Physicists control quantum particles by looking at them

The state of the nuclear spin, visualised by the arrow, after
measurements with varying strengths (light blue is very weak,
dark blue is very strong). For increasing measurement
strength, the state rotates towards the classical up
 state (arrow pointing up). This data is post-selected on one
specific measurement outcome of the electron spin.
For the other outcome, the arrow rotates downwards.

Scientists from the FOM Foundation and TU Delft have manipulated a quantum particle, merely by looking at it in a smart way. By adjusting the strength of their measurement according to earlier measurement outcomes, they managed to steer the particle towards a desired state. The scientists published their results online on 16 February 2014 in Nature Physics.

In two states at once

Quantum mechanics describes the behaviour of microscopic particles, such as atoms and electrons. When we compare it to our observations in everyday life, nature behaves very strangely at the scale of these particles. For instance, an electron can be in two states at the same time.

Schrödinger’s cat in a box, being
both dead and alive at the
same time. When the box is
opened completely, the state
of the cat will be either dead or
alive. By slightly lifting the lid,
it is possible to acquire only a little
bit of information, while maintaining
the fragile quantum state. In this
experiment, the nucleus plays the
role of the cat.

To demonstrate how peculiar this property is, the physicist Erwin Schrödinger proposed a famous thought experiment where the state of a quantum particle is linked to the fate of a cat.

The two are situated in a sealed box. The quantum particle acts as a switch that can either open (switch on) or close (switch off) a small flask of poison. As long as the quantum particle can be simultaneously in two states (on and off), the flask with poison is open and closed, and the cat is both dead and alive at the same time.  

But the weirdness doesn’t end there: as soon as the box is opened to observe the state of the cat, this situation changes. The act of measurement forces the animal to be either dead or alive. This is called the quantum mechanical measurement back-action: the state (of the particle as well as the imaginary cat) is inevitably perturbed by the measurement and collapses to a classical state. In this work, the scientists investigated what happens when the box is only slightly opened. Is it possible to peek at the cat, without destroying the fragile quantum state?

Peeking at Schrödinger’s cat

Instead of a cat, the scientists in the group of FOM workgroup leader prof.dr.ir. Ronald Hanson used a nucleus in diamond. These particles carry an intrinsic property called spin that behaves like a small magnet. The spin of the nucleus can point up (cat alive) or down (cat dead). 

In earlier work the group showed that it is possible to measure the orientation of a single spin, in analogy to fully opening Schrödinger’s box. To partially open the box, the scientists used a trick. Instead of directly measuring the nucleus, they first coupled the state of the nucleus to a nearby electron. They then determined the state of the electron. 

By varying the strength of the coupling between the nucleus and the electron, the scientists could carefully tune the measurement strength. A weaker measurement reveals less information, but also has less back-action. An analysis of the nuclear spin after such a weak measurement showed that the nuclear spin remained in a (slightly altered) superposition of two states. In this way, the scientists verified that the change of the state (induced by the back-action) precisely matched the amount of information that was gained by the measurement. 

Steering by peeking

The scientists realised that it is possible to steer the nuclear spin by applying sequential measurements with varying measurement strength. Since the outcome of a measurement is not known in advance, the researchers implemented a feedback loop in the experiment. They chose the strength of the second measurement depending on the outcome of the first measurement. In this way the scientists could steer the nucleus towards a desired superposition state by only looking at it. 

This result provides new insight in the role of measurements in quantum mechanics. Furthermore the combination of measurements and feedback, as demonstrated here, form an essential building block for the future quantum computer. Finally, these techniques can increase the sensitivity of magnetic field sensors. 

More information
Reference: Manipulating a qubit through the backaction of sequential partial measurements and real-time feedback, M.S. Blok, C. Bonato, M.L. Markham, D.J. Twitchen, V.V. Dobrovitski, R. Hanson, Nature Physics. DOI: 10.1038/nphys2881.

Source: http://home.tudelft.nl/en/current/latest-news/article/detail/sturen-door-gluren-fysici-bedwingen-quantumdeeltjes-door-ze-te-bekijken/

Researchers Find Unambiguous Evidence for Coherent Phonons in Superlattices

Surface topography of a 200 nanometer thick
strontium titanate/ calcium titanate superlattice
film on a strontium titanate substrate.

We all learn in high school science about the dual nature of light – that it exists as both waves and quantum particles called photons. It is this duality of light that enables the coherent transport of photons in lasers. 

Sound at the atomic-scale has the same dual nature, existing as both waves and quasi-particles known as phonons. Does this duality allow for phonon-based lasers? Some theorists say yes, but the point has been argued for years. Recently a large collaboration, in which Berkeley Lab scientists played a prominent role, provided the first “unambiguous demonstration” of the coherent transport of phonons.

Ramamoorthy Ramesh, a senior scientist with Berkeley Lab’s Materials Sciences Division, was a co-leader with Arun Majumdar, a former Associated Laboratory director at Berkeley Lab and currently VP for Energy at Google, of an experiment in which phonons underwent particle-to-wave crossovers in superlattices of perovskite oxides.

“Our observations open up new opportunities for studying the wave-like nature of phonons, particularly phonon interference effects,” says Ramesh. “Such research should have potential applications in thermoelectrics and thermal management, and in the long run could help the development of phonon lasers.”

Unlike elementary particles such as electrons and photons, whose wave nature and coherent properties are well-established, experimental demonstration of coherent wave-like properties of phonons has been limited. This is because phonons are not true particles, but the collective vibrations of atoms in a crystal lattice that can be quantized as if they were particles. However, understanding the coherent wave nature of phonons is of fundamental importance to thermoelectrics, materials that can convert heat into electricity, or electricity into heat, which represent a potentially huge source of clean, green energy.

“Lower thermal conductivity is one of the keys to improving the efficiency of thermoelectric materials and the key to thermal conductivity in semiconductors is phonon transport,” Majumdar says. “Nanostructures such as superlattices are the ideal model systems for the study of phonon transport, particularly the wave-particle crossover, because the wavelength of the most relevant phonons are in the range of one to 10 nanometers.”

Electron microscopy-spectroscopy images of a strontium titanate/barium titanate superlattice film reveal the presence of atomically sharp interfaces with minimal intermixing. Superlattice is color-coded with strontium (orange) barium (purple) and titanium (green).

Superlattices are artificial periodic structures consisting of  two dissimilar semiconductors in alternating layers a few nanometers thick. For this demonstration, the collaboration synthesized high-quality superlattices of electrically insulating perovskite oxides on various single-crystal oxide substrates. Interface densities in these superlattices were systematically varied using two different epitaxial growth techniques. Thermal conductivity was measured as a function of interface density.

“Our results were in general agreement with theoretical predictions of crossover from incoherent particle-like to coherent wave-like phonon transport,” Ramesh says. “We also found sufficient evidence to eliminate extraneous or spurious effects, which could have alternatively explained the observed thermal conductivity minimum in these superlattices.”

Capitalizing on the wave behavior of phonons should enable new advances in new heat transfer applications, the collaborators say. Furthermore, perovskite superlattice-based heterostructures could also serve as basic building blocks for the development of lasers in which beams of coherent phonons rather than coherent photons are emitted. Phonon lasers could provide advanced ultrasound imaging or highly accurate measuring devices, among other possibilities.

Ramesh is a corresponding author of a Nature Materials paper describing this research titled “Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices.”

This research was primarily supported by U.S. Department of Energy’s Office of Science.

Source: http://newscenter.lbl.gov/science-shorts/2014/02/05/coherent-phonons-in-superlattices/