Mechanical Quanta See the Light


Highlights

Interconnecting different quantum systems is important for future quantum computing architectures, but has proven difficult to achieve. Researchers from the Delft University of Technology (TU Delft), FOM and the University of Vienna have now realized a first step towards a universal quantum link based on quantum-mechanical vibrations of a nanomechanical device. The researchers’ findings were published today in Nature.

A stylization of the researcher’s nanomechanical device. By way of vibrating back-and-forth, the hole-filled silicon beam converts quantum particles of light into quantum vibrations, and later back into light. (Image courtesy: Jonas Schmöle, University of Vienna)

Quantum physics is increasingly becoming the scientific basis for a large number of new technologies. These new quantum technologies promise to fundamentally change the way we communicate, as well as radically enhance the performance of sensors and of our most powerful computers. One of the open challenges for practical applications is how to make different quantum technologies talk to each other. Presently, in most cases, different quantum devices are incompatible with one another, preventing these emerging technologies from linking, or connecting, to one another. One solution proposed by scientists is to build nanometer-sized mechanical objects that vibrate back-and-forth, just like a tiny vibrating tuning fork. These ‘nanomechanical devices’ could be engineered such that their vibrations are the mediator between otherwise different quantum systems. For example, mechanical devices that convert their mechanical vibrations to light could connect themselves (and other devices) to the world’s optical fibre networks, which form the Internet. An outstanding challenge in quantum physics has been building a nanomechanical device that convert quantum-mechanical vibrations to quantum-level light, thus allowing one to connect quantum devices to a future quantum Internet.

Nanomechanical device leads to fundamental physics
Researchers led by Prof. Simon Gröblacher at TU Delft and Prof. Markus Aspelmeyer at the University of Vienna have now realized just such a nanomechanical device. It converts individual particles of light, known as photons, into quantum-mechanical vibrations, known as phonons, and then back again, as reported in the journal Nature. Traditionally, the probability to first convert a photon into a phonon has been far too small to be useful. But this joint-team applied a trick: Whenever their nanomechanical device first converted a photon to a phonon, their device created a ‘signalling’ photon. By first looking for this signalling photon, the researchers knew exactly when their nanomechanical device had succeeded in the conversion – it had converted light into quantum-mechanical vibrations of their device. Afterwards, using lasers, the researchers then had their device convert its phonon back into light, and emit a photon. Finally, by carefully counting the signalling photons and the emitted photons, the researchers demonstrated that the entire conversion process happened at the quantum level – a single particle at a time. “Not only is this exactly what is necessary to convert and store quantum bits; what I also find amazing,” explains Ralf Riedinger, lead author on the study, “is the implications for fundamental physics. We normally think of mechanical vibrations in terms of waves, like waves travelling across a lake, as water vibrates up and down. But our measurements are clear evidence that mechanical vibrations also behave like particles. They are genuine quantum particles of motion. It’s wave-particle duality, but with a nano-sized tuning fork.”

Produced at TU Delft
The nanomechanical device itself is a tiny silicon beam, only half a micrometer wide, and contains a regular pattern of holes, which traps light and mechanical vibrations in the same spot. This nano-sized beam vibrates back-and-forth billions of times each second. It was fabricated at TU Delft by Gröblacher’s team on a silicon chip and uses infrared wavelengths of light, exactly as industry-standard fibre optic networks, integrated electronic, and emerging photonic circuits.

Great potential for the future
“We clearly also see the long-term technological potential”, says Gröblacher. “Such quantum mechanical vibrations could eventually be used as a ‘memory’ to temporarily store quantum information inside quantum networks or computers.” One grand future vision is to establish a quantum Internet in which quantum bits, instead of classical bits, are distributed and processed all around the world. Just like in today’s Internet, light will be used for global exchange of quantum information. How it can be converted to a large variety of different quantum devices that will be available for storage and computation remains a major open question. “Our research indicates that nanomechanical devices are a promising candidate to form this link”, reflects Gröblacher.


R. Riedinger*, S. Hong*, R. A. Norte, J. A. Slater, J. Shang, A. G. Krause, V. Anant, M. Aspelmeyer, and S. Gröblacher
Non-classical correlations between single photons and phonons from a mechanical oscillator
Nature advance online publication, 18.01.2016 (DOI 10.1038/nature16536)

 

Two-impurity Kondo paper in Nature Comms


Highlights

The Kondo effect – an intricate quantum phenomenon involving the spins of many electrons surrounding a magnetic atom – is already quite intriguing by itself. But an even higher level of complexity is reached when two coupled atoms are together Kondo-screened. Depending on the competition between the exchange interaction and the screening strength, combined with an external magnetic field, a variety of different correlated phases can be realised. So far, some of these phases existed only on paper.

Today, Anna Spinelli and coworkers from the Otte Lab show in Nature Communications that the complete phase diagram of the two-impurity Kondo problem in a magnetic field can be covered experimentally. In order to achieve this, a range of different pairs of Co atoms was designed and assembled through atom manipulation. This work forms the basis for advanced experiments on e.g. Kondo chains and Kondo lattices.


Phase diagram of the two-impurity Kondo problem. Horizontal axis: exchange interaction ranging from
ferromagnetic (left) to antiferromagnetic (right). Vertical axis: applied magnetic field. The two-impurity Kondo screening phase was not reached experimentally before.

 

Nature Communications: 3D Optomechanics


Highlights

In optomechanics, light is used to control and detect mechanical motion. In order to achieve quantum superposition states of motion, researchers are in search of a platform which promises strong coupling between light and motion at single-photon level.

Recently, Yuan et al. from the SteeleLab have introduced a new optomechanical platform based on 3D superconducting cavities in Nature Communications. For the first time, researchers couple the mechanical motion of a silicon nitride membrane to electromagnetic field inside the 3D microwave cavity. Exploiting the large quality factors of both the cavity and the membrane, they are able to use microwave photons to cool the motion of the millimetre scale membrane resonator, visible to the naked eye, to a record low mode temperature of 34 microkelvin.

The result demonstrates the potentials of 3D optomechanics. With optimization, this platform offers the possibility to reach the regime of single-photon strong coupling, opening up a new generation of experiments.

For more information, see the article on the Nature Communications website, as well as the press release from TU Delft.


3D optomechanical cavity   Millimeter sized membrane

Left: A silicon nitride membrane embedded in an aluminium 3D cavity. The membrane is held on a 5mm X 5mm silicon frame. Right: An optical microscope image looking from the top showing an aluminium coated silicon nitride membrane and the underlying aluminium antenna pads. The size of the membrane is 1mm X 1mm.

Controlled closure of cotunneling paths


Highlights

When electrons tunnel through an atom, they may lose energy in the process. Such inelastic cotunneling events render the atom in an excited state, either with a flipped spin or with an entirely different orbital filling. In a recent paper in Nano Letters of the Otte Lab, we report the observation of both types of cotunneling events. In addition, we demonstrate how both inelastic cotunneling processes may be switched off entirely through a controlled modification of the immediate environment of the atom. This work provides starting points for research on engineering the slowdown of decoherence in atomic spin systems.

Tunneling spectroscopy measurements performed on individual cobalt atoms assembled into a chain. Inelastic cotunneling events are observed as steps in the spectroscopy. For atoms in the inside of the chain, complete suppression of these steps is observed.

Counting of phonons


Highlights

Simon Gröblacher co-authors a Hanbury Brown and Twiss type experiment with phonons performed at Caltech, which was recently published in the renowned journal Nature. Pioneering photon counting experiments, such as the intensity interferometry performed by Hanbury Brown and Twiss to measure the angular width of visible stars, have played a critical role in our understanding of the full quantum nature of light. Over the last years the field of optomechanics, where a mechanical oscillator is coupled to light via the radiation-pressure force, has emerged as a leading candidate to observe complex quantum phenomena in truly macroscopic, massive systems.

Here we use an optical probe and single-photon detection to study the acoustic emission and absorption processes in a silicon nano-mechanical resonator, and perform a measurement similar to that used by Hanbury Brown and Twiss to measure correlations in the emitted phonons as the resonator undergoes a parametric instability formally equivalent to that of a laser. With straightforward improvements to this method, a variety of quantum state engineering tasks using mechanical resonators can be enabled, including the generation and heralding of single-phonon Fock states and the quantum entanglement of remote mechanical elements.

Figure: Finite element simulation of the nanobeam structure used to measure the phonon statistics of a mechanical oscillator using a Hanbury Brown and Twiss type experiment.

Nature Communications: Nanotube Decoherence


Highlights

Coherence is a widely used concept in quantum mechanics. When a quantum system interacts with its environment, the loss of information about the phase of quantum states is defined as decoherence. Decoherence plays a crucial role in quantum information processing: in contrast to classical bits that suffer only from relaxation in the form of bit flips (loss of energy), quantum bits are much more fragile as their state can also be lost by decoherence of the phase information stored in quantum superpositions. As mechanical objects are now reaching the quantum regime, it is interesting to think about how the concept of decoherence can be applied to the motion of a mechanical resonator.

In their recent work in Nature Communications, Schneider et al. have explored the concept of decoherence applied to a mechanical resonator. By studying very sensitive nanotube mechanical resonators at low temperatures using a new, high-speed detection technique, they were able to observe and identify a process of decoherence in the motion of a carbon nanotube.

Figure: Electron microscope image of a suspended carbon nanotube (white curling string) suspended between two metal electrodes (blue). The carbon nanotube is only 1 nm (about 10 atoms) in diameter. Near the trench, the nanotube sticks to the metal and is pulled into a tight string that can vibrate like a guitar string. The blue electrodes are also used for injecting and detecting current flowing through the nanotube, which is used to sense the nanotube motion. Below the nanotube is a local gate (yellow) that is used to shake the nanotube string, making it vibrate..

What does “decoherence” mean for a mechanical resonator? Similar to the loss of information of the phase of a quantum superposition of a qubit, decoherence in a mechanical resonator corresponds to the loss of information about the phase of the oscillations in the position of the mechanical object. In the animation shown below, we show side-by-side the processes of oscillation, relaxation, and decoherence for a quantum bit and a mechanical resonator. In both cases, the loss of phase information comes in through random fluctuations of the frequency of the motion, where “motion” for the qubit corresponds to oscillations of the Bloch vector, while for the mechanical resonator, “motion” is simply the oscillations of it’s position.

Movie sequence: A sequence of videos showing (i) how to relate the evolution of a quantum superposition of a qubit to the motion of a mechanical resonator (ii) how to visualize the decay of a qubit and the damping of motion, (iii) the effect of dephasing on the coherence of a qubit and on the motion of a vibrating string, and (iv) the combined effects of dissipation and dephasing on qubits and mechanical motion. See the description box on the youtube pages for a detailed explanation of the videos.

To visualize this, it is useful to think about the motion of a guitar string. When you pluck a guitar string, it will vibrate up and down at its resonance frequency, and after some time, this motion will slowly decay as they lose energy. The vibrations in the string lose energy due to damping from the air and transmitting sounds into the guitar body and the room. This process of losing energy is called dissipation.

Another related effect, which is often overlooked in the response of mechanical resonators, is dephasing. Dephasing, which leads to decoherence, and in this case, rather than losing energy, instead the frequency of the oscillation is changing randomly in time: imagine, for example, that someone was turning the tuning screw on the guitar string while it was vibrating without you knowing about it. These random fluctuations in the frequency will make you lose track of the phase of the oscillations of the string since sometimes it will vibrate a bit faster, and sometimes a bit slower. You can “hear” the type of sound that such a “bad” qubit would make in this youtube video here:

In the experiment, by measuring the mechanical response of the nanotube in two different ways, one “spectral” technique by slowly sweeping the frequency, and one “ring down” technique by plucking the nanotube and looking at how low it takes for the sound to decay, Schneider et al observed that the nanotube in their experiment was not only subject to dissipation (energy loss), as assumed in all previous nanotube experiments, but it is also influenced by dephasing that caused decoherence of its motion.

Although the experiments were performed in the classical regime, the concepts identified in the classical motion would also apply directly to quantum superpositions of mechanical motion, a goal of the current research in quantum mechanical resonators.

For more information, see the article here in Nature Communications and also the press release on the TU Delft website.

Nature Communications: Electrons rolling “uphill”


Highlights

In physics, we are used to thinking about the motion of electrons driven by electric fields: electrons are attracted to regions of lower electrostatic potential, like a ball rolling down a hill. When we put a voltage across a resistor, electrons feel a force from the electric field and flow like a river to the place of lowest potential. The effect of electrons flowing “downhill” is also the basis of solar cells, in which they flow in response to the built-in electrostatic voltage in a semiconductor PN junction and generate a current when it is exposed to light.

In their recent work reported in Nature Communications, Gilles Buchs and Salvatore Bagiante, working with Gary Steele in the SteeleLab, have demonstrated that this is not always the case, and in particular, electrons can sometimes be induced to flow “uphill”.

Left: Schematic of the photocurrent imaging experiment with a double-gated suspended carbon nantoube. Right: Observed photocurrent for different doping configurations induced by the gates. In the upper right “pp” region, the sign of the photocurrent indicates that electrons are flowing “uphill” in response to the light

Their work is based on a double-gated semiconducting carbon nanotube, in which the doping level in different parts of the device can be controlled using local gates. Measuring the photo-induced current across the device as the doping levels were changed, Buchs et al. showed that at certain gate voltages, the sign of the current indicated that electrons were flowing “uphill”, against an electric field. This results from the photothemoelectric effect, in which the flow of electrons is driven by differences in chemical potential rather than electrostatic voltage.

The new work solves contradicting reports in the nanotube community about photocurrent generation mechanisms in semiconducting carbon nanotubes, and lays an important framework for interpreting signals in scannning photocurrent microscopy, a technique widely applied to other materials such as graphene, 2D semiconductors, and semiconducting nanowires.

For more information, see the article in Nature Communications, also covered in an article on nanowerk.comThe authors would also like to thank Val Zwiller for experimental support during the project.

 

Nature Nanotechnology: Graphene Optomechanics


Highlights

Graphene is famous for the relativistic way that electrons in the material move, but more recently, researchers have started studying how the graphene sheet itself moves when you make it into a mechanical resonator like a drumhead.

In our recent work in Nature Nanotechnology, we have used a superconducting microwave cavity coupled to a graphene membrane to study the drum’s motion at milliKelvin temperatures. By peeling off a graphene flake on top of a superconducting metal used to trap microwave photons, Singh et al from the SteeleLab have used microwave “light” to detect the position of the membrane. By bouncing this light off of the graphene sheet, which acts as a moving “mirror”, the researchers were also able to “beat the drum”, shaking it using the radiation pressure from the momentum of light.

For more information, including an animation of the microwave photons bouncing off the drum, see our press release on the TU-Delft website.

Left: Colorized electron microscope image of the graphene drum used in the experiment. The blue shows the graphene sheet, the yellow shows the superconducting metal that forms the cavity. Right: An artist impression of microwave photons (yellow comets and glow) propagating in the gaps of the superconducting waveguide and interacting with the graphene drum (transparent sheet).

Nature Materials: Spin waves observed


Highlights

Spin waves are the elementary excitations of any magnetic material and play an essential role in all magnetic dynamics, for example in the flipping of a bit on a hard disk. In Nature Materials, Spinelli et al. of the Otte Lab report the observation of individual spin waves in a self-built magnetic bit of only 6 iron atoms. Not only can they discern standing waves of various wavelengths for different energies in our bit; they can also study the switching behaviour of the bit and identify the role of the spin waves in the switching process.

More information can be found in the News & Views article, written by Rohart and Rodary, that accompanies the paper.

Impression of different spin wave modes in a six atom chain, driven at low (top) and higher energy (bottom).

 

Cantilever NEMS switches in Nature Communications


Highlights

The cantilever is a prototype of a highly compliant mechanical system and has an instrumental role in nanotechnology, enabling surface microscopy, and ultrasensitive force and mass measurements. Here we report fluctuation-induced transitions between two stable states of a strongly driven microcantilever. Geometric nonlinearity gives rise to an amplitude-dependent resonance frequency and bifurcation occurs beyond a critical point. The cantilever response to a weak parametric modulation is amplified by white noise, resulting in an optimum signal-to-noise ratio at finite noise intensity. This stochastic switching suggests new detection schemes for cantilever-based instrumentation, where the detection of weak signals is mediated by the fluctuating environment. For ultrafloppy, cantilevers with nanometer-scale dimensions operating at room temperature—a new transduction paradigm emerges that is based on probability distributions and mimics nature.

More information

Stochastic switching of cantilever motion, W.J. Venstra, H.J.R. Westra, H.S.J. van der Zant, Nature Communications  4 | Article number:  2624 | doi:10.1038/ncomms3624 | Published 31 October 2013.

This work is funded by the European Union’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement no. 318287, project LANDAUER.