Tag Archives: superpositions

Capturing light in a bottle, a single-atom light switch, and superpositioning

A Nov. 5, 2013 Vienna University of Technology press release (also available on EurekAlert) describes research that may make quantum optical switches possible,

With just a single atom, light can be switched between two fibre optic cables at the Vienna University of Technology. Such a switch enables quantum phenomena to be used for information and communication technology.

The press release goes on to describe a ‘light in a bottle’ technique which leads, the researchers hope, that they may have discovered how to create a quantum light switch,

Professor Arno Rauschenbeutel and his team at the Vienna University of Technology capture light in so-called “bottle resonators”. At the surface of these bulgy glass objects, light runs in circles. If such a resonator is brought into the vicinity of a glass fibre which is carrying light, the two systems couple and light can cross over from the glass fibre into the bottle resonator.

“When the circumference of the resonator matches the wavelength of the light, we can make one hundred percent of the light from the glass fibre go into the bottle resonator – and from there it can move on into a second glass fibre”, explains Arno Rauschenbeutel.

A Rubidium Atom as a Light Switch
This system, consisting of the incoming fibre, the resonator and the outgoing fibre, is extremely sensitive: “When we take a single Rubidium atom and bring it into contact with the resonator, the behaviour of the system can change dramatically”, says Rauschenbeutel. If the light is in resonance with the atom, it is even possible to keep all the light in the original glass fibre, and none of it transfers to the bottle resonator and the outgoing glass fibre. The atom thus acts as a switch which redirects light one or the other fibre.

Both Settings at Once: The Quantum Switch
In the next step, the scientists plan to make use of the fact that the Rubidium atom can occupy different quantum states, only one of which interacts with the resonator. If the atom occupies the non-interacting quantum state, the light behaves as if the atom was not there. Thus, depending on the quantum state of the atom, light is sent into either of the two glass fibres. This opens up the possibility to exploit some of the most remarkable properties of quantum mechanics: “In quantum physics, objects can occupy different states at the same time”, says Arno Rauschenbeutel. The atom can be prepared in such a way that it occupies both switch states at once. As a consequence, the states “light” and “no light” are simultaneously present in  each of the two glass fibre cables. [emphasis mine]

For the classical light switch at home, this would be plain impossible, but for a “quantum light switch”, occupying both states at once is not a problem. “It will be exciting to test, whether such superpositions are also possible with stronger light pulses. Somewhere we are bound to encounter a crossover between quantum physics and classical physics”, says Rauschenbeutel.

This light switch is a very powerful new tool for quantum information and quantum communication. “We are planning to deterministically create quantum entanglement between light and matter”, says Arno Rauschenbeutel. “For that, we will no longer need any exotic machinery which is only found in laboratories. Instead, we can now do it with conventional glass fibre cables which are available everywhere.”

Darrick Chang offers a good introduction (i.e., it’s challenging but you don’t need a physics degree to read it) and some analysis of this work in his Nov. 4, 2013 article for Physics (6, 121 (2013) DOI: 10.1103/Physics.6.121) titled: Viewpoint: A Single-Atom Optical Switch.

Quantum scientists over the past two decades have dreamt of realizing powerful new information technologies that exploit the laws of quantum mechanics in their operation. While many approaches are being pursued, a prevailing choice consists of using single atoms and particles of light—single photons—as the fundamental building blocks of these technologies [1]. In this paradigm, one envisions that single atoms naturally act as quantum processors that produce and interface with single photons, while the photons naturally act as wires to carry information between processors. Reporting in Physical Review Letters, researchers at the Vienna University of Technology, Austria, have taken an important step forward in this pursuit, by experimentally demonstrating a microphotonic optical switch that is regulated by just a single atom [2].

This article is open access.

For those willing to tackle a more challenging paper, here’s a link to and a citation for the Vienna University of Technology researchers’ paper,

Fiber-Optical Switch Controlled by a Single Atom by Danny O’Shea, Christian Junge, Jürgen Volz, and Arno Rauschenbeute. Phys. Rev. Lett. 111, 193601 (2013) [5 pages]

This work is behind a paywall.

Minutes after publishing: here’s an image that illustrates superpositioning in a quantum switch,

The Quantum Light Switch: It can occupy both possible states at the same time. Courtesy Vienna University of Technology

The Quantum Light Switch: It can occupy both possible states at the same time. Courtesy Vienna University of Technology

Environmental decoherence tackled by University of British Columbia and California researchers

The research team at the University of British Columbia (UBC) proved a theory for the prediction and control of environmental decoherence in a complex system (an important step on the way to quantum computing) while researchers performed experiments at the University of California Santa Barbara (UCSB) to prove the theory.  Here’s an explanation of decoherence and its impact on quantum computing from the July 20, 2011 UBC news release,

Quantum mechanics states that matter can be in more than one physical state at the same time – like a coin simultaneously showing heads and tails. In small objects like electrons, physicists have had success in observing and controlling these simultaneous states, called “state superpositions.”

Larger, more complex physical systems appear to be in one consistent physical state because they interact and “entangle” with other objects in their environment. This entanglement makes these complex objects “decay” into a single state – a process called decoherence.

Quantum computing’s potential to be exponentially faster and more powerful than any conventional computer technology depends on switches that are capable of state superposition – that is, being in the “on” and “off” positions at the same time. Until now, all efforts to achieve such superposition with many molecules at once were blocked by decoherence.

The UBC research team, headed by Phil Stamp, developed a theory for predicting and controlling environmental decoherence in the Iron-8 molecule, which is considered a large complex system.

Iron-8 molecule (image provided by UBC)

This next image represents one of two states of decoherence, i. e., the molecule ‘occupies’ only one of two superpositions, spin up or spin down,

 

Decoherence: occupying either the spin up or spin down position (image provided by UBC)

Here’s how the team at the UCSB proved the theory experimentally,

In their study, Takahashi [Professor Susumu Takahashi is now at the University of Southern California {USC}] and his colleagues investigated single crystals of molecular magnets. Because of their purity, molecular magnets eliminate the extrinsic decoherence, allowing researchers to calculate intrinsic decoherence precisely.

“For the first time, we’ve been able to predict and control all the environmental decoherence mechanisms in a very complex system – in this case a large magnetic molecule,” said Phil Stamp, University of British Columbia professor of physics and astronomy and director of the Pacific Institute of Theoretical Physics.

Using crystalline molecular magnets allowed researchers to build qubits out of an immense quantity of quantum particles rather than a single quantum object – the way most proto-quantum computers are built at the moment.

I did try to find definitions for extrinsic and intrinsic decoherence unfortunately the best I could find is the one provided by USC (from the news item on Nanowerk),

Decoherence in qubit systems falls into two general categories. One is an intrinsic decoherence caused by constituents in the qubit system, and the other is an extrinsic decoherence caused by imperfections of the system - impurities and defects, for example.

I have a conceptual framework of sorts for a ‘qubit system’, I just don’t understand what they mean by ‘system’. I performed an internet search and virtually all of the references I found to intrinsic and extrinsic decoherence cite this news release or, in a few cases, papers written by physicists for other physicists. If anyone could help clarify this question for me, I would much appreciate it.

Leaving extrinsic and intrinsic systems aside, the July 20, 2011 news item on Science Daily provides a little more detail about the experiment,

In the experiment, the California researchers prepared a crystalline array of Iron-8 molecules in a quantum superposition, where the net magnetization of each molecule was simultaneously oriented up and down. The decay of this superposition by decoherence was then observed in time — and the decay was spectacularly slow, behaving exactly as the UBC researchers predicted.

“Magnetic molecules now suddenly appear to have serious potential as candidates for quantum computing hardware,” said Susumu Takahashi, assistant professor of chemistry and physics at the University of Southern California.

Congratulations to all of the researchers involved.

ETA July 22, 2011: I changed the title to correct the grammar.