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Twice as much information

Technique doubles the amount of data that can be recovered from a quantum system

Intertwined laser beams: a set of mirrors that form a type of resonance cavity allow measurement of the slight oscillations of photons.

Eduardo CesarIntertwined laser beams: a set of mirrors that form a type of resonance cavity allow measurement of the slight oscillations of photons.Eduardo Cesar

A team of Brazilian physicists demonstrated that the use of an alternative technique to retrieve the information stored in particles of light—photons—doubles the data transmission capacity of quantum systems, an improvement that may accelerate the development of quantum bit-driven computing. The characteristics and potential of the method, called cavity-assisted measurement, were described in two papers both published on November 14, 2013, in the on-line versions of two scientific journals: Physical Review Letters and Physical Review A.

“The technique allows us to recover a portion of the information that was previously lost in the system,” says Marcelo Martinelli, of the University of São Paulo Institute of Physics (IF-USP), a member of the group that carried out the research under the National Institute of Quantum Information Science and Technology (INCT-IQ), a partnership between FAPESP and the National Council for Scientific and Technological Development (CNPq). A co-author on the papers was French physicist Claude Fabre of Université Pierre et Marie Curie-Paris 6 who is working with the Brazilians on a project funded by the Foundation and the Centre National de la Recherche Scientifique (CNRS).

The improved performance of the method—which is not new, but was enhanced by Martinelli and his colleagues—derives from its greater ability to bypass a kind of communication noise that limits the decoding of all information stored in processes such as cryptography and quantum teleportation. Photons in similar quantum states, but with slightly different frequency-spectrum oscillation patterns, are indistinguishable using what is known as the homodyne detection technique, typically used to “read” data in this type of system today. The adoption of cavity-assisted measurement—in which a beam of light passes through a set of mirrors, forming a “sounding board” before its detection—enables measurement of the slight photon quantum fluctuations. In this way, we can distinguish almost identical particles of light that, using the homodyne detection technique, appear to have the same characteristics. “With the previous technique, we were intrinsically unable to reconstruct all the information stored,” says Paulo Nussensveig, also at IF-USP, and another co-author of the articles.

The researchers draw a parallel between the effect produced by the use of cavity-assisted quantum measurement and the repercussions in the audio industry due to the adoption of devices capable of reproducing stereo sound. An FM station, for example, transmits songs recorded using two independent, simultaneous sound channels that operate at extremely close—but slightly different—frequencies. Thus each channel can broadcast different information at the same time. In instrumental music, the bass and drums can be recorded on the right channel while the piano and guitar are recorded on the left.

Something similar occurs in a laser beam with entangled photons, which is a kind of quantum correlation that can be exploited to store, process and transmit information. In experiments performed at USP, physicists generated a system with three interwoven beams of different colors and, therefore, of different wavelengths. Incidentally, the Brazilians were the first to produce and partly control an entangled system with these characteristics (see Pesquisa FAPESP Issue No. 164, October 2009). The frequency of each beam was approximately 300 terahertz, 1 million times higher than that of an FM radio transmission. Strictly speaking, the quantum information is not stored and transmitted through this principal light channel, but rather through two small 20 MHz channels located slightly above and below the main frequency.

Stereo instead of mono
As with a stereo system, each secondary channel is capable of carrying specific information. “The homodyne detection technique does not record these channels independently,” says Alessandro Villar, of the Federal University of Pernambuco (UFPE), another author of the study. “It takes an average of the data in both fields. We knew it was not ideal, but there was no alternative. “Returning to the radio comparison, it’s like someone used a device that played only one channel, in mono mode, to listen to a stereo broadcast. Can the broadcast message be understood even in mono? Yes, but some details are lost.

This is more or less what happens when the quantum information in a system is recovered using the traditional homodyne detection technique, according to the researchers. That is, while the Brazilians’ experiment with three laser beams would register only three channels of quantum information using the conventional method, six channels were obtained using cavity-assisted measurement, one pair of data fields per beam.

1. Quantum information processing with continuous variables (2010/52282-1); Grant Mechanism FAPESP-CNRS Cooperation Agreement; Coord. Marcelo Martinelli, USP and Claude Fabre, Université Pierre et Marie Curie-Paris 6; Investment R$ 34,859.16 (FAPESP) and R$ 35,000 (CNRS);
2. Teleportation of quantum information between different colors (2009/52157-5); Grant Mechanism Regular Line of Research Project Award; Coord. Marcelo Martinelli, USP; Investment R$ 171,938.32 and US$ 43,800.00 (FAPESP).

Scientific articles
BARBOSA, F.A.S. et al. Beyond spectral homodyne detection: complete quantum measurement of spectral modes of light. Physical Review Letters. Nov. 14, 2013.
BARBOSA, F.A.S. et al. Quantum state reconstruction of spectral field modes: Homodyne and resonator detection schemes. Physical Review A. Nov. 14, 2013.