New dimensions of quantum information added through hyperentanglement
In quantumcryptography, encoding entangled photons with particular spin states is atechnique that ensures data transmitted over fiber networks arrives at itsdestination without being intercepted or changed. However, as each entangledpair is usually only capable of being encoded with one state (generally thedirection of its polarization), the amount of data carried is limited to justone quantum bit per photon. To address this limitation, researchers have nowdevised a way to "hyperentangle" photons that they say can increase the amount of data carried by a photon pair by as much as 32 times.
In this research, a team led by engineers from UCLA has verified that it is possible to break up and entangle photon pairs into many dimensions using properties such as the photons' energy and spin, with each extra dimension doubling the photons' data carrying capacity. Using this technique, known as "hyperentanglement",each photon pair is able to be programmed with far more data than waspreviously possible with standard quantum encoding methods.
To achieve this, the researchers transmitted hyperentangled photons in theform of a biphoton frequency comb (essentially a series of individual,equidistantly-arranged frequencies) that divided up the entangled photons intosmaller parts. An extension of the technique of wavelength-division multiplexing(the process used to simultaneously transmit things such as multiple video signals over asingle optical fiber), the biphoton frequency comb demonstrates the usefulapplications of such methods not just at macro levels, but quantum ones as well.
"We show that an optical frequencycomb can be generated at single photon level,” said ZhendaXie, an associateprofessor of electrical engineering at UCLA and research scientist on the project. "Essentially, we’releveraging wavelength division multiplexing concepts at the quantum level."
Working on previous theories mooted by Professor Jeff Shapiro of MIT on thepossibilities of quantum entanglement being utilized in the formation of comb-likeproperties of light, it is only with recent adaptations of ultrafast photondetectors and the advancement of the various supporting technologies requiredto generate hyperentanglement that such hypotheses could be physically tested.
"We are fortunate to verify adecades-old theoretical prediction by Professor Jeff Shapiro of MIT, thatquantum entanglement can be observed in a comb-like state," said Chee Wei Wong, a UCLA associate professor of electrical engineering who was the research project’s principal investigator. "With the help of state-of-the-art high-speed single photondetectors at NIST and support from Dr. Franco Wong, Dr. Xie was able to verify thehigh-dimensional and multi-degrees-of-freedom entanglement of photons. Theseobservations demonstrate a new fundamentally secure approach for denseinformation processing and communications."
If this research can be successfully and continuously replicated, quantum encoding will no longer be bound to the limitations of a single quantum bit (qubit). Rather, quantum hyperentanglementresearch can now move into the realm of the qudit (a unit of quantum information encoded in any number of d states, where d is a variable),where the quantum-encoded information able to be transmitted can theoretically beincreased manifold times by simultaneously encoding energy levels, spin states and other parameters inherent in the quantum attributes of a photon.
Aside from the usual applications in secure communication and informationprocessing, and high-capacity, minimal error data transfer, the team believesthat a raft of other technologies could benefit from this breakthrough. Theenormously data-intensive needs of medical computer servers, government informationtransfer, financial data, ultra-secure military communications channels, distributedquantum computing, and quantum cloud communications are just a few of the areasthe researchers say may usefully employ this new method.
Engineers at UCLA were the research project’s principal investigators, with assistance provided from researchers at MIT, Columbia University, the University of Maryland, andthe National Institute of Standards and Technology.
The results of this research were recently published in the journal NaturePhotonics.