Quantum Electromagnetically Induced Transparency

This article reports the realization of a powerful quantum optical
effect -known as Electromagnetically Induced Transparency- with an ensemble
of electrons in a semiconductor. The study used optical transitions of
electrons bound at donor sites, and a resonant laser field is usually
absorbed in this medium. This absorption was suppressed by driving a
parallel optical transition at the same time with another laser. The
absorption disappears because destructive quantum interference in the
systems' dynamics prohibits excitation, which can occur when the electrons
are driven in a quantum superposition of spin-up and spin-down that is just
right for avoiding absorption. Realizing this effect in an ensemble with
many electrons removes the need for using optical cavities for controlling
strong interactions between spin states and optical fields. Work in the
field of atomic physics (with vapors of Rubidium or Cesium) showed that this
gives access to very robust quantum optical control, and realizations of
slow-light, storage of light, quantum communication, and preparing quantum
entanglement between spins that are separated by a large distance. By
implementing this physics in a semiconductor these applications can be
strongly miniaturized.


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LT12358

Measuring Qubits

In this paper, we have discovered a device that can be probed at both extremely low powers, where transitions between single quantum levels dominate, and high powers, where the system responds classically. Surprisingly, the macroscopic response of the system for large drive powers is dictated by the quantum state occupied by the system immediately before the measurement. Intrinsically quantum effects are thus easily observed in a classical signal. Moreover, the system is extremely simple: it requires only a single Josephson junction coupled to a resonator with no extraneous parts needed to control the transition to the classical regime, as in other circuits; essentially, it is a "qubit", or artificial atom, which acts as its own amplifier. We have also shown how this novel behavior easily lends itself to joint measurements of three or more qubits simultaneously. This measurement scheme is a significant improvement over prior readout schemes, taking readout fidelity from ~5% in this device to almost 90%. The phenomenon we report is unique in the simplicity and elegance that intrinsically quantum effects can express themselves macroscopically.

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LT12770

Quantum plasticity and supersolidity

We have discovered that a helium-4 crystal with no impurity at all is anomalously soft. Its plasticity is large, due to quantum effects. This is because it contains dislocations which can move macroscopic distances (a fraction of a millimeter) at high speed (several meters per second). Dislocations are lines running through the crystal, where the stacking of atoms is disordered compared to the rest of the crystal. In classical crystals all atoms are completely frozen at low temperature. But in quantum crystals such as helium-4, quantum fluctuations are large and atoms can jump by quantum tunneling from site to site, especially along dislocations where the packing is not as compact as elsewhere. Quantum tunneling is a well documented process in which particles go through energy barriers without any dissipation because of their wave-like character. Highly mobile dislocations are able to reduce the stiffness of helium-4 crystals by one order of magnitude.
However, very tiny traces of helium-3 impurities are sufficient to stop the motion of dislocations when they attach to them below temperatures of order 100 millikelvin. Apparently, this is what drives these crystals to a Òsupersolid stateÓ, a highly debated new state of matter which may be the consequence of mass flow along the core of dislocations when they stop moving.


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LF12863B

'Dirty' Device Offers New Perspective on Electron Transport


The transistors that have enabled the information age have been so successful in part because of the near-perfection with which they can be made. Some effects, however, require a little dirt. Taking an unconventional approach, researchers have now designed a semiconductor device to be extremely sensitive to material imperfections (and, for good measure, they deliberately added impurities to the sample). In the resulting device, electrons confined to a single layer, called a quantum well, became a hundred times more likely to escape from the layer if they are scattered by a defect. The researchers then showed that this unusual behavior could be used like a periscope to probe how the electrons were moving in the layer simply by measuring how quickly they escaped. This demonstration is interesting not only because it highlights a relatively unexplored type of electron transport but also because it offers a fundamentally new approach to investigating the many peculiar quantum effects, such as the quantum Hall effect, that occur when electrons are trapped in a single layer of a semiconductor.

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LQ12519

The unbearable weight of the vacuum

Modern physics has unveiled an incredibly rich structure for what
was thought in classical physics to be the most uninteresting of
the states: the vacuum. From the early Dirac sea of negative-energy
states to the picture of virtual particles constantly being created
and annihilated, the vacuum has acquired conceptual importance for a
consistent description of nature. Yet, from the observational
point of view, its existence continues to be almost as evasive as in
classical physics, demanding carefully designed experiments in order
to detect its subtle effects (e.g., the Casimir effect). Here, in
contrast, we use the recently unveiled “vacuum awakening” mechanism
[Lima and Vanzella, Phys. Rev. Lett. 104, 161102 (2010)] to show
that the gravitational field of some neutron stars may force the
quantum vacuum to become, in the lapse of a few milliseconds, as
dense in energy as the stars themselves (which are already the most
dense objects known to exist). This "extra weight" would destabilize
the star, possibly leading to events of astrophysical proportions
fueled by the vacuum. Reversing the argument, the mere observation
of stable neutron-star configurations may be used to rule out the
existence of certain kinds of fields in nature (those for which the
vacuum awakening effect would have been triggered). In a Universe
where 95% of the energy content is unknown, such a simple method to
discard fields provides a great deal of information.