Nav: Home

Long-distance quantum information exchange -- success at the nanoscale

March 18, 2019

At the Niels Bohr Institute, University of Copenhagen, researchers have realized the swap of electron spins between distant quantum dots. The discovery brings us a step closer to future applications of quantum information, as the tiny dots have to leave enough room on the microchip for delicate control electrodes. The distance between the dots has now become big enough for integration with traditional microelectronics and perhaps, a future quantum computer. The result is achieved via a multinational collaboration with Purdue University and the University of Sydney, Australia, now published in Nature Communications.

Size matters in quantum information exchange even on the nanometer scale

Quantum information can be stored and exchanged using electron spin states. The electrons' charge can be manipulated by gate-voltage pulses, which also controls their spin. It was believed that this method can only be practical if quantum dots touch each other; if squeezed too close together the spins will react too violently, if placed too far apart the spins will interact far too slowly. This creates a dilemma, because if a quantum computer is ever going to see the light of day, we need both, fast spin exchange and enough room around quantum dots to accommodate the pulsed gate electrodes.

Normally, the left and right dots in the linear array of quantum dots (Illustration 1) are too far apart to exchange quantum information with each other. Frederico Martins, postdoc at UNSW, Sydney, Australia, explains: "We encode quantum information in the electrons' spin states, which have the desirable property that they don't interact much with the noisy environment, making them useful as robust and long-lived quantum memories. But when you want to actively process quantum information, the lack of interaction is counterproductive - because now you want the spins to interact!" What to do? You can't have both long lived information and information exchange - or so it seems. "We discovered that by placing a large, elongated quantum dot between the left dots and right dots, it can mediate a coherent swap of spin states, within a billionth of a second, without ever moving electrons out of their dots. In other words, we now have both fast interaction and the necessary space for the pulsed gate electrodes ", says Ferdinand Kuemmeth, associate professor at the Niels Bohr Institute.

Collaborations are an absolute necessity, both internally and externally

The collaboration between researchers with diverse expertise was key to success. Internal collaborations constantly advance the reliability of nanofabrication processes and the sophistication of low-temperature techniques. In fact, at the Center for Quantum Devices, major contenders for the implementation of solid-state quantum computers are currently intensely studied, namely semiconducting spin qubits, superconducting gatemon qubits, and topological Majorana qubits.

All of them are voltage-controlled qubits, allowing researchers to share tricks and solve technical challenges together. But Kuemmeth is quick to add that "all of this would be futile if we didn't have access to extremely clean semiconducting crystals in the first place". Michael Manfra, Professor of Materials Engineering, agrees: "Purdue has put a lot of work into understanding the mechanisms that lead to quiet and stable quantum dots. It is fantastic to see this work yield benefits for Copenhagen's novel qubits".

The theoretical framework of the discovery is provided by the University of Sydney, Australia. Stephen Bartlett, a professor of quantum physics at the University of Sydney, said: "What I find exciting about this result as a theorist, is that it frees us from the constraining geometry of a qubit only relying on its nearest neighbours". His team performed detailed calculations, providing the quantum mechanical explanation for the counterintuitive discovery.

Overall, the demonstration of fast spin exchange constitutes not only a remarkable scientific and technical achievement, but may have profound implications for the architecture of solid-state quantum computers. The reason is the distance: "If spins between non-neighboring qubits can be controllably exchanged, this will allow the realization of networks in which the increased qubit-qubit connectivity translates into a significantly increased computational quantum volume", predicts Kuemmeth.

University of Copenhagen

Related Quantum Dots Articles:

Graphene and quantum dots put in motion a CMOS-integrated camera that can see the invisible
ICFO develops the first graphene-based camera, capable of imaging visible and infrared light at the same time.
Platelets instead of quantum dots
A team of researchers led by ETH Zurich professor David Norris has developed a model to clarify the general mechanism of nanoplatelet formation.
Quantum dots illuminate transport within the cell
Biophysicists from Utrecht University have developed a strategy for using light-emitting nanocrystals as a marker in living cells.
'Flying saucer' quantum dots hold secret to brighter, better lasers
By carefully controlling the size of the quantum dots, the researchers can 'tune' the frequency, or color, of the emitted light to any desired value.
'Flying saucer' colloidal quantum dots produce brighter, better lasers
A multi-institutional team of researchers from Canada and the US has demonstrated steady state lasing with solution-processed nanoparticles called 'colloidal quantum dots,' an important step on the path to improving laser tools for fiber optics, video projectors and more accurate medical testing technology.
Quantum dots with impermeable shell: A powerful tool for nanoengineering
Depending on their applications, quantum dots need to be tailored in terms of their structure and properties.
USC quantum computing researchers reduce quantum information processing errors
USC Viterbi School of Engineering scientists found a new method to reduce the heating errors that have hindered quantum computing.
A new form of hybrid photodetectors with quantum dots and graphene
ICFO researchers develop a hybrid photodetector comprising an active colloidal quantum dot photodiode integrated with a graphene phototransistor.
ORNL demonstrates large-scale technique to produce quantum dots
ORNL demonstrates a method to produce significant amounts of semiconducting nanoparticles for light-emitting displays, sensors, solar panels and biomedical applications.
First single-enzyme method to produce quantum dots revealed
Three Lehigh University engineers have successfully demonstrated the first precisely controlled, biological way to manufacture quantum dots using a single-enzyme, paving the way for a significantly quicker, cheaper and greener production method.

Related Quantum Dots Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Changing The World
What does it take to change the world for the better? This hour, TED speakers explore ideas on activism—what motivates it, why it matters, and how each of us can make a difference. Guests include civil rights activist Ruby Sales, labor leader and civil rights activist Dolores Huerta, author Jeremy Heimans, "craftivist" Sarah Corbett, and designer and futurist Angela Oguntala.
Now Playing: Science for the People

#521 The Curious Life of Krill
Krill may be one of the most abundant forms of life on our planet... but it turns out we don't know that much about them. For a create that underpins a massive ocean ecosystem and lives in our oceans in massive numbers, they're surprisingly difficult to study. We sit down and shine some light on these underappreciated crustaceans with Stephen Nicol, Adjunct Professor at the University of Tasmania, Scientific Advisor to the Association of Responsible Krill Harvesting Companies, and author of the book "The Curious Life of Krill: A Conservation Story from the Bottom of the World".