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Reflecting on quantum memory
Materials Today Volume 18, Number 4 May 2015
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away from equilibrium using a strong electric field. The system then reconvened to a maximal entropy through two distinct intermediate stages, with spontaneous formation of self-assembled conducting nanotube chains along the way. Of course, the maximum entropy cannot be achieved in this experiment since it would correspond to a complete discharge of the battery running the experiment. In the first stage, conductive chains align themselves according to the polarity of the applied field. This allows the system to carry a current and so lose energy through resistive heating and so produce entropy. But, the nanotubes also sprout appendages connecting them, again increasing entropy production. Excessive heat causes destructive cascades or ‘avalanches’ that tear them apart, but the appendages sometimes retract before this happens and regrow once the energy is shed. ‘The avalanches were apparent in the changes of the electric current over time,’ explains Bezryadin. In the second intermediate there are no destructive avalanches and the system is much more stable hinting at how a system
the so-called ‘Dyson sphere’. Such a sphere might be constructed by a technologically advanced civilization in order to consume all power supplied by the sun. The team now needs to scale-up its systems to confirm that the principle holds more widely and to show that their selforganizing and self-connecting nanotube systems could somehow self-replicate. Self-replication can be expected on general grounds, since it would allow the system to further increase the entropy production. ‘The general trend of the evolution of biological systems seems to be this: more advanced life forms tend to dissipate more energy by broadening their access to various forms of stored energy,’ Bezryadin explains. ‘Thus a common underlying principle can be suggested between our self-organized clouds of nanotubes, which generate more and more heat by reducing their electrical resistance and thus allow more current to flow, and the biological systems which look for new means to find food, either through biological adaptation or by inventing more technologies.’ David Bradley
might evolve past a critical point, all the while generating entropy. This stable phase occurs after the systems evolves to the point that it is able to consume and convert into heat the maximum possible power provided by the battery and limited by a fixed resistor. Interestingly, such a tendency to approach the maximum power consumption and the subsequent stabilization of the system resembles a philosophical concept
New transistor switches superconductivity using light A new type of superconducting transistor able to be switched reversibly between on and off positions using light irradiation has been developed by a team of scientists in Japan. This switching capability, based on organic superconducting field-effect transistors (FETs), could result in a new generation of high-speed switching devices and highly sensitive optical sensors. The researchers, led by Hiroshi Yamamoto from Japan’s Institute for Molecular Science, created the first organic superconducting FETs a couple of years ago, bringing attention to their flexibility and designability. FETs are a standard switching element that controls electrical current in electronic circuits, and are now used in many electronic devices, including smart phones and computers. Much research is being carried out into superconducting FETs as a key technology for computations using quantum states. This study, as reported in Science [Suda et al., Science (2015), doi:10.1126/sci-
ence.1256783], developed a novel photoswitchable transistor by replacing the gate electrode in the conventional FET with a spiropyran thin film. Spiropyran is a photoactive organic molecule that shows intramolecular electrical polarization by ultraviolet (UV) light irradiation. On shining a pale UV light on the transistor, it demonstrated a quick decrease in electrical resistance and turned into a superconducting state after 180 seconds. However, as researcher Masayuki Suda points out, ‘it can be operated much faster in principle because the switching speed depends on the timescale of the photochromic reactions.’ In this process, carriers for the superconductivity can be accumulated by UV light-induced electrical polarization of the spiropyran film, while the device can be switched to the superconducting state through both gate-voltage control and light-irradiation control. This multi-mode
operation indicates the high designability of organic systems. Although superconducting transistors have been developed using electric-double layer capacitors, modulations of the carrier density have been limited to the high-temperature regime because of the freezing of ionic motion below 200 K, since the heating and cooling process is required to switch the superconductivity. For this transistor, direct in situ switching is possible even in cryogenic conditions. The study demonstrates that a voltage source is not necessary for field-effect transistors, and that it is possible to access other energy sources such as light to operate transistors. Although still at a basic research stage, it illustrates the concept of superconductivity being switched by optical stimuli could drive innovation in the field of fast switching devices and very sensitive optical sensors. Laurie Donaldson
efficiency with which photons transmit information about the electronic spin states
of those impurity atoms. The production of these spin-photon interfaces could be
Reflecting on quantum memory Tiny ‘mirrors’’ that can trap light around impurities within a diamond can boost the
179
NEWS
Materials Today Volume 18, Number 4 May 2015
essential to the development of interconnected quantum memory devices that might be used in quantum computation and long-distance cryptographic systems. Dirk Englund’s team at the Massachusetts Institute of Technology in Cambridge USA, working with colleagues at Brookhaven National Laboratory in Long Island, New York, have demonstrated that the memory encoded in the electron spin state, the spin-coherence time, can persist for 200 microseconds or more; this is a record for quantum memories in such photonic traps [Englund et al., Nat. Commun. (2015), http://dx.doi.org/10.1038/ncomms7173]. ‘Our research demonstrates a technique to extend the storage time of quantum memories in solids that are efficiently coupled to photons, which is essential to scaling up such quantum memories for functional quantum computing systems and networks,’ explains Englund. The impurity atoms present in the diamond crystals studied by Englund and colleagues are nitrogen-vacancy (NV) centers. These consist of a nitrogen atom in the place of a carbon atom, adjacent to a crystal
about the electron spin state via photons, but we have to make the interface between the photons and electrons more efficient,’ Englund explains. Unfortunately, photons and electrons interact only very weakly. To boost the interaction, the team built an optical cavity around the NV to trap the photons using a transferred hard mask lithography technique. The cavity, nanofabricated at BNL by MIT graduate student Luozhou Li, working with BNL staff scientist Ming Lu, is made from layers of diamond and air tightly spaced around the impurity atom of the NV center. Reflection occurs at each interface between the layers so that photons entering bounce back and forth up to 10000 times, which boosts the interaction with the electrons in the NV center. ‘These methods have given us a great starting point for translating information between the spin states of the electrons among multiple NV quantum memories,’ explains Englund. ‘These results are an important part of validating the scientific promise of NV-cavity systems for quantum networking.’ David Bradley
vacancy within the diamond lattice. The spin state of the center can be either up or down thus providing the ‘0’ or ‘1’ of binary code. Microwaves radiation can be used to manipulate the spin state and because the ‘0’ state has a greater fluorescence than the ‘1’ state, the researchers can use an optical microscope to read the quantum memory. However, in order to be useful for carrying out logical operations of the kind that underpin computation, the spin states must be stable for a sufficient length of time. ‘It is already possible to transfer information
The economics of solar photovoltaic systems The US Energy Department’s National Renewable Energy Laboratory (NREL) has published two new reports that explore the economic viability of owning or leasing a solar photovoltaic (PV) system in the United States. The reports examine PV systems in terms of the economic options available when making decisions on financing commercial or residential solar energy systems, and are timely, especially with the market for solar-specific loan financing on the increase, and new loan product announcements and programs being launched all the time. Their main finding was that companies using low-cost financing to buy PV systems, and households taking out solar-specific loans, can achieve savings of up to 30% compared to leasing a PV system through a conventional third-party owner. The first report, ‘Banking on Solar: An Analysis of Banking Opportunities in the US Distributed Photovoltaic Market’, presents a high-level overview of developing US solar loan products, and the consumer and commercial loan products available for financing solar. It looks at both existing and potential solar lending institutions, loan 180
products, loan program structures and post-loan origination options, as well as reviewing the various risks of the solar asset class for lenders. In addition, the report explains how solar loan financing arrangements differ from third-party ownership, especially for the retention of ownership rights by the system host, and associated incentives, as well as the market players operating in distributed solar loans, and how solar loans with a range of maturities compare against thirdparty financing. The report demonstrates that the levelized cost of energy (LCOE) for residential systems with solar loans was less than for residential systems with power purchase agreements (PPAs) by as much as 29%, depending on the term of the loan. As co-author Travis Lowder said, ‘Using the lower cost rates provided by the loans could help to make solar power more affordable to more consumers, and more competitive with utility rates in more states.’ The second report, ‘To Own or Lease Solar: Understanding Commercial Retailers’ Decisions to Use Alternative Financing Models’, investigates the trade-offs involved between
financing methods for businesses installing onsite PV systems. It analyses the financial drivers when choosing a PV financing strategy and how they can vary depending on different assumptions based around case studies on two companies: IKEA, who selffinance their PV systems; and Staples, which leases its PV through PPAs. IKEA’s method suits their corporate culture, as careful process management allows it to own long-term, high-quality assets, while Staples finds leasing its PV systems through PPAs benefits their risk–return preferences, with the expertise the third-party PV owner brings meaning they can deploy PV rapidly. In terms of the financial considerations, the report concluded that the most suitable PV financing option can depend on the characteristics and circumstances of each business. As lead author David Feldman pointed out, ‘A company must work across its different business groups to decide what is most appropriate for its situation. With that said, if a company has less expensive sources of financing and is comfortable with the risks, it can often save on its energy bills by owning a PV system.’ Laurie Donaldson
NEWS
away from equilibrium using a strong electric field. The system then reconvened to a maximal entropy through two distinct intermediate stages, with spontaneous formation of self-assembled conducting nanotube chains along the way. Of course, the maximum entropy cannot be achieved in this experiment since it would correspond to a complete discharge of the battery running the experiment. In the first stage, conductive chains align themselves according to the polarity of the applied field. This allows the system to carry a current and so lose energy through resistive heating and so produce entropy. But, the nanotubes also sprout appendages connecting them, again increasing entropy production. Excessive heat causes destructive cascades or ‘avalanches’ that tear them apart, but the appendages sometimes retract before this happens and regrow once the energy is shed. ‘The avalanches were apparent in the changes of the electric current over time,’ explains Bezryadin. In the second intermediate there are no destructive avalanches and the system is much more stable hinting at how a system
the so-called ‘Dyson sphere’. Such a sphere might be constructed by a technologically advanced civilization in order to consume all power supplied by the sun. The team now needs to scale-up its systems to confirm that the principle holds more widely and to show that their selforganizing and self-connecting nanotube systems could somehow self-replicate. Self-replication can be expected on general grounds, since it would allow the system to further increase the entropy production. ‘The general trend of the evolution of biological systems seems to be this: more advanced life forms tend to dissipate more energy by broadening their access to various forms of stored energy,’ Bezryadin explains. ‘Thus a common underlying principle can be suggested between our self-organized clouds of nanotubes, which generate more and more heat by reducing their electrical resistance and thus allow more current to flow, and the biological systems which look for new means to find food, either through biological adaptation or by inventing more technologies.’ David Bradley
might evolve past a critical point, all the while generating entropy. This stable phase occurs after the systems evolves to the point that it is able to consume and convert into heat the maximum possible power provided by the battery and limited by a fixed resistor. Interestingly, such a tendency to approach the maximum power consumption and the subsequent stabilization of the system resembles a philosophical concept
New transistor switches superconductivity using light A new type of superconducting transistor able to be switched reversibly between on and off positions using light irradiation has been developed by a team of scientists in Japan. This switching capability, based on organic superconducting field-effect transistors (FETs), could result in a new generation of high-speed switching devices and highly sensitive optical sensors. The researchers, led by Hiroshi Yamamoto from Japan’s Institute for Molecular Science, created the first organic superconducting FETs a couple of years ago, bringing attention to their flexibility and designability. FETs are a standard switching element that controls electrical current in electronic circuits, and are now used in many electronic devices, including smart phones and computers. Much research is being carried out into superconducting FETs as a key technology for computations using quantum states. This study, as reported in Science [Suda et al., Science (2015), doi:10.1126/sci-
ence.1256783], developed a novel photoswitchable transistor by replacing the gate electrode in the conventional FET with a spiropyran thin film. Spiropyran is a photoactive organic molecule that shows intramolecular electrical polarization by ultraviolet (UV) light irradiation. On shining a pale UV light on the transistor, it demonstrated a quick decrease in electrical resistance and turned into a superconducting state after 180 seconds. However, as researcher Masayuki Suda points out, ‘it can be operated much faster in principle because the switching speed depends on the timescale of the photochromic reactions.’ In this process, carriers for the superconductivity can be accumulated by UV light-induced electrical polarization of the spiropyran film, while the device can be switched to the superconducting state through both gate-voltage control and light-irradiation control. This multi-mode
operation indicates the high designability of organic systems. Although superconducting transistors have been developed using electric-double layer capacitors, modulations of the carrier density have been limited to the high-temperature regime because of the freezing of ionic motion below 200 K, since the heating and cooling process is required to switch the superconductivity. For this transistor, direct in situ switching is possible even in cryogenic conditions. The study demonstrates that a voltage source is not necessary for field-effect transistors, and that it is possible to access other energy sources such as light to operate transistors. Although still at a basic research stage, it illustrates the concept of superconductivity being switched by optical stimuli could drive innovation in the field of fast switching devices and very sensitive optical sensors. Laurie Donaldson
efficiency with which photons transmit information about the electronic spin states
of those impurity atoms. The production of these spin-photon interfaces could be
Reflecting on quantum memory Tiny ‘mirrors’’ that can trap light around impurities within a diamond can boost the
179
NEWS
Materials Today Volume 18, Number 4 May 2015
essential to the development of interconnected quantum memory devices that might be used in quantum computation and long-distance cryptographic systems. Dirk Englund’s team at the Massachusetts Institute of Technology in Cambridge USA, working with colleagues at Brookhaven National Laboratory in Long Island, New York, have demonstrated that the memory encoded in the electron spin state, the spin-coherence time, can persist for 200 microseconds or more; this is a record for quantum memories in such photonic traps [Englund et al., Nat. Commun. (2015), http://dx.doi.org/10.1038/ncomms7173]. ‘Our research demonstrates a technique to extend the storage time of quantum memories in solids that are efficiently coupled to photons, which is essential to scaling up such quantum memories for functional quantum computing systems and networks,’ explains Englund. The impurity atoms present in the diamond crystals studied by Englund and colleagues are nitrogen-vacancy (NV) centers. These consist of a nitrogen atom in the place of a carbon atom, adjacent to a crystal
about the electron spin state via photons, but we have to make the interface between the photons and electrons more efficient,’ Englund explains. Unfortunately, photons and electrons interact only very weakly. To boost the interaction, the team built an optical cavity around the NV to trap the photons using a transferred hard mask lithography technique. The cavity, nanofabricated at BNL by MIT graduate student Luozhou Li, working with BNL staff scientist Ming Lu, is made from layers of diamond and air tightly spaced around the impurity atom of the NV center. Reflection occurs at each interface between the layers so that photons entering bounce back and forth up to 10000 times, which boosts the interaction with the electrons in the NV center. ‘These methods have given us a great starting point for translating information between the spin states of the electrons among multiple NV quantum memories,’ explains Englund. ‘These results are an important part of validating the scientific promise of NV-cavity systems for quantum networking.’ David Bradley
vacancy within the diamond lattice. The spin state of the center can be either up or down thus providing the ‘0’ or ‘1’ of binary code. Microwaves radiation can be used to manipulate the spin state and because the ‘0’ state has a greater fluorescence than the ‘1’ state, the researchers can use an optical microscope to read the quantum memory. However, in order to be useful for carrying out logical operations of the kind that underpin computation, the spin states must be stable for a sufficient length of time. ‘It is already possible to transfer information
The economics of solar photovoltaic systems The US Energy Department’s National Renewable Energy Laboratory (NREL) has published two new reports that explore the economic viability of owning or leasing a solar photovoltaic (PV) system in the United States. The reports examine PV systems in terms of the economic options available when making decisions on financing commercial or residential solar energy systems, and are timely, especially with the market for solar-specific loan financing on the increase, and new loan product announcements and programs being launched all the time. Their main finding was that companies using low-cost financing to buy PV systems, and households taking out solar-specific loans, can achieve savings of up to 30% compared to leasing a PV system through a conventional third-party owner. The first report, ‘Banking on Solar: An Analysis of Banking Opportunities in the US Distributed Photovoltaic Market’, presents a high-level overview of developing US solar loan products, and the consumer and commercial loan products available for financing solar. It looks at both existing and potential solar lending institutions, loan 180
products, loan program structures and post-loan origination options, as well as reviewing the various risks of the solar asset class for lenders. In addition, the report explains how solar loan financing arrangements differ from third-party ownership, especially for the retention of ownership rights by the system host, and associated incentives, as well as the market players operating in distributed solar loans, and how solar loans with a range of maturities compare against thirdparty financing. The report demonstrates that the levelized cost of energy (LCOE) for residential systems with solar loans was less than for residential systems with power purchase agreements (PPAs) by as much as 29%, depending on the term of the loan. As co-author Travis Lowder said, ‘Using the lower cost rates provided by the loans could help to make solar power more affordable to more consumers, and more competitive with utility rates in more states.’ The second report, ‘To Own or Lease Solar: Understanding Commercial Retailers’ Decisions to Use Alternative Financing Models’, investigates the trade-offs involved between
financing methods for businesses installing onsite PV systems. It analyses the financial drivers when choosing a PV financing strategy and how they can vary depending on different assumptions based around case studies on two companies: IKEA, who selffinance their PV systems; and Staples, which leases its PV through PPAs. IKEA’s method suits their corporate culture, as careful process management allows it to own long-term, high-quality assets, while Staples finds leasing its PV systems through PPAs benefits their risk–return preferences, with the expertise the third-party PV owner brings meaning they can deploy PV rapidly. In terms of the financial considerations, the report concluded that the most suitable PV financing option can depend on the characteristics and circumstances of each business. As lead author David Feldman pointed out, ‘A company must work across its different business groups to decide what is most appropriate for its situation. With that said, if a company has less expensive sources of financing and is comfortable with the risks, it can often save on its energy bills by owning a PV system.’ Laurie Donaldson