Quantum Rainbow Photon Gun Unveiled

  Published by MIT

A photon gun capable of reliably producing single photons of different colours could become an important building block of a quantum internet

We've heard much about the possibility of a quantum internet which uses single photons to encode and send information protected by the emerging technology of quantum cryptography.

The main advantage is of such a system is perfect security, the kind of thing that governments, the military, banks and assorted other groups would pay handsomely to achieve.

One of the enabling technologies for a quantum internet is a reliable photon gun that can fire single photons on demand. That's not easy.

One of the significant weaknesses of current quantum cryptographic systems is the finite possibility that today's lasers emit photons in bunches rather than one at a time. When this happens, an eavesdropper can use these extra photons to extract information about the data being transmitted.

So there's no shortage of interest in developing photon guns that emit single photons and indeed various groups have made significant progress towards this.

Against this background, Michael Fortsch at the Max Planck Institute for the Science of Light in Erlangen, Germany, and a few pals today say they've made a significant breakthrough. These guys reckon they've built a photon emitter with a range of properties that make it far more flexible, efficient and useful than any before--a kind of photon supergun.

The gun is a disc-shaped crystal of lithium niobate zapped with 582nm light from a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser. Lithium niobate is a nonlinear material that causes single photons to spontaneously convert into photon pairs.

So the 582nm photons ricochet around inside the disc and eventually emerge either as unchanged 582nm photons or as a pair of entangled photons with about twice the wavelength (about 1060nm). This entangled pair don't have quite the same wavelength and so all three types of photon can be easily separated.

The 582 nm photons are ignored. Of the other pair, one is used to transmit information and the other is picked up by a detector to confirm that the other photon is ready form transmission.

So what's so special about this photon gun? First and most important is that the gun emits photons in pairs. That's significant because the detection of one photon is an unambiguous sign that another has also been emitted. It's like a time stamp that says a photon is on its way.

This so-called photon herald means that there can be no confusion over whether the gun is secretly leaking information to a potential eavesdropper.

This gun is also fast, emitting some 10 million pairs of photons per second per mW and also two orders of magnitude more efficient than other photon guns.

These guys can also change the wavelength of the photons the gun emits by heating or cooling the crystal and thereby changing its size. This rainbow of colours stretches over 100nm (OK, not quite a rainbow but you get the picture).

That's important because it means the gun can be tuned to various different atomic transitions allowing physicists and engineers to play with a variety of different atoms for quantum information storage.

All in all, an impressive feat and clearly an enabling step along the way to more powerful quantum information processing tools.

Ref: arxiv.org/abs/1204.3056: A Versatile Source of Single Photons for Quantum Information Processing

Newscribe : get free news in real time

Related posts:

Rightways: Quantum strategy offers game-winning advantages ...

Rightways: IBM Scalable Quantum Computing

Rightways: Can The Human Brain See Quantum Images?

Rightways: Quantum Computing Thrives on Chaos

IBM takes giant step to faster, quantum computers

IBM researchers presenting the results at this week’s American Physical Society meeting in Boston said that quantum computing “has the potential to deliver computational power that is unrivaled by any supercomputer today.” – Reuters File Photo

WASHINGTON: IBM researchers have taken a leap in computing by using quantum mechanics to harness the power of atoms and molecules, a move likely to lead to vast increases in speed and security of computers and other devices.

IBM researchers presenting the results at this week’s American Physical Society meeting in Boston said that quantum computing “has the potential to deliver computational power that is unrivaled by any supercomputer today.” The new type of computing uses information encoded into quantum bits or qubits, putting into use a theory that scientists have been discussing for decades.

“The special properties of qubits will allow quantum computers to work on millions of computations at once, while desktop PCs can typically handle minimal simultaneous computations,” an IBM statement said.

“For example, a single 250-qubit state contains more bits of information than there are atoms in the universe.” “The quantum computing work we are doing shows it is no longer just a brute force physics experiment,” said IBM scientist Matthias Steffen, manager of the research team that is working on applications for quantum computing systems.

“It’s time to start creating systems based on this science that will take computing to a new frontier.” Quantum computing expands on the most basic piece of information that a typical computer understands — a bit. While a normal bit can have only one of two values: “1” or “0,” qubits can hold a value of “1” or “0” as well as both values at the same time.

“Described as superposition, this is what allows quantum computers to perform millions of calculations at once,” IBM says.

A problem for scientists is that qubits have a short life of several billionths of a second, but IBM has succeeded in developing “three dimensional” superconducting qubits which retain their quantum states up to 100 microseconds – an improvement of two to four times prior records.

“Based on this progress, optimism about superconducting qubits and the possibilities for a future quantum computer are rapidly growing,” says IBM.

To harness the power of quantum computing, scientists have had to work to minimize errors in calculations caused by interference from factors such as heat, electromagnetic radiation, and materials defects.

The use of quantum computing “will have widespread implications foremost for the field of data encryption where quantum computers could factor very large numbers like those used to decode and encode sensitive information,” IBM said.

“Other potential applications for quantum computing may include searching databases of unstructured information, performing a range of optimization tasks and solving previously unsolvable mathematical problems.”

Related posts:

Quantum strategy offers game-winning advantages, even without entanglement
IBM Scalable Quantum Computing
Can The Human Brain See Quantum Images?
A quantum connection between light and motion
Quantum Computing Thrives on Chaos
The Quantum Physics Behind The Death Of Osama Bin ...  

IBM Scalable Quantum Computing

IBM Paves The Way Towards Scalable Quantum Computing

Alex Knapp, Forbes Staff

Three superconducting qubits. (Credit: IBM Research)

IBM has announced today that it’s achieved a breakthrough in its work to develop scalable quantum computing by developing a superconducting qubit made from microfabricated silicon that maintains coherence long enough for practical computation.

And now that I’ve thrown a ton of information at you in one tiny sentence, let’s break it all down. I had a chance to talk with IBM scientist Matthias Steffen about this new technology, and he broke it down for me. Let’s start with the qubit. Classical computing, as you probably know, is based on the bit. A bit can exist in one of two possible states, which are typically referred to as “0″ or “1″. A qubit is the equivalent of a bit for quantum computing. It can be in three possible states – “0″ or “1″ or both. The “both” state is known as the superposition. Now, the difference may seem subtle, but mathematically, it’s huge. A few hundred qubits can contain more classical bits of information than the the universe has atoms.

IBM Shrinks Computer Memory Into Only Twelve Atoms
  Alex Knapp Forbes Staff
 
What makes quantum computing challenging is the problem of decoherence. When a qubit is moved from the 0 state to either 1 or the superposition, it will decohere to state 0 due to interference from other parts of the computer. In order for quantum computing to be scalable and practical, the qubits have to be coherent for a long enough time that error-correction techniques can be employed to make sure that the decoherence doesn’t prevent accurate computation.

“In 1999, coherence times were about 1 nanosecond,” Steffen told me. “Last year, coherence times were achieved for as long as 1 to 4 microseconds. With these new techniques, we’ve achieved coherence times of 10 to 100 microseconds. We need to improve that by a factor of 10 to 100 before we’re at the threshold we want to be. But considering that in the past ten years we’ve increased coherence times by a factor of 10,000, I’m not scared.”

 

Alex Knapp Forbes Staff
 MIT's Scott Aaronson Explains Quantum Computing

The IBM team has taken two approaches to quantum computing, both of which factor into the breakthroughs announced here. The first approach is building a 3-D qubit made from superconducting, microfabricated silicon. Steffen notes that the benefit of using silicon for these qubits is that the manufacturing equipment and know-how already exists – new techniques don’t have to be developed. 3-D qubits were pioneered by the Schoelkopf Lab at Yale, and Steffen expressed his admiration for that work. Building on the Yale techniques, the IBM team was able to maintain coherence for 95 microseconds. (“But you could round that to 100 for the piece if you want,” Steffen joked.)

How To Make A Cheaper Quantum Computer
  Alex Knapp Forbes Staff

 The second approach involved a traditional 2-D qubit, which IBM’s scientists used to build a “Controlled NOT gate” or CNOT gate, which is a building block of quantum computing. A CNOT gate connects two qubits such that the second qubit will change state if the first qubit changes its state to 1. For example, if qubit A’s state is changed from 0 to 1, and qubit B’s state is 1, it will flip to state 0. But if qubit A’s state is changed from 1 to 0, qubit B is unaffected. That seems simple enough, but when you scale multiple logic gates like this together, you have a very real basis for computation. The CNOT gates were able to maintain coherence times of 10 microseconds, which is long enough to show a 95% accuracy rate. The previous accuracy record for CNOT gates was 81% accuracy, so this is a huge step.  Of course, Steffen was quick to note that there’s still a ways to go before this can be implemented as a computing solution. That makes common sense, since 95% is accurate, but in the long run you need the accuracy to be as close to 100% as possible. 
 

The Inner Workings of a Quantum von Neumann Computer Alex Knapp Forbes Staff

Given the rapid progress that IBM has made, scalable quantum computing is starting to look like a real possibility. As error-correction protocols improve and coherence times lengthen, accurate quantum computing becomes a real possibility. But don’t expect to have a quantum smartphone anytime soon using this technique. In order to get the results the IBM team has seen in either the 2-D or 3-D configuration, the qubits have to be cooled down to less than a degree above absolute zero.

“There’s a growing sense that a quantum computer can’t be a laptop or desktop,” said Steffen. “Quantum computers may well just being housed in a large building somewhere. It’s not going to be something that’s very portable.  In terms of application, I don’t think that’s a huge detriment because they’ll be able to solve problems so much faster than traditional computers.”

The next steps for the team is to improve coherence and error-correction protocols to the point where the accuracy is over 99.9%. That means they’ll have achieved a “logical qubit” – one that, for practical purposes, doesn’t experience decoherence. From that point, the next step is to develop a quantum computing architecture. IBM is considering some possibilities here, including developing some quantum memory architechture. But what encourages Steffen in these endeavors is that these are questions of engineering, not of theory.

“We are very excited about how the quantum computing field has progressed over the past ten years,” he told me. “Our team has grown significantly over past 3 years, and I look forward to seeing that team continue to grow and take quantum computing to the next level.”

Newscribe : get free news in real time