- An important step toward ultra-long-distance, low-power and high-capacity communications - (June 24, 2011)
National Institute of Information and Communications Technology (hereinafter called NICT, President: Hideo Miyahara), in collaboration with National Institute of Advanced Industrial Science and Technology (hereinafter called AIST) and Nihon University, has succeeded in the world’s first demonstration of beating the bit error rate bound of optical communication theory, by using a new technology of “quantum receiver”. By replacing a conventional optical receiver with this quantum receiver, one will be able to realize large capacity communications without increasing the signal power in a fiber, as well as to extend a communication distance in space. Our result is an important step toward realizing those tasks. This achievement has been reported over the official website and a magazine of Physical Review Letters*, on June 24, 2011.
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Our recent research result on quantum superposition control of light wave has been selected as one of major achievements in Optics in 2010, and made the cover of the journal "Optics and Photonics News."
Qubit realizations in photonics so far have mainly based on polarization or "which-path" degrees of freedom of a single photon. Our team could have succeeded in realizing an optical qubit of two orthogonal states of light wave, which lie in the infinite dimensional space associated with continuous variables (quadrature amplitude and phase). This kind of qubit may be referred to as "continuous variable qubit." Such qubits would be useful, when used as local oscillators in optical coherent communication, to realize quantum decoder for the ultimate capacity communication.
This qubit state can be represented by a Wigner function distribution locating at a point in a Bloch sphere, symbolizing the continuous and discrete nature of this new state of light.
The National Institute of Information and Communications Technology (NICT, President Dr. Hideo Miyahara) in cooperation with its commissioned research partners NEC, Mitsubishi Electric and NTT has started with the operation of the world’s fastest quantum key distribution (QKD) network using part of NICT’s open fiber testbed network JGN2plus. The Tokyo QKD Network boasts key generation rates at around 100kbps allowing perfectly secure one-time pad encryption of video data in real time. In this field network operation, Toshiba Research Europe Ltd and other European organizations have also participated in an effort to promote standardization of the interconnection technology between Japanese and overseas QKD systems.
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We have demonstrated an important milestone protocol, called “entanglement distillation” applicable to continuous variable quantum communications. Quantum entanglement is a strong correlation formed between two or more quantum systems. This can be used to realize secure and high capacity communication that are impossible by classical systems. Because the entanglement is very susceptible to damping, distilling stronger entanglement from the degraded one is indispensable after transmission. While distillation for entangled single photons was previously demonstrated, the distillation for more wave-like, continuous variable (CV) states had not been realized until now. This is important because current optical networks are based on CV technology, and eventually it would deliver ultra-high capacity communications with minimum power in the current infrastructure. This result will be published in the UK science journal, "Nature Photonics."
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We develop a source of hybrid entanglement pairs between two different degrees of freedom, a 1550 nm time-bin qubit and an 810 nm polarization qubit. The polarization qubit at 810 nm is transformed by an asymmetric Mach-Zehnder interferometer consisted of a Glan laser prism and a polarization-maintaining fiber. We obtained visibilities of 95.8% and 88% with tolerance ±0.2 and ±1% along Z-Z and X-X axes on the Poincare sphere, respectively with a coincidence count rate of more than 800 c/s after entanglement format transformation. These values are well above the threshold of 70.7% needed to violate a Bell inequality and allow distilling a secure key in the quantum key distribution.
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--- Advancement toward realizing cryptographic key distribution with the ultimate information security ---
We have achieved ultra fast quantum key distribution (QKD)* over a 97-km installed telecom fiber. The cryptographic key generation rate was 100 times higher than the previous field experiments and about 10 times higher than the best rate ever reported including laboratory experiments.
Cryptography used in the internet now is public key cryptography. This method is, however, always threatened by advancement of computers, which allows an eavesdropper to decrypt the key with ultrafast computers. In contract, QKD can ensure unbreakable cryptographic scheme regardless of how technologies would advance in future.
Recently leakage of confidential information from information systems becomes a serious concern. This accelerates an effort to construct a network in which confidential data are managed in a data center, and terminals are linked through secure lines with it. QKD demonstrated here is suitable for such a network.
Most of QKD experiments so far were done in ideal laboratory environment. Our experiment is, however, done in an installed telecom fiber in a city. We have achieved the key generation rate 100 times higher than the previous field experiments, and 10 times higher than the previous labolatory experiments. This is a significant step toward realizing cryptographic key distribution with the ultimate information security.
(This experiment was done in the field test-bed optical fiber network of NICT, called JGN2, with the cooperation of NEC Corporation and National Institute of Standards and Technology (NIST).)
Single photons exhibit quantum mechanical properties, such as superposition and no-cloning character. These properties supply the basis for the use of single photos as an ideal carrier of quantum information. Special schemes are necessary for generating single photons with a fixed temporal spacing, since they tend to bunch together, or appear randomly even when a laser is used. A single calcium ion 40Ca+ trapped and cooled in an ion trap has been used to generate single photons with fixed timing as well as controlled waveform for the first time, in collaboration with Prof. Herbert Walther's group at Max-Planck Institute of Quantum Optics in Germany. Stimulated Raman scattering by cavity is the mechanism of this achievement. This single-photon emission process can be extended to a basic quantum mechanical protocol of mapping quantum states between ions and photons. The protocol might supply a crucial building block for quantum networks which connect quantum computers, and synchronize quantum atomic clocks securing network transactions.Original paper
Nature 431, 1075 (2004)
Narrow-band green coherent radiation has been successfully generated by a low-cost, small-sized and robust frequency doubled diode laser instead of a conventional frequency-doubled YAG laser. The non-critical phase matching condition is exploited by combining a 50mW diode laser at 1080nm with an a-cut KTP crystal. Green coherent radiation at 540nm in an ideal Gaussian beam profile is obtained with a conversion efficiency of 51.6%. Resonant optical feedback guarantees intensity fluctuation less than 0.3% and a linewidth less than 82kHz. The green radiation was used to pump an optical parametric oscillator comprising of an a-cut KTP crystal to yield 5.1mW of quantum-correlated twin beams exhibiting intensity-difference squeezing of 63%. This is the first demonstration of generation of non-classical light by a frequency-doubled diode laser. Generation of non-classical light by diode laser is advantageous in versatile, efficient, compact and low-cost quantum communications.
Related new resources
News Breaks in "Laser Focus World", Sep. 1. 2004
Optics Express 12, 3567 (2004)
Optics Letters 29, 1665 (2004)
Compression of signals is the operation to remove the redundancy of the original message and shorten the total code length. The redundancy and the compression limit usually depend on the properties of the signals. For quantum signals, it was recently predicted by quantum information theory that there exists a compression scheme that exceeds the limit predicted by the conventional (classical) information theory. In general, quantum carriers include the redundancy due to the overlap of wavefunctions between the carriers. The idea to remove such additional redundancy is to control quantum correlations between these carriers appropriately. In this experiment, we have performed the compression and decompression operations of three qubit signals encoded in a single photon's orthogonal polarization and it's four optical paths. The linear optics compressor entangles these qubits and compressed the three qubit signals into two qubit signals, and then the signals are decompressed again by the linear optics decompressor. We have observed the higher decompression fidelity for our scheme compared to the scheme that have not employed any entangling operations. Fundamental coding theorems in quantum information theory are quantum source coding theorem and quantum channel coding theorem. Now, we have succeeded to experimentally demonstrate these theorems. The present result demonstrated the former and our previous result (demonstration of the super-additive quantum coding gain) demonstrated the latter.For more detail
This super-additive quantum coding gain is, in the simplest saying, such that transmissible information can increase even more than twice when the amount of transmission resources such as the signal power or the bandwidth is doubled for fixed noise characteristics of a channel. According to conventional communication theory, on the other hand, the amount of transmissible information can be increased twice at most. This quantum gain can be particularly remarkable for the signals at the quantum level conveyed by photons and electrons. We have demonstrated a proof-of-principle experiment of the super-additive quantum coding gain by using the ternary symmetric states of a single photon. We prepare the ternary symmetric signals by modulating polarization and spatial paths of a photon, and decode them at the receiver forming quantum entanglement between the degrees of freedom of polarization and spatial paths. We have then confirmed that the amount of information can be extracted more than double the amount that can be obtained when only one of those, the polarization signal or the spatial mode signal, is used for transmission. Our result is related to a new principle that beats the conventional technological limits toward large capacity communications and weak signal detections.
Multivalued polarized wave modulation signals due to a single photon are the simplest signal systems when performing a principal demonstration of quantum signal detection theory. On the other hand, because limited errors must be accompanied by discriminating signals, using this reversibly also enables us to constitute protocols of quantum channel coding. In spite of being predicted of the presence so far against such signal systems, we succeeded in proving a group quantum optimal detection strategy that had not previously been experimentally proven. The strategy outputs up to ternary, and this facilitates extraction of the maximum information volume no matter what the value modulation signal numbers are. This signal detection circuit consists of simple polarization Mach-Zehnder interference systems and proved detection performance approximating 96% of the quantum optimal detection limit in prediction."Quantum communication"