How can quantum information be stored as long as possible? An important step forward in the development of quantum memories has been achieved by a research team of TU Wien.
Conventional memories used in today’s computers only differentiate between the bit values 0 and 1. In quantum physics, however, arbitrary superpositions of these two states are possible. Most of the ideas for new quantum technology devices rely on this “Superposition Principle”. One of the main challenges in using such states is that they are usually short-lived. Only for a short period of time can information be read out of quantum memories reliably, after that it is irrecoverable.
A research team at TU Wien has now taken an important step forward in the development of new quantum storage concepts. In cooperation with the Japanese telecommunication giant NTT, the Viennese researchers lead by Johannes Majer are working on quantum memories based on nitrogen atoms and microwaves. The nitrogen atoms have slightly different properties, which quickly leads to the loss of the quantum state. By specifically changing a small portion of the atoms, one can bring the remaining atoms into a new quantum state, with a lifetime enhancement of more than a factor of ten. These results have now been published in the journal “Nature Photonics”.
Nitrogen in diamond
“We use synthetic diamonds in which individual nitrogen atoms are implanted”, explains project leader Johannes Majer from the Institute of Atomic and Subatomic Physics of TU Wien. “The quantum state of these nitrogen atoms is coupled with microwaves, resulting in a quantum system in which we store and read information.”
However, the storage time in these systems is limited due to the inhomogeneous broadening of the microwave transition in the nitrogen atoms of the diamond crystal. After about half a microsecond, the quantum state can no longer be reliably read out, the actual signal is lost. Johannes Majer and his team used a concept known as “spectral hole burning”, allowing data to be stored in the optical range of inhomogeneously broadened media, and adapted it for supra-conducting quantum circuits and spin quantum memories.
Dmitry Krimer, Benedikt Hartl and Stefan Rotter (Institute of Theoretical Physics, TU Wien) have shown in their theoretical work that such states, which are largely decoupled from the disturbing noise, also exist in these systems. “The trick is to manoeuver the quantum system into these durable states through specific manipulation, with the aim to store information there,” explains Dmitry Krimer.
Excluding specific energies
“The transitions areas in the nitrogen atoms have slightly different energy levels because of the local properties of the not quite perfect diamond crystal”, explains Stefan Putz, the first author of the study, who has since moved from TU Wien to Princeton University. “If you use microwaves to selectively change a few nitrogen atoms that have very specific energies, you can create a “Spectral Hole”. The remaining nitrogen atoms can then be brought into a new quantum state, a so-called “dark state”, in the center of these holes. This state is much more stable and opens up completely new possibilities.”
“Our work is a ‘proof of principle’ – we present a new concept, show that it works, and we want to lay the foundations for further exploration of innovative operational protocols of quantum data,” says Stefan Putz.
With this new method, the lifetime of quantum states of the coupled system of microwaves and nitrogen atoms increased by more than one order of magnitude to about five microseconds. This is still not a great deal in the standard of everyday life, but in this case it is sufficient for important quantum-technological applications. “The advantage of our system is that one can write and read quantum information within nanoseconds,” explains Johannes Majer. “A large number of working steps are therefore possible in microseconds, in which the system remains stable.”
Sharp metal needles can be used to emit electrons. A quantum effect opens up new possibilities of controlling electron emission with extremely high accuracy.
In an electron microscope, electrons are emitted by pointy metal tips, that way the can be steered and controlled with high precision. Recently, such metal tips have also been used as high precision electron sources for generating x-rays. A team of researchers at TU Wien (Vienna), together with colleagues from the FAU Erlangen-Nürnberg (Germany), have developed a method of controlling electron emission with higher precision than ever before. With the help of two different laser pulses it is now possible to switch the flow of electrons on and off on extremely short time scales.
It’s Just the Tip of the Needle
“The basic idea resembles a lightning rod”, says Christoph Lemell (TU Wien). “The electrical field around a needle is always strongest right at the tip. That’s why the lightning always strikes the tip of a rod, and for the same reason, electrons leave a needle right at the tip.”
Extremely pointy needles can be fabricated with the methods of modern nanotechnology. Their tip is just a few nanometres wide, so the point at which the electrons are emitted is known with very high accuracy. In addition to that, it is also important to control at which point in time the electrons are emitted.
This kind of temporal control has now become possible, using a new approach: “Two different laser pulses are fired at the metal tip”, explains Florian Libisch (TU Wien). The colours of these two lasers are chosen such that the photons of one laser have exactly twice the energy of the other laser’s photons. Also, it is important to ensure that both light waves oscillate in perfect synchronicity.
With the help of computer simulations, the team from TU Wien was able to predict that a small time delay between the two laser pulses can serve as a “switch” for electron emission. This prediction has now been confirmed by experiments performed by Professor Peter Hommelhoff’s research group at FAU Erlangen-Nürnberg. Based on these experiments, it is now possible to understand the process in detail.
When a laser pulse is fired at the metal tip, its electrical field can rip electrons out of the metal – that is a well-known phenomenon. The new idea is that a combination of two different lasers can be used to control the emission of the electrons on a femtosecond time scale.
There are different ways an electron can gain enough energy to leave the metal tip: It can absorb one photon from the high-energy laser and two photons from the low-energy laser or four electrons from the low-energy laser. Both mechanisms lead to the same result. “Much like a particle in a double slit experiment, which travels on two different paths at the same time, the electron can take part in two different processes at the same time”, says Professor Joachim Burgdörfer (TU Wien). “Nature does not have to pick one the two possibilities – both are equally real and interfere which each other.”
By carefully tuning the two lasers, it is possible to control whether the two quantum physical processes amplify each other, which leads to an increased emission of electrons, or whether they cancel each other, which means that hardly any electrons are emitted at all. This is a simple and effective way of controlling electron emission.
It is not just a new method of performing experiments with high energy electrons, the new technology should open the door to controlled x-ray generation. “Innovative x-ray sources are already being built, using arrays of narrow metal tips as electron sources”, says Lemell. “With our new method, these nano tips could be triggered in exactly the right way so that coherent x-ray radiation is produced.”
It is the Philosopher’s Stone of Nanotechnology: using a technological trick, scientists at TU Wien (Vienna) have succeeded in creating nanostructures made of pure gold.
This work is a giant leap forward for 3D nano-printing of gold structures which will be the core part of nanoplasmonics and bioelectronics devices
The idea is reminiscent of the ancient alchemists’ attempts to create gold from worthless substances: Researchers from TU Wien (Vienna) have discovered a novel way to fabricate pure gold nanostructures using an additive direct-write lithography technique. An electron beam is used to turn an auriferous organic compound into pure gold. This new technique can now be used to create nanostructures, which are needed for many applications in electronics and sensor technology. Just like with a 3D-printer on the nanoscale, almost arbitrary shapes can be created.
The long search for the right production process
“Gold is not only a noble metal of exceptional beauty, but also a highly desired material for functional nanostructures”, says Professor Heinz Wanzenböck from TU Wien. Especially patterned gold nanostructures are key enabling structures in plasmonic devices, for biosensors with immobilized antibodies and as electrical contacts. For decades the fabrication of pure gold nanostructures on non-planar surfaces as well as of 3-dimensional gold nanostructures has been the bottleneck. Up to now, only 2-dimensional gold nanostructures on planar surfaces were achievable by resist based lithography.
The new technology, developed at TU Wien, can now solve this problem. The principle is the local decomposition of a metalorganic precursor by the focused electron beam of an electron microscope. With extremely high precision, the electron beam can decompose the organic compound at exactly the right position, leaving behind a 3D-trail of solid gold.
The final obstacle was getting the material purity right, as the electron-induced decomposition of metalorganic precursors has typically yielded metals with high carbon contaminations. This last bottleneck on the road to custom-designed, pure gold nanostructures has now been overcome as described in the work on “Highly conductive and pure gold nanostructures grown by electron beam induced deposition” published in Scientific Reports.
While conventional gold deposition usually contains about 70 atomic % carbon and only 30 atomic % gold, the new approach developed by a research group lead by Dr. Heinz Wanzenboeck at TU Wien has allowed to fabricate pure gold structures by in-situ addition of an oxidizing agent during the gold deposition. “The whole community has been working hard for the last 10 years to directly deposit pure gold nanostructures”, says Heinz Wanzenböck. At last, the group’s expertise in engineering and chemical reactions paid off and direct deposition of pure gold was successful. “It’s a bit like discovering the legendary philosopher’s stone that turns common, ignoble material into gold” joked Wanzenboeck.
This deposited pure gold structure exhibits extremely low resistivity near that of bulk gold. Generally, a FEBID gold structure has a resistivity around 1-Ohm-cm which is about 1 million times worse than the resistivity of purest bulk gold. However, this specially enhanced FEBID process produces a resistivity of 8.8 micro-Ohm-cm which is only a factor 4 away from the bulk resistivity of purest gold (2.4 micro-Ohm-cm).
The authors of the paper Dr. Mostafa Moonir Shawrav and Dipl.Ing. Philipp Taus stated, “This highly conductive and pure gold structure will open a new door for novel nanoelectronic devices. For example, it will be easier to produce pure gold structures for nanoantennas and biomolecule immobilization which will change our everyday life”. Dr. Shawrav added “it is remarkable how a regular SEM (Scanning Electron Microscope) nowadays can deposit nanostructures compared to 20 years back when it was only a characterization device”. And with pure gold direct deposition available now, he expects nanodevices to be deposited directly and utilized in many different applications for technological revolution. Concluding, this work is a giant leap forward for 3D nano-printing of gold structures which will be the core part of nanoplasmonics and bioelectronics devices.
Learn more: Nanostructures Made of Pure Gold
A laser and detector in one: a microscopic sensor has been developed at TU Wien, which can be used to identify different gases simultaneously.
As humans, we sniff out different scents and aromas using chemical receptors in our noses. In technological gas detection, however, there are a whole host of other methods available. One such method is to use infrared lasers, passing a laser beam through the gas to an adjacent separate detector, which measures the degree of light attenuation it causes. TU Wien’s tiny new sensor now brings together both sides within a single component, making it possible to use the same microscopic structure for both the emission and detection of infrared radiation.
Circular quantum cascade lasers
“The lasers that we produce are a far cry from ordinary laser pointers ,” explains Rolf Szedlak from the Institute of Solid State Electronics at TU Wien. “We make what are known as quantum cascade lasers. They are made up of a sophisticated layered system of different materials and emit light in the infrared range.”
When an electrical voltage is applied to this layered system, electrons pass through the laser. With the right selection of materials and layer thicknesses, the electrons always lose some of their energy when passing from one layer into the next. This energy is released in the form of light, creating an infrared laser beam.
“Our quantum cascade lasers are circular, with a diameter of less than half a millimetre,” reports Prof. Gottfried Strasser, head of the Center for Micro- and Nanostructures at TU Wien. “Their geometric properties help to ensure that the laser only emits light at a very specific wavelength.”
“This is perfect for chemical analysis of gases, as many gases absorb only very specific amounts of infrared light,” explains Prof. Bernhard Lendl from the Institute of Chemical Technologies and Analytics at TU Wien. Gases can thus be reliably detected using their own individual infrared ‘fingerprint’. Doing so requires a laser with the correct wavelength and a detector that measures the amount of infrared radiation swallowed up by the gas.
A laser that also detects
“Our microscopic structure has the major advantage of being a laser and detector in one,” professes Rolf Szedlak. Two concentric quantum cascade rings are fitted for this purpose, which can both (depending on the operating mode) emit and detect light, even doing so at two slightly different wavelengths. One ring emits the laser light which passes through the gas before being reflected back by a mirror. The second ring then receives the reflected light and measures its strength. The two rings then immediately switch their roles, allowing the next measurement to be carried out.
In testing this new form of sensor, the TU Wien research team faced a truly daunting challenge: they had to differentiate isobutene and isobutane – two molecules which, in addition to confusingly similar names, also possess very similar chemical properties. The microscopic sensors passed this test with flying colours, reliably identifying both of the gases.
“Combining laser and detector brings many advantages,” says Gottfried Strasser. “It allows for the production of extremely compact sensors, and conceivably, even an entire array – i.e. a cluster of microsensors – housed on a single chip and able to operate on several different wavelengths simultaneously.” The application possibilities are virtually endless, ranging from environmental technology to medicine.
Learn more: The quantum sniffer dog
Scientists at TU Wien (Vienna) found a way to compress ultrashort laser pulses, increasing its peak power to half a terawatt – which is equivalent to the output of hundreds of nuclear reactors.
It is a very unusual kind of laser: researchers at the photonics institute at TU Wien (Vienna) have built a device which emits ultrashort flashes of infrared light with extremely high energy. “It is very hard to combine these three properties – long infrared wavelength, short duration and high energy”, says Valentina Shumakova. “But this combination is exactly what we need for many interesting strong-field applications.”
Now the team has achieved a major breakthrough: By sending very energetic pulses in the infrared regime through a solid medium, the pulses can be compressed in time and space. The energy stays roughly the same, but it can now be deposited in an even shorter period of time, resulting in an incredible peak power of up to half a terawatt. This power corresponds to the output of hundreds of nuclear reactors. But unlike power plants, which produce the power steadily, the compressed laser pulse only lasts 30 femtoseconds (millionths of a billionth of a second). The new results have now been published in the journal “Nature communications”.
Playing with Invisible Colours
“Under certain conditions, laser pulses can self-compress and become shorter. This is a well-known phenomenon in laser science”, says Audrius Pugzlys. “But until now, people used to believe that self-compression in solid media at extremely high intensities is impossible.”
Unlike the light of a simple laser pointer, an ultrashort laser pulse does not only have one specific colour. It is a mixture of a spectrum of different wavelengths – in this case centred around 3.9 micrometers, in the long infrared region, invisible to the human eye.
In vacuum, light always travels at the same speed, regardless of its wavelength. But this is not the case for light traversing a solid material. “The material causes some components of the laser pulse to move faster than others. If this effect is cleverly used, the laser pulse is compressed, it becomes shorter just by travelling through the material”, says Skirmantas Alisauskas.
This technique, however, is not always applicable. “If a pulsed laser beam of very high intensities is sent through a material, the beams tends to collapse chaotically into many separate filaments”, says Audrius Pugzlys. “It is like a bolt of lightning that spontaneously breaks up into various branches.” Each of the branches only carries a small part of the energy of the original beam, the resulting laser beam cannot be used for advanced strong-field laser experiments any more.
Breaking the filamentation threshold by four orders of magnitude
The Viennese research group, in collaboration with researchers from Moscow state university, has now identified conditions which lead to self-compression and an extremely high peak power without causing the beam to collapse into filaments. “As it turns out, we are dealing with two different length scales”, says Valentina Shumakova. “The length scale of the unwanted filamentation is longer than the length on which self-compression occurs. Therefore, it is possible to find a parameter regime in which the pulse is compressed but filamentation does not yet set in.” The power of the lase pulse is 10,000 times higher than the filamentation threshold – and still it does not collapse.
The team used an Yttrium aluminium garnet (YAG) crystal with a width of only a few millimetres – and the results are remarkable: By sending the laser pulse through the crystal, its duration decreases from 94 femtoseconds to a mere 30 femtoseconds. Its energy stays almost the same, and the power (energy per time) increases by a factor three, to almost half a terawatt. “As the pulse is very short, its extremely high power opens the door to many exciting experiments and maybe even to new technologies in laser science”, says Audrius Pugzlys.
Learn more: With Great Power Comes Great Laser Science
Electrons reveal their quantum properties when they are confined to small spaces. Scientists from TU Wien (Vienna), Aachen and Manchester have created tiny quantum dots in Graphene
In a tiny quantum prison, electrons behave quite differently as compared to their counterparts in free space. They can only occupy discrete energy levels, much like the electrons in an atom – for this reason, such electron prisons are often called “artificial atoms”. Artificial atoms may also feature properties beyond those of conventional ones, with the potential for many applications for example in quantum computing. Such additional properties have now been shown for artificial atoms in the carbon material graphene. The results have been published in the journal “Nano Letters”, the project was a collaboration of scientists from TU Wien (Vienna, Austria), RWTH Aachen (Germany) and the University of Manchester (GB).
Building Artificial Atoms
“Artificial atoms open up new, exciting possibilities, because we can directly tune their properties”, says Professor Joachim Burgdörfer (TU Wien, Vienna). In semiconductor materials such as gallium arsenide, trapping electrons in tiny confinements has already been shown to be possible. These structures are often referred to as “quantum dots”. Just like in an atom, where the electrons can only circle the nucleus on certain orbits, electrons in these quantum dots are forced into discrete quantum states.
Even more interesting possibilities are opened up by using graphene, a material consisting of a single layer of carbon atoms, which has attracted a lot of attention in the last few years. “In most materials, electrons may occupy two different quantum states at a given energy. The high symmetry of the graphene lattice allows for four different quantum states. This opens up new pathways for quantum information processing and storage” explains Florian Libisch from TU Wien. However, creating well-controlled artificial atoms in graphene turned out to be extremely challenging.
Cutting edge is not enough
There are different ways of creating artificial atoms: The simplest one is putting electrons into tiny flakes, cut out of a thin layer of the material. While this works for graphene, the symmetry of the material is broken by the edges of the flake which can never be perfectly smooth. Consequently, the special four-fold multiplicity of states in graphene is reduced to the conventional two-fold one.
Therefore, different ways had to be found: It is not necessary to use small graphene flakes to capture electrons. Using clever combinations of electrical and magnetic fields is a much better option. With the tip of a scanning tunnelling microscope, an electric field can be applied locally. That way, a tiny region is created within the graphene surface, in which low energy electrons can be trapped. At the same time, the electrons are forced into tiny circular orbits by applying a magnetic field. “If we would only use an electric field, quantum effects allow the electrons to quickly leave the trap” explains Libisch.
The artificial atoms were measured at the RWTH Aachen by Nils Freitag and Peter Nemes-Incze in the group of Professor Markus Morgenstern. Simulations and theoretical models were developed at TU Wien (Vienna) by Larisa Chizhova, Florian Libisch and Joachim Burgdörfer. The exceptionally clean graphene sample came from the team around Andre Geim and Kostya Novoselov from Manchester (GB) – these two researchers were awarded the Nobel Prize in 2010 for creating graphene sheets for the first time.
The new artificial atoms now open up new possibilities for many quantum technological experiments: “Four localized electron states with the same energy allow for switching between different quantum states to store information”, says Joachim Burgdörfer. The electrons can preserve arbitrary superpositions for a long time, ideal properties for quantum computers. In addition, the new method has the big advantage of scalability: it should be possible to fit many such artificial atoms on a small chip in order to use them for quantum information applications.
Learn more: “Artificial Atom“ Created in Graphene
When rain falls on a lotus leaf, the leaf doesn’t get wet. Thanks to its special structure, the water drops roll off without wetting the surface. Artificial materials can be made water-repellent, too. It is, however, extremely challenging to produce a surface with switchable wetting. Now, a research team from TU Wien, KU Leuven and University of Zürich has managed to manipulate a surface of a single layer of boron nitride in such a way that it can be switched back and forth between states with high and low wetting and adhesion.
Hexagons making waves
One of the most interesting physical properties of a surface is its stiction or static friction” says Stijn Mertens (Institute of Applied Physics at the Vienna University of Technology, and associated with KU Leuven in Belgium). „This force has to be overcome for an object on the surface to start sliding.” The nanostructure of the surface determines its stiction to a large extent: the details of the contact between the surface and another object (for example, a drop of liquid) depend on the geometry of its atoms and other properties. This in turn is crucial for adhesion, stiction and wetting. The relationship between stiction and wetting, however, is so far only poorly understood.
“Just as the material graphene consists of only one layer of carbon atoms, our boron nitride — which contains as many boron as nitrogen atoms — has a thickness of only one atomic layer”, explains Thomas Greber from the Physics Institute at the University of Zürich. This ultrathin layer can be grown on a rhodium single crystal. The atoms on the rhodium surface and in the boron nitride form a hexagonal pattern, but the distances between the atoms in the two materials are different. Thirteen atoms in boron nitride take the same space as twelve rhodium atoms, so that the two crystals do not fit together perfectly. Because of this mismatch, the boron nitride hexagons must bend, they appear as a frozen wave with a wavelength of 3.2 nanometres and a height of about 0.1 nanometres.
Precisely this two-dimensional nanowave influences the wetting of the surface by water”, says Stijn Mertens. In any case, the boron nitride superstructure can be made flat with a simple trick: by putting the material in acid and applying an electrical voltage, hydrogen atoms creep under the boron nitride layer and change the bond between nitrogen and rhodium. This makes the boron nitride flat. Suddenly the adhesion of a water drop on the surface changes dramatically – even though the drop is 100’000 times bigger than the tiny waves in the boron nitride. If the voltage is decreased, this effect is reversed: „We can switch the surface again and again between these two states”, explains Stijn Mertens.
The Vienna University of Technology (German: Technische Universität Wien) is one of the major universities in Vienna, the capital of Austria.
Founded in 1815 as the “Imperial-Royal Polytechnic Institute” (k. k. Polytechnisches Institut), it currently has about 26,200 students (19% foreign students/30% women), eight faculties and about 4,000 staff members (1,800 academics). The university’s teaching and research is focused on engineering and natural sciences.
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