New methods for exploiting quantum information in photons

New methods for exploiting quantum information in photons
© iStock/Ramberg

The Quantum Information Science and Quantum Computation Lab at the University of Vienna is unravelling new ways to encode, manipulate, and detect quantum information in photons – and is already delivering some exciting results.

In the Quantum Information Science and Quantum Computation Lab at the University of Vienna we study new ways to encode, manipulate, and detect quantum information in photons – particles of light. Our goal is two-fold: first, we wish to learn the physical limits of these process, and, then, we aim to push these limits to create a quantum device capable of outperforming any of today’s comparable devices.  To perform these studies we leverage both cutting-edge telecom technology and novel concepts from foundational physics.

Quantum technologies

Quantum technologies are the way of the future. There are two reasons for this. First, the progressive miniaturisation of classical computer chips has reached a point wherein the size of an individual transistor is now so small that quantum effects can no longer be ignored. The second, and perhaps more exciting, reason is that it has been shown that quantum devices exist which are intrinsically better than any of their corresponding classical counterparts.

In these quantum devices information is manipulated in a way that is fundamentally different than in classical information and communication technology. Our lab’s motivation is to develop experimental techniques to reach the quantum limits of information processing and to then apply these techniques to technological applications. Our group maintains a strong focus on understanding the limitations that quantum mechanics places on information processing as this can lead to new and sometimes surprising results.

For the past 20 years or so, ‘quantum advantages’ have been studied academically in scientific labs, but, as of yet, no device has been created that is unambiguously faster than its classical counterpart.  However, current quantum technologies are on the verge of this objective, which has sparked significant interest in these technologies from major companies. Additionally, there are numerous quantum technology start-ups whose sole focus is to deliver practical quantum technologies. The European Commission has also recognised this market with a €1bn funding initiative known as the Quantum Flagship.

Broadly speaking, a quantum technology is any device that makes use of quantum mechanics to achieve something that couldn’t otherwise be done. This is accomplished by encoding information in a quantum system, manipulating that information in some way, and then reading the processed information out of the quantum system.

One of the best-known examples of a quantum technology is a quantum computer. In a standard classical computer, bits can either be a zero or a one. However, in a quantum computer, the classical bits are replaced with quantum bits (qubits) that are allowed to be placed in superposition of these states, meaning that they can simultaneously be both a zero and a one. Then, these qubits are made to interact via a series of ‘quantum logic gates’ which allow, for example, one qubit to change the state of another qubit. Finally, after the computation the qubits are measured. Quantum computers are tantalising as they can efficiently solve several important computing problems which are known to be impossible to solve on a classical computer.

Quantum computers are not the only quantum technology. The field of quantum communication promises more secure communication, while quantum metrology studies how to perform more precise measurements and has potential applications in microscopy and medical sensing. For many of these quantum technologies photonic quantum information has a unique advantage related to the fact that light can be made to controllably interact with stationary systems, and it can transmit information over long distances.

Quantum photonics

A wide range of physical systems can be used to encode quantum information, from single atoms to super-conducting wires to single particles of light (photons). All of these various systems have pros and cons, and it is not yet known which system will be used to construct a quantum device capable of outperforming its classical counterpart. It is likely that some combination of these systems will be required for different aspects of quantum technologies. In our group at the University of Vienna, we study different ways to encode quantum information in photons.

Photons have several properties that make them special when it comes to quantum information processing. Most noticeably, photons do not sit still. Photons can be easily transmitted between two points just as easily as a laser pointer travels across a room. This is essentially how communication with satellites orbiting the planet is done. Moreover, optical fibres, used for worldwide classical communication, can also transmit quantum information in encoded in photons.

Another feature unique to photonic quantum information is that photons barely interact with their surroundings. This means that once quantum information is written into a photon, it remains more or less intact. With other quantum systems much more care must be taken to shield the quantum information from the outside environment. These two features make photons the only near-term option for transmitting quantum information. From this perspective it seems that photons will play an integral role as information carriers in any future quantum network.

While photons are ideally suited for communication, several challenges remain to efficiently manipulate photonic quantum information as required for a quantum computer. To address these challenges our group investigates techniques to miniaturise the required components. We apply technologies that take advantage of telecom fabrication techniques to create a reprogrammable ‘photonic processor’. With these devices we can manipulate many optical modes in a footprint of only a few microns.

Combined with photonic technologies’ exceptional ability to transmit quantum information, these new methods provide a route towards a new tier of photonic quantum information manipulation. They could one day enable quantum cloud computing, wherein clients securely delegate computation tasks to a quantum server.

On top of all their technical advantages, photons have a long history in the study of experimental quantum information.

Quantum foundations

Although quantum technologies are now on the cusp of potentially providing exciting applications, the field was born from a very different perspective. Quantum information processing was set into motion by considering very foundational questions about quantum entanglement and Einstein’s famous ‘spooky action at distance’. In these now-famous discussions Einstein noticed that entangled particles seem to have some ability to instantaneously affect one another. As this is a valid prediction from quantum mechanics, he thus posited that quantum mechanics must be somehow wrong.

Almost 50 years later these effects were finally experimentally tested, by entangling pairs of photons.  The experimental techniques used to test this question formed a starting point for modern quantum photonics. Variants of these techniques have since been used to study a myriad of foundational questions, ultimately leading to the fields of quantum information and quantum computation. Thus, experimental quantum information is still very influenced by the foundations of quantum physics. Indeed, in our group we have several projects dedicated to experimentally testing such foundational concepts.

In one such project, for example, we have been studying the role of causal orders in quantum mechanics. A causal order is essentially a list stating the order in which events occur. Until very recently, it was always assumed that causal orders were always fixed – i.e. given two parties, Alice and Bob, Alice always acts before Bob, or vice versa. Recently, our collaborators noticed that quantum mechanics allows for the parties to act in a quantum superposition of both orders at the same time.  In other words, quantum mechanics allows for causal orders to be indefinite.

Taking advantage of the inherent mobility of photons, we were able to route a photon to two parties in a superposition of both orders at the same time. This set-up allowed us to experimentally prove that quantum mechanics does indeed allow for indefinite causal orders. Applying this concept to quantum computing quickly led to the discovery that indefinite causal orders can also be used as a new resource.

In a standard quantum computer the gates act in a fixed order. But if one applies the concept of an indefinite causal order to quantum computers, allowing the gates to act in a superposition of order, it is possible to gain an even greater advantage. We were also able to show this experimentally. By placing the gates in a quantum computer in a superposition, we ran a simple algorithm using fewer gates than normal.

As is often the case, once one application is found, more follow. Soon after our original work, it was discovered that superimposing the order of a communication channel can allow improved communication over noisy channels—an idea that we are very excited to pursue experimentally.

Thus, we remain interested in testing both foundational and practical concepts, and photonic quantum information processing remains an ideal platform for such tests.

 

Lee Rozema

Philip Walther

Vienna Center for Quantum Science and Technology (VCQ)

Faculty of Physics

University of Vienna

http://walther.univie.ac.at

This is a commercial article that will appear in SciTech Europa Quarterly issue 27, which will be published in June, 2018.

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