Quantum erasure-correcting code (Nature Photonics)

Sunday 30 Jan 11

We have experimentally realized a new quantum error correction code for continuous variable quantum systems that is capable of protecting highly fragile quantum states from complete erasure. The article can be found here and a popular introduction is available here.



Quantum error-correcting codes


Millions and billions of bits are being processed every day around the world. Skype calls and facebook messages are flowing through the internet coded as streams of bits, SMSs are sent through the telecommunication network as bit messages and DVDs and CDs are being scanned on drivers that read and process bits. However, in this big mesh of bits zigzagging through the world, errors inevitably occurs. Those errors might result in “flicker” on the TV screen, crackle on the telephone line or mistunes when playing a CD. In good communication systems these errors are minimized by using a special trick: The information that we need to sent over the Atlantic or process in a computer, can be coded in some special code words that is particularly robust to frequently occurring transmission and processing errors. 

These standard code words however do not work in quantum communication and computational systems. In these systems, information is encoded in quantum bits (so-called qubits) rather than as classical bits, 0 and 1. Due to the radically different nature of these qubits, the standard coding methods known for classical computation and communication are simply useless for quantum information processing. Quantum information processing was therefore seen as a field of interest mostly for theoreticians without any practical significance.  However, this changed radically in 1996 with the discovery of quantum error-correcting codes (QECC) by P. Shor and A. Steane. A new bright era in quantum information processing was born. Quantum information processing was now seen as a potentially practical alternative to classical information processing. 

Following these discoveries, immense theoretical work was carried out but only a few experimental implementations demonstrating quantum error correction have been carried out to date, e.g., in nuclear magnetic resonance systems, in an ion-trap system, and in a pure optical system. All these works have reported on the correction of errors, which are the manifestation of line noise. However, it is very often the loss of photons in a transmission line (corresponding to it erasures in an information theoretic language) that is the main obstacle to the survival of quantum coherence. Erasure-correcting codes have long been known in classical coding theory, and their quantum counterparts have also been theoretically developed. In our work published Nature Photonics, we have implemented the first experimental realization of a quantum erasure-correcting code which simultaneously protects two independent continuous-variable (CV) quantum systems against complete photon losses in the transmission channel. 

For detailed information about the actual protocol and how it was implemented and verified, please consult our Nature Photonics paper here (or arXiv: 1006.3941). The theoretical proposal can be found here (or arXiv: 0710.4858). This work was supported by the Danish Research Council (FTP) and the EU (COMPAS).