Understanding the ultimate limit of quantum dot linewidths


mix-id328072.jpgColloidal quantum dots are potentially useful as artificial atoms for applications in emerging quantum technologies. However, previous measurements indicated that these nanocrystals are prone to significant decoherence (as they transition from quantum to classical behaviour). The origins behind this phenomenon remained a mystery, but researchers at the University of Bordeaux in France now provide a possible explanation. Thanks to a novel light absorption-based technique, which reveals that the decoherence is caused by spontaneous charge noise in the environment surrounding the nanocrystals, decoherence-limited linewidths of approximately one gigahertz have been found. The finding should aid in the design of quantum photonic structures containing nanocrystals.

In the quantum regime, particles can act like waves and interfere with each other. However, this quantum interference vanishes as we approach macroscopic length scales as the particles begin to interact with their environment. Physicists usually try to avoid this phenomenon, which is known as decoherence.

Colloidal quantum dots for their part are plagued by spectral instabilities, known as spectral diffusion, which are detrimental to their application in quantum technologies. Spectral diffusion comes about as the excited quantum dot shifts its emission frequency in response to slight changes in its local environment. The phenomenon is unfortunate since colloidal quantum dots could offer some unique advantages in this field thanks to their being compatible with a wide range of photonic structures and the fact that they can be accurately integrated within these structures. Understanding the origin of spectral diffusion is thus important and would ultimately help researchers mitigate these effects. 

A team has now studied the fast spectral diffusion process using a resonant photoluminescence excitation technique in which a narrow-band laser is scanned across an absorption line of a single quantum dot and the signal detected The shape of the measured spectral line can show whether the spectral diffusion is caused by the absorption of a photon or not, and the Bordeaux researchers have found that at the highest resolution the spectral diffusion process does not depend on photon absorption. 

In this work, the properties of charge noise in disordered media were used to demonstrate that a single colloidal quantum dot is capable of detecting spontaneous changes in the environmental charge distribution via the quantum confined Stark effect. Such fluctuations were found to be compatible with the gigahertz linewidths previously reported. Fast spectral diffusion in quantum dots can thus be attributed to spontaneous environmental charge noise within the disordered local environment, something that ultimately sets a limit on the linewidth that can be obtained with colloidal quantum dots.

More information can be found in the journal Nanotechnology (in press).

Further reading

Quantum dot blend gives wide-bandwidth FET-based photodetector (Jul 2012) Novel recombination layers improve multijunction photovoltaics (Jun 2012) How far can charge carriers travel in CQD films? (Jun 2013) Nanoparticles pinpoint brain activity (Jan 2006)

About the author

The research was conducted in the Nanophotonics Group at Bordeaux headed by Professor Brahim Lounis at the University of Bordeaux. Dr Mark Fernee is an invited researcher specializing in the photophysical properties of nanocrystals with a particular emphasis on applications in quantum technologies. Chiara Sinito is a PhD student in the group, who together with Dr Yann Louyer participated in the experiments. Professor Philippe Tamarat is an expert in single molecule and single nanoparticle detection.

Nanobiotix Announces EUR 2.8 Million Grant From bpifrance to Accelerate Development of NBTXR3 in a Third New Indication


Through ETPN (European Technology Platform of Nanomedicine), Nanobiotix Is Driving Nanomedicine in Europe and Is Now Involved in the First Nanomedicine Consortium in France With the launch of NICE Project
201306047919620PARIS–(Marketwired – Jul 3, 2013) – NANOBIOTIX (EURONEXT PARIS: NANO), a clinical-stage nanomedicine company pioneering novel approaches for the local treatment of cancer, announces today it has secured c.EUR 9 million in funding from bpifrance (formerly OSEO) of which EUR 2.8 million is directly attributable to the Company. This grant, awarded through bpifrance’s Strategic Industrial Innovation (ISI) program, will accelerate the clinical and industrial development of the Company’s lead product NBTXR3 in a new indication, liver cancer (hepatocellular carcinoma). Liver cancer is a major health problem that causes one of the greatest number of deaths each year worldwide, c.695,000 deaths per annum.

This grant supports the launch of NICE (Nano Innovation for CancEr), the first consortium of nanomedicine stakeholders in France focused on characterization and industrialization aspects. The consortium has been accredited by the Medicen Paris Region, a competitive cluster for innovative therapies in Ile-de-France (www.medicen.org).

Consisting of five public and private partners, the NICE consortium includes partners with in depth expertise in the field of nanomedicine. Its mission is to build a platform to accelerate the development and industrialization of nanomedicine in France by capitalizing on the strong and complementary expertise of each partner.

Nanobiotix’s lead product NBTXR3, based on NanoXray, is currently under clinical development for advanced soft tissue sarcoma and has received authorization from the French Medicine Agency, ANSM, to start a second clinical trial in patients with locally advanced cancers of the oral cavity or oropharynx (head and neck cancer). NBTXR3 will benefit fully from this platform of expertise and funding received from bpifrance by being able to accelerate its clinical development. The purpose of this project is the start of a new Phase I clinical study with NBTXR3 in patients with primary liver cancer.

Watch the Video from Nanobiotix here:

http://www.youtube.com/watch?v=kxSX6YJTS2I&feature=share&list=PL9C30814198614279

In addition to Nanobiotix, the consortium includes BioAlliance Pharma, leader of the consortium which is developing Livatag®, a doxorubicin nanoparticle currently in Phase III clinical trial for treatment of primary liver cancer; CEA-Leti, the developer of cancer-detecting nanoparticles based on Leti’s Lipidots® platform; DBI, a company specialized in the production of nanomedicine pharmaceutical products and the Institut Galien Paris Sud (University Paris Sud/CNRS), which has an academic-excellence team specialized in nanoparticle research.

Today, nanomedicine is considered as one of the major growth drivers of the global pharmaceutical industry and it is essential that the industry is structured at the local level to be competitive,” said Laurent Levy, CEO of Nanobiotix. “In turn, Nanobiotix will benefit from the consortium in two ways; if the sector is structured right to be able to accelerate the development and industrialization of products, and in the development of a new indication of NBTX3 which will help patients and build shareholder value.”

About BPIFRANCE’s Strategic Industrial Innovation (ISI) program

The Strategic Industrial Innovation program promotes the emergence of European champions. It supports ambitious, innovative collaborative projects through to industrialization, driven by innovative medium-sized companies (less than 5000 employees) and small businesses (less than 250 employees). These highly promising projects are aimed at the commercialization of products which result from technological breakthroughs and which not be possible without fostering measures from the public sector. Funding is generally in the EUR 3-10 million range, as grants-in aid and loans which are repayable if the project is a success.

About NANOXRAY

Nanobiotix’s first-in-class, proprietary technology called NanoXray is at the forefront of a new era of nanomedicine, where nanoparticles are not just a vehicle for targeted drug delivery, but have become the principal active element. The NanoXray technology is based on the physical properties of hafnium-oxide nanoparticles and is used to enhance the efficacy of radiotherapy treatment for a variety of cancer indications.

Nanoparticles are designed to enter tumor cells and, upon activation by a standard dose of radiation, they emit large amounts of electrons resulting in the generation of free radicals that destroy cancer cells (the same mode of action than radiotherapy but largely amplified). Nanoparticle-enhanced radiotherapy therefore amplifies the lethal dose of energy locally within the tumor without changing the effect of the dose passing through surrounding healthy tissues.

By changing the coating of the nanoparticles, Nanobiotix is developing three different products that can be administered either by direct injection into the tumor (NBTXR3), intravenous injection (NBTX-IV) or topical application to fill tumor cavities after surgery (NBTX-TOPO). The product applied will depend on type of tumor and the patient’s specific clinical needs. NanoXray products are classified as a medical device in Europe and as a drug in the US. They are compatible with current radiotherapy methods with respect to equipment and protocols, as well as with older radiotherapy equipment or any radiation-based therapy.

About NANOBIOTIX

Nanobiotix (Euronext: NANO / ISIN: FR0011341205) is a clinical-stage nanomedicine company pioneering novel approaches for the local treatment of cancer. The Company’s first-in-class, proprietary technology, NanoXray, enhances radiotherapy energy to provide a new, more efficient treatment for cancer patients. NanoXray products are compatible with current radiotherapy treatments and are meant to treat a wide variety of cancers via multiple routes of administration. Nanobiotix’s lead product NBTXR3, based on NanoXray, is currently under clinical development for soft tissue sarcoma. The Company has partnered with PharmaEngine for clinical development and commercialization of NBTXR3 in Asia. The Company is based in Paris, France.

Laurent Levy, CEO and co-founder of Nanobiotix, is the Vice-President of the European Technology Platform of Nanomedicine (ETPN).

For more information, please visit www.nanobiotix.com

Press release (PDF): http://hugin.info/157012/R/1714109/569130.pdf

International Nanotechnology conference 2013: Dubai


QDOTS imagesCAKXSY1K 8SETCOR Nanotech Dubai 2013 brings together leading scientists, researchers, engineers, practitioners, technology developers and policy makers in nanotechnology to exchange information on their latest research progress, innovation and business opportunities. It’s among the most important events in terms of international regulatory policies and it’s opened to the participation of private companies. It’s unique venue for companies to promote equipment and technology.
The conference covers all frontier topics in nanotechnology. The conference includes plenary lectures and invited talks by eminent personalities from around the world in addition to contributed papers both oral and poster presentations.
SETCOR Nanotech Dubai 2103 conference organizing committee is looking forward to welcoming you in Dubai- United Arab Emirates.
Conference Scope

Themes of the 3 days event will be as follow:

  • Advanced Materials
  • Nanoscale Electronics
  • Nanotech in Life Sciences & Medicine
  • Energy & Environment
  • Fabrication, Characterization & Tools
  • Nanotechnology safety
  • Nano Application

 

 

Keynote Speakers

Speakers

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Prof. Andrea C. Ferrari

Cambridge Graphene Centre, Engineering Department, University of Cambridge– UK

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Dr. Bouzid Menaa

Fluorotronics, Inc, San Diego, CA– USA

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Prof. Erich Sackmann

Institute of Molecular and Cellular Biophysics, Technische Universitat Munchen- Germany

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Prof. Farzaneh Arefi-Khonsari

University of Pierre & Marie Curie, Paris, France

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Prof. Adnane Abdelghani

National Institute of Applied Science and Technology, Tunis-Tunisia

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Prof. Axel Lorke

Faculty of Physics and CeNIDE, University of Duisburg-Essen-Germany

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Prof. Jackie Ying

Executive Director, Institute of Bio-engineering and Nanotechnology, Singapore.

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Dr. Vasco Teixeira

Associate Professor in Materials Physics, University of Minho

“Spin States” and Quantum Dots and Why … It’s Important


QDOTS imagesCAKXSY1K 8New experiments show how to optically initialize a specific spin state of a single manganese atom placed inside a quantum dot for spintronics applications.

Spin dynamics of a Mn atom in a semiconductor quantum dot under resonant optical excitation S. Jamet, H. Boukari, and L. Besombes

Published June 10, 2013 | PDF (free)
8447cb8b9e31ef1b The ability to control individual spins (the intrinsic units of angular momentum carried by electrons) in semiconductors is an important requirement for a new generation of devices based on spin rather than charge logic. Single magnetic ions are promising for this type of application because of their long spin coherence times.

 

Figure 1 Scheme of the optical transitions in a quantum dot containing an individual magnetic atom and 0 (Mn ), 1 (X−Mn ), or 2 (X2−Mn ) excitons. The exciton states are split by the exchange interaction with the Mn spin, whereas in the ground (Mn ) and biexciton (X2−Mn ) states, the energy levels result from the fine and hyperfine structure of the Mn spin. Spin-population trapping on the 5/2 level is achieved by carefully tuning a resonant CW laser (green arrow). Spin readout: A direct resonant excitation of the biexciton is performed by a pulsed two-photon absorption through an intermediate virtual state (blue arrows). The biexciton photon emission allows monitoring all six Mn spin levels simultaneously.

The difficulty lies in addressing these stable but isolated spins, a feat that can be achieved by placing a single magnetic ion like manganese (Mn ) inside a semiconductor quantum dot [1]. This arrangement strongly mixes the states of the Mn spin and the charge carriers trapped inside the nano-object. As a result, optical initialization and readout of an Mn spin state can be achieved [2] using resonant laser excitation, as employed previously in single-electron and hole spin-pumping schemes used in quantum dots [3, 4]. A key feature of these established spin-pumping techniques is a depletion of the spin level that is resonantly excited, which may at first seem counterintuitive.

Segolene Jamet and colleagues at the French National Center for Scientific Research (CNRS) and Joseph Fourier University, France, have now experimentally demonstrated a new spin-population-trapping scheme for a single Mn spin state [5] monitored via a new readout technique (Fig. 1). Writing in Physical Review B, they show how the Mn atom is directly pumped into the spin state that is resonantly excited, in stark contrast to existing methods [2, 3, 4]. Tuning the laser energy and power allows varying the strength of the coherent coupling between Mn spin levels that is at the origin of the spin-population trapping.

Researchers have demonstrated efficient optical and electrical control of individual electron and nuclear spin states in semiconductor quantum dots that allow for the integration with standard semiconductor circuits [6]. Placing an isolated Mn ion at a fixed lattice position inside a quantum dot adds important new opportunities for spintronics because of the possibility of manipulating Mn spins in this environment with well-established optical and electrical quantum-dot control techniques [7, 8]. When a single Mn
impurity is introduced into a II-VI quantum-dot material like CdTe, as used by Jamet et al. [5], an interesting situation arises. An Mn atom has five electrons on the d shell, which results in a possible total spin of up to S=5/2 (in units of ħ ). Just as a free electron can have a spin (projected on a given axis) of +1/2 or −1/2 , there are six different spin states for a Mn atom from −5/2 , −3/2 , −1/2 , … up to +5/2 .

Storing information in these six spin states can be thought of as a “quantum die,” as opposed to a two-quantum-level system called “quantum bit.” Therefore the system investigated by Jamet et al. is interesting in the context of quantum computing because 15 pairs of different quantum bits can be defined for a single Mn impurity.

A general advantage of an isolated Mn atom in a semiconductor matrix is that these spin states can be well separated from each other in energy, even in modest magnetic fields, through what is termed the “giant Zeeman effect” [9]. Applying magnetic fields is not always feasible in real-life applications, therefore Jamet et al. lift the spin-state degeneracy through photoexcitation: The optically created electron-hole pair (exciton) strongly interacts with the 5d electrons of the Mn via an effect known as the Coulomb exchange interaction. As a result the quantum-dot emission, usually one single line, is split into six well-separated components. In the absence of Mn spin pumping, such as that applied by Jamet et al., the spin state of the Mn atom following excitation of the dot by a nonresonant laser is completely arbitrary. As a result, all six possible lines are observed in the time-averaged optical spectra [1].

Using laser excitation that is slightly off resonance with respect to the 5/2 spin state (green arrow in Fig.1), Jamet et al. are able to populate mainly this targeted spin state, to which the populations from the 1/2 and −1/2 states have been transferred. In order for this transfer to occur, first, they needed to match the energy of the spin states. Jamet et al. arranged this through resonant excitation with a laser.

The strong coupling between the electromagnetic radiation and the Mn system shifts the states in energy as dressed states are formed [5]. Second, they needed to establish population transfer toward the 5/2
state. Spin systems are, in general, very sensitive to the exact symmetry (spherical, cubic,…) of their environment. CdTe dots in a ZnTe matrix are subject to lattice strain.

This results in an anisotropic local environment for the electronic Mn spin system inside a strained quantum dot. As a direct consequence of the anisotropic strain distribution [10], spin states separated by two units of ħ are coupled (i.e., 5/2 coupled to 1/2 not 3/2 ). As the 5/2 and 1/2 states dressed by the laser field are brought into resonance, this coherent coupling induces a population transfer from the 1/2
to the 5/2 states. This population transfer is irreversible once the photon has left the dot; i.e., the optically dressed state has recombined.

The novel spin-population-trapping scheme introduced by Jamet et al. is controlled by the presence of coherent coupling between different Mn spin states. In future experiments, this coupling can be optimized through strain engineering, i.e., using different dot-barrier material combinations with a variety of lattice parameters. Also the application of a small, external magnetic field in the dot plane will modify the coupling between spin states.

The spin-population trapping for the Mn electron also involves flips with the spin of the Mn
nucleus. Optical pumping of the spin of a single Mn nucleus with long spin-memory times is a natural extension of the current work. In principle, the spin-population trapping introduced by Jamet et al. can be applied to other solid-state and atomic systems provided that a coherent coupling between the spin sublevels is present or can be induced.