Nav: Home

Quantum transfer at the push of a button

June 14, 2018

Data transmission is the backbone of the modern information society, on both the large and small scale. On the internet, data are exchanged between computers all over the world, most often using fibre optic cables. Inside a computer, on the other hand, information has to be shuttled back and forth between different processors. A reliable exchange of data is also of great importance for the new quantum information technologies that are currently being developed - but at the same time it is also fiendishly difficult. At the ETH in Zurich, a team of physicists led by Andreas Wallraff of the Laboratory for Solid State Physics has now succeeded in transmitting quantum information, at the push of button and with high fidelity, between two quantum bits roughly a metre apart. Their results are published in the scientific journal Nature this week.

Flying quantum bits

The main peculiarity of quantum information technologies, such as quantum computers and quantum cryptography, is the use of quantum bits or «qubits» as the elementary unit of information. Differently from classical bits, qubits cannot just have the value 0 or 1, but also take on so-called superposition states. On the one hand, this results in the possibility to build extremely powerful computers that make use of those superposition states to perform calculations much more efficiently and faster than classical computers. On the other hand, those states are also very sensitive and cannot be transmitted simply using conventional techniques. The problem is that the state of a stationary qubit first has to be transformed into a so-called "flying" qubit, for instance a photon, and then back into another stationary qubit. A few years ago researchers were able to transmit the quantum state of an atom in this way. Wallraff and his co-workers have now succeeded in realizing such a transmission also from one superconducting solid-state qubit to another one some distance away.

To do so, the physicists connected two superconducting qubits using a coaxial cable of the kind that is also used to connect to antenna terminals. The quantum state of the first qubit, which is defined by the number of superconducting electron pairs (also known as Cooper pairs) contained in it, was first transferred to a microwave photon of a resonator using very precisely controlled microwave pulses. From that resonator the photon could then fly through the coaxial cable to a second resonator, inside of which microwave pulses, once more, transferred its quantum state onto the the second qubit. Similar experiments were recently carried out at Yale University.

Deterministic rather than probabilistic

"The important point of our method is that the transmission of the quantum state is deterministic, which means that it works at the push of a button", Philipp Kurpiers, a PhD student in Wallraff's lab, emphasizes. In some earlier experiments a transfer of quantum states could already be realized, but that transmission was probabilistic: sometimes it worked, but most of the time it didn't. A successful transmission could, for instance, be signalled by a "heralding photon". Whenever the transmission hadn't worked, one simply tried again. In that way, the effective quantum transmission rate was, of course, strongly reduced. For practical applications, therefore, deterministic methods such as the one now demonstrated at ETH are clearly advantageous.

"Our transmission rate for quantum states is among the highest ever realized, and at 80% our transmission fidelity is very good in the first realization of the protocol", says Andreas Wallraff. Using their technique, the researchers were also able to create a quantum mechanical entanglement between the qubits as many as 50,000 times per second. The transmission procedure itself took less than a millionth of a second, which means that there is quite a bit of room for improvement in the transmission rate. Quantum mechanical entanglement creates an intimate link between two quantum objects even across large distances, a feature that is used for cryptography or quantum teleportation.

Quantum transfer for quantum computers

As a next step, the researchers want to try to use two qubits each as transmitter and receiver, which makes entanglement swapping between the qubit pairs possible. Such a process is useful for larger quantum computers, which are supposed to be built in the next few years. So far, they only consist of a handful of qubits, but when trying to build larger computers, already for a few hundred qubits one will have to worry about how to connect them most effectively in order to exploit the advantages of a quantum computer in the best possible way.

Much like clusters of single computers used today, quantum computer modules could then be connected together using Wallraff's technique. The transmission distance, which is currently about a metre, could certainly be increased. Wallraff and his colleagues recently demonstrated that an extremely cold, and thus superconducting, cable could transmit photons over distances of several tens of metres with very little loss. Wiring together a quantum computing centre, therefore, seems to be quite feasible.
-end-
Reference

Kurpiers P, Magnard P, Walter T, Royer B, Pechal M, Heinsoo J, Salathé Y, Akin A, Storz S, Besse J-C, Gasparinetti S, Blais B, Wallraff A. Deterministic quantum state transfer and remote entanglement using microwave photons. Nature (2018), published online 13th June,

ETH Zurich

Related Quantum Computers Articles:

Study takes step toward mass-producible quantum computers
Study takes step toward mass-producible quantum computers.
Testing quantum field theory in a quantum simulator
Quantum field theories are often hard to verify in experiments.
Refrigerator for quantum computers discovered
Researchers at Aalto University have invented a quantum-circuit refrigerator, which can reduce errors in quantum computing.
New quantum liquid crystals may play role in future of computers
First 3-D quantum liquid crystals may have applications in quantum computing.
'Virtual' interferometers may overcome scale issues for optical quantum computers
A team of researchers from RMIT, the University of Sydney and UTS have devised an entirely new way of implementing large-scale interferometers that will dramatically miniaturize optical processing circuitry.
Further improvement of qubit lifetime for quantum computers
An international team of scientists has succeeded in making further improvements to the lifetime of superconducting quantum circuits.
Construction of practical quantum computers radically simplified
Scientists at the University of Sussex have invented a ground-breaking new method that puts the construction of large-scale quantum computers within reach of current technology.
New quantum states for better quantum memories
How can quantum information be stored as long as possible?
A new class of materials could realize quantum computers
Scientists at EPFL and PSI have discovered a new class of materials that can prove ideal for the implementation of spintronics.
New 3-D wiring technique brings scalable quantum computers closer to reality
Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.

Related Quantum Computers 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

Digital Manipulation
Technology has reshaped our lives in amazing ways. But at what cost? This hour, TED speakers reveal how what we see, read, believe — even how we vote — can be manipulated by the technology we use. Guests include journalist Carole Cadwalladr, consumer advocate Finn Myrstad, writer and marketing professor Scott Galloway, behavioral designer Nir Eyal, and computer graphics researcher Doug Roble.
Now Playing: Science for the People

#529 Do You Really Want to Find Out Who's Your Daddy?
At least some of you by now have probably spit into a tube and mailed it off to find out who your closest relatives are, where you might be from, and what terrible diseases might await you. But what exactly did you find out? And what did you give away? In this live panel at Awesome Con we bring in science writer Tina Saey to talk about all her DNA testing, and bioethicist Debra Mathews, to determine whether Tina should have done it at all. Related links: What FamilyTreeDNA sharing genetic data with police means for you Crime solvers embraced...