What is it like to be a Schrödinger cat?

dn22336-1_300Hrvoje Nikolic
The possibility of quantum interference of a composite object with many internal degrees of freedom is studied from the point of view of the object itself.
The internal degrees play a role of an internal environment.
In particular, if the internal degrees have a capacity for an irreversible record of which-path information, then the internal-environment induced decoherence prevents external experimentalists from observing interference. Interference can be observed only if the interfering object is sufficiently isolated from the external environment, so that the object cannot record which-path information.
Extrapolation to a hypothetical interference experiment with a conscious object implies that being a Schrödinger cat would be like being an ordinary cat living in a box without any information about the world external to the box….
… Read more at http://arxiv.org/pdf/1406.3221v1.pdf

Cool for Cats

Cooling for a cat In this figure we show, by making use of Wigner functions, the effect of two different environmentson a ring prepared in a coherent state biased at zero  ux. Each graph contains a top down view with a three dimensional plot of the function as a not to fixed scale inset.

Cooling for a cat In this figure we show, by making use of Wigner functions, the effect of two different environments
on a ring prepared in a coherent state biased at zero flux. Each graph contains a top down view with a three dimensional plot of the function as a not to fixed scale inset.

M. J. Everitt, T. P. Spiller, G. J. Milburn, R. D. Wilson, A. M. Zagoskin
The iconic Schrödinger’s cat state describes a system that may be in a superposition of two macroscopically distinct states, for example two clearly separated oscillator coherent states. Quite apart from their role in understanding the quantum classical boundary, such states have been suggested as offering a quantum advantage for quantum metrology, quantum communication and quantum computation. As is well know these applications have to face the difficulty that the irreversible interaction with an environment causes the superposition to rapidly evolve to a mixture of the component states in the case that the environment is not monitored. Here we show that by engineering the interaction with the environment the system can evolve irreversibly to a cat state. To be precise we show that it is possible to engineer an irreversible process so that the steady state is close to a pure Schrödinger’s cat state by using a bistable system and an environment comprising two-photon (or phonon) absorbers. We also show that it should be possible to prolong the lifetime of a Schrödinger’s cat state exposed to the destructive effects of a conventional single-photon decohering environment. Our protocol should make it easier to prepare and maintain Schrödinger cat states which would be useful in applications of quantum metrology and information processing as well as being of interest to those probing the quantum to classical transition.
Read more: http://arxiv.org/pdf/1212.4795v1.pdf

Quantum measurements leave Schrödinger’s cat alive

Lisa Grossman
Schrödinger’s cat, the enduring icon of quantum mechanics, has been defied. By making constant but weak measurements of a quantum system, physicists have managed to probe a delicate quantum state without destroying it – the equivalent of taking a peek at Schrodinger’s metaphorical cat without killing it. The result should make it easier to handle systems such as quantum computers that exploit the exotic properties of the quantum world.

Quantum objects have the bizarre but useful property of being able to exist in multiple states at once, a phenomenon called superposition. Physicist Erwin Schrödinger illustrated the strange implications of superposition by imagining a cat in a box whose fate depends on a radioactive atom. Because the atom’s decay is governed by quantum mechanics – and so only takes a definite value when it is measured – the cat is, somehow, both dead and alive until the box is opened.

Superposition could, in theory, let quantum computers run calculations in parallel by holding information in quantum bits. Unlike ordinary bits, these qubits don’t take a value of 1 or 0, but instead exist as a mixture of the two, only settling on a definite value of 1 or 0 when measured.

But this ability to destroy superpositions simply by peeking at them makes systems that depend on this property fragile. That has been a stumbling block for would-be quantum computer scientists, who need quantum states to keep it together long enough to do calculations.

Gentle measurement

Researchers had suggested it should be possible, in principle, to make measurements that are “gentle” enough not to destroy the superposition. The idea was to measure something less direct than whether the bit is a 1 or a 0 – the equivalent of looking at Schrödinger’s cat through blurry glasses. This wouldn’t allow you to gain a “strong” piece of information – whether the cat was alive or dead – but you might be able to detect other properties.

Now, R. Vijay of the University of California, Berkeley, and colleagues have managed to create a working equivalent of those blurry glasses. “We only partially open the box,” says Vijay.

The team started with a tiny superconducting circuit commonly used as a qubit in quantum computers, and put it in a superposition by cycling its state between 0 and 1 so that it repeatedly hit all the possible mixtures of states.

Next, the team measured the frequency of this oscillation. This is inherently a weaker measurement than determining whether the bit took on the value of 1 or 0 at any point, so the thought was that it might be possible to do this without forcing the qubit to choose between a 1 or a 0. However, it also introduced a complication.

Quantum pacemaker

Even though the measurement was gentle enough not to destroy the quantum superposition, the measurement did randomly change the oscillation rate. This couldn’t be predicted, but the team was able to make the measurement very quickly, allowing the researchers to inject an equal but opposite change into the system that returned the qubit’s frequency to the value it would have had if it had not been measured at all.

This feedback is similar to what happens in a pacemaker: if the system drifts too far from the desired state, whether that’s a steady heartbeat or a superposition of ones and zeros, you can nudge it back towards where it should be.

Vijay’s team was not the first to come up with this idea of using feedback to probe a quantum system, but the limiting factor in the past had been that measurements weak enough to preserve the system gave signals too small to detect and correct, while bigger measurements introduced noise into the system that was too big to control.

Error correction

Vijay and colleagues used a new kind of amplifier that let them turn up the signal without contaminating it. They found that their qubit stayed in its oscillating state for the entire run of the experiment. That was only about a hundredth of a second – but, crucially, it meant that the qubit had survived the measuring process.

“This demonstration shows we are almost there, in terms of being able to implement quantum error controls,” Vijay says. Such controls could be used to prolong the superpositions of qubits in quantum computing, he says, by automatically nudging qubits that were about to collapse.

The result is not perfect, points out Howard Wiseman of Griffith University in Brisbane, Australia, in an article accompanying the team’s paper. “But compared with the no-feedback result of complete unpredictability within several microseconds, the observed stabilization of the qubit’s cycling is a big step forward in the feedback control of an individual qubit.”

Journal reference: Nature, DOI: 10.1038/nature11505 – http://arxiv.org/abs/1205.5591

Read more: www.newscientist.com

Physicists break record for extreme quantum state

Optical equipment used by Jian-Wei Pan and team to create an eight-photon Schrödinger's cat state

Physicists in China have broken their own record for the number of photons entangled in a “Schrödinger’s cat state”. They have managed to entangle eight photons in the state, beating the previous record of six, which they set in 2007. The Schrödinger’s cat state plays an important role in several quantum-computing and metrology protocols. However, it is very easily destroyed when photons interact with their surroundings, prompting the researchers to describe its creation in eight photons as “state of the art” in quantum control.
In Erwin Schrödinger’s famous thought experiment of 1935, all of the molecules in a cat are in a superposition of two extreme states – living and dead – and an observer cannot tell which until a measurement puts the cat into one of the two states. Today physicists use the term “Schrödinger’s cat state” (or Greenberger–Horne–Zeilinger state) to describe any multi-particle quantum system that is in a superposition of extreme states…. Continue reading Physicists break record for extreme quantum state

Topological Schrödinger cats

A Schrödinger kink in a quantum Ising chain. (a) A topological defect in a non-local superposition and (b) The analogue of a double-slit experiment. A double-well potential (left) is used to create a topological defect, such as a domain wall, superimposed in two locations (here, defect is represented by its probability distribution in space). To carry out the double slit experiment, the two potential wells are eliminated, allowing the defect to move. In isolation, the two wavepackets emerging from the "slits" interfere creating fringes of high and low probability for the location of the kink. However, when the system interacts with the environment, such a superposition will decohere at rate proportional to the distance L - the unzipped part of the chain shown in (a), which corresponds to the size of the Schrodinger kink - resulting in a classical distribution for the defect.

Topological defects (such as monopoles, vortex lines, or domain walls) mark locations where disparate choices of a broken symmetry vacuum elsewhere in the system lead to irreconcilable differences. They are energetically costly (the energy density in their core reaches that of the prior symmetric vacuum) but topologically stable (the whole manifold would have to be rearranged to get rid of the defect). We show how, in a paradigmatic model of a quantum phase transition, a topological defect can be put in a non-local superposition, so that – in a region large compared to the size of its core – the order parameter of the system is “undecided” by being in a quantum superposition of conflicting choices of the broken symmetry. We demonstrate how to exhibit such a “Schrödinger kink” by devising a version of a double-slit experiment suitable for topological defects. Coherence detectable in such experiments will be suppressed as a consequence of interaction with the environment. We analyze environment-induced decoherence and discuss its role in symmetry breaking…………….

Read more: http://arxiv.org/PS_cache/arxiv/pdf/1106/1106.2823v1.pdf