Haas et al. tackled this problem using a high-finesse optical cavity formed by two opposing mirrors at the ends of optical fibers. Photons typically bounce between these mirrors more than 10,000 times. Precisely tuning the resonator length to the wavelength of a weak impinging laser beam leads to light transmission with a small line width. Such a resonator is extremely amenable to even the slightest change of refractive index of its contents. In fact, a single atom in an excited state changes the effective optical path length for the light field sufficiently to render the resonator opaque, while atoms in other internal states are transparent for the light field. Thus, even for many atoms in one internal state, the laser beam is fully transmitted, but if an external microwave field injects just a single excitation, the transmission vanishes, heralding the presence of the entangled W state (see the figure). Full information about a quantum state— even about a single quantum system—is in
Research into larger and other entangled states not only drives the development of emerging quantum applications; it elucidates the fundamental question, “Why does quantum physics explain perfectly everything we know about the microscopic world but is never observed in our everyday macroscopic life?” Only with experiments creating and analyzing larger and larger entangled states will we be able to track, and perhaps steer, the quantum-to-classical transition. References 1. A. Einstein, B. Podolsky, N. Rosen, Phys. Rev. 47, 777 (1935). 2. F. Haas et al., Science 343, 180 (2014); 10.1126/ science.1248905. 3. R. Blatt, D. Wineland, Nature 453, 1008 (2008). 4. T. Monz et al., Phys. Rev. Lett. 106, 130506 (2011). 5. O. Mandel et al., Nature 425, 937 (2003). 6. J. Estève, C. Gross, A. Weller, S. Giovanazzi, M. K. Oberthaler, Nature 455, 1216 (2008). 7. M. F. Riedel et al., Nature 464, 1170 (2010). 8. H. Häffner et al., Nature 438, 643 (2005). 10.1126/science.1251472
MATERIALS SCIENCE
Materials both Tough and Soft Jian Ping Gong Tough elastomers are created by adapting an approach previously used for hydrogels.
H
ydrogels and elastomers are soft materials that have similar network structures but very different affinities to water. Consisting mostly of water, hydrogels resemble biological soft tissues and have great potential for use in biomedical applications; they tend to be very brittle, like fragile jellies. Elastomers are formed of nonhydrated polymer networks and are widely used as load-dispersing and shock-absorbing materials. They are stretchable but break easily along a notch. On page 186 of this issue, Ducrot et al. (1) show that the toughness of elastomers can be improved substantially by Faculty of Advanced Life Science, Hokkaido University, Sapporo, 060-0810, Japan. E-mail: [email protected]. ac.jp
combining two different network materials, an approach previously applied to hydrogels. Double-network hydrogels contain 80 to 90 weight percent (wt %) of water, yet are both hard and strong, with mechanical properties comparable to that of rubbers and cartilages (2, 3). The gels consist of two interpenetrating polymer networks with contrasting mechanical properties. The first network is highly stretched and densely cross-linked, making it stiff and brittle. The second network is flexible and sparsely cross-linked, making it soft and stretchable. The toughness of a material is its ability to absorb mechanical energy and deform without fracturing. One definition of material toughness is the fracture energy, which is the energy per unit area required to make
www.sciencemag.org SCIENCE VOL 344 11 APRIL 2014 Published by AAAS
161
