It is well-known that quantum computing is a work in progress at the moment. However, a real-world application of quantum information exists and it is quantum cryptography. QC allows secure data transmission independent form the computing power of the attacker. Nowadays qc works up to 100 km, something more, on optical fibers. I saw some news abot Durban, south africa, that has been completly cabled for qc, and London will be the next city. But the problem of effective quantum repeaters must be solved. Many reaserchers work on the possibility to transmit on the air up to satellites and back to the earth.
QC performances captured the interest of banks, big companies and institutions
The cost of this kind of system is nowadays around 100000 dollars. It is expected to get less expensive and reach longer distances. But the need of secure transactions is not only from big companies, but also from home users and normal citizens that use internet for online commercial transactions and ATM terminals evry day. This is why a compact and low-cost qc system is an important achievement. We aim to insert the system into a smartphone, an iphone or blakberry like.
The basic idea of qc goes back to 1969, when a PhD studnet proposed the protocol to Charles Bennet. Bennet formilized it with Gilles Brassard many years later in 1984. It is known that in qm the act of observation affects any quantum system and corrupts its information content. So if Alice (the transmitter ) sends a quantum information to Bob (the receiver), any eardropper E (Eve) will corrupt the information sequence, whatever his computing power. The protocol is based on the polarization properties of photons. You see here two polarized filters that allow photons to pass or stop them depending on the photon polarization of the photons.
Let’s imagina now that A and B can communicate through a quantum channel (photons) and aa public channel too (e.g. internet or phone). A chooses a random series of polarized photons and records them and send them to B. They are polarized with 4 different polarizations: vertical, hrizontal, +45°, -45° Bob usese an analyzer at random to analyze the photons, and records the seqquence and the polarizations used.
On the public channel B communicates A his polarizations sequence. A checks the sequence and on the public channel communicates B which bits are to be deleted because their polarization does not match, but does not communicates any information about the polarization itself.
A and B share now a correct sequence of polarizations, i.e. the secret key. Really bright (smart)
You can also settle if E the eardropper has tried to intercept the message, A and B via public channel share a portion of the sequence and see if it is coincident. There is a specific error statistics for this kind of transmission. For the mq laws , if E intercepts the photn sequence, the error statistics changes dramatically and the transaction can be immediately stopped. Errors can be further minimized by means of standard or quantum error correction and privacy amplification techniques.
Our syste cconsists of two custom circuit boards, transmitter and receiver. The transmitter is an electronic circuit that drives four high-performance LEDs. Light is suitably attenuated to obtain “raindrops” photons. LEDs are covered by 4 polarized filters. The circuit generates random casual logical signals. The random logic signals are applied to two pins of the parallel port, then sent to the transmission circuit This generates four bits that pilot a transistor that turns on the LEDs in sequence. The synchronism signal allows the bits to last the same time. An acquisition card sends to the second PC the bits after amplification and level adaptation.
The four channel receiver is equipped with high sensitivity photodiodes. It must establish the s equence of bits starting from the received photons. The photodiodes transform photons into electrical signals, then into bits. We used a logic state analyzer that revelas the voltage peaks coming from the photodiodes. The current intensity of the signal coming ffrom the photodiodes is extremely low, so they have to be amplified The receiver front-end uses the IVC102 integrated that is endowed with high-impedance inputs that can detect fA currents. We use four single channels on the same card, with four IVC102 Il front-end del ricevitore utilizza l'integrato IVC102 il quale è dotato di ingressi ad alta impedenza a FET che consentono di rilevare correnti a livello di fA , segue una seconda amplificazione e un circuito di trigger. After the integration process, the output voltage of the IVC102 is proportional to the input current. The integration value adopted is 100pF.
The two cards are driven by two separated computers equipped with a software written in C, that generates and decodes the signals. In the first PC the sogftware generates psudo-casual numbers, generates and synchronizes the sequence of bits Through the paralle port the second PC acquires the sequence of bits.
On the second PC the logic state analyzer reconstructs the signals and the software reads and synchronize them at the same clock frequency as in the transmitter We also built a software that simulates the comparison on public channel between generated sequences. In this way we obtain the in-clear secret secure key. A logic analyzer is the digital counterpart of an analog oscilloscope. It allows a number of digital input signals to be sampled and stored sequentially in a high-speed memory or buffer. A logic analyzer can also recognize a condition, or sequence of conditions, on the input data and use that combination of events to trigger data storage. The information acquired is displayed as oscilloscope-like waveforms or as list of numbers representing a sequence of logic states
At the moment the system is a prototype on optical bench, but in the futuro it can be adapted to work on optical fibers or directly on ATM terminals. The adopted optics can vary depending on the applications. Currently we carry out experiments if free air under darkness conditions, placing TX and RX in front one to each otehr at a distance between 30 and 100 cm. The performances of the system can be improved by substituting components and software with more effective models.
We are already acquiring new avalanche photodiodes to ensure single-photon performances. Of course the best performances are ensured at -20°C reducing thermal noise. The random number generator will be replaced with the portable IdQuantique hardware device or a custom hardware, both at a low cost. Finally, robust error correction and privacy amplification algorithms will be applied. The
An affordable Quantum Cryptography system
Rita PizziDepartment of Information Technology Università degli Studi di Milano
PROTOTYPE OF A QUANTUMCRYPTOGRAPHY SYSTEM FOR THE END USER
QUANTUM INFORMATION The quantum computer does non exist yet But a real world application based on quantum information exists: QUANTUM CRYPTOGRAPHY It allows the secure transmission of data, independent from algorithms and computing power of the attacker It is possible to detect any intrusion immediately Nowadays optical fiber systems exist that reach distances of 100 km Methods to increase distances and usability areunderway (quantum repeaters for optical fibers / satellite transmissions)
QUANTUM CRYPTOGRAPHY TODAYQuantum cryptography performances captured theinterest of banks, big companies and institutions.Systems already on sale:• MagiQ Technologies New York• idQuantique Geneve• SmartQuantum York• QinetiQ UK (defence)• Toshiba Corp Tokio• National Institute of Standards and Technology (US government agency )are acquiring this technology
QUANTUM CRYPTOGRAPHY TODAYSome cities (Durban, Madrid, London) are going to be completely cabled to apply quantum cryptography Today the cost of a system is around 100.000 $ Less expensive applications are interesting, affordable for the end user: ATM terminals, online internet transactions We developed our prototype for this purpose: a compact and cheap system that could be embedded in a smartphone
THE BB84 PROTOCOL(Bennet Brassard 1984) In quantum physics the act of observation modifies in an unpredictable way the observed system Thus any external action in the system will corrupt the flow of information, revealing the intrusion The BB84 protocol is based on the polarization properties of the photons
THE BB84 PROTOCOL (Bennet Brassard 1984) Alice chooses randomly a sequence of 1 and 0 bits, turns them into photons, applies to each bitone of the possible polarizations, then sends them to Bob.Bob chooses randomly a polarization to examineeach of the received photons, turns them into bits and records the results of his observations.
THE BB84 PROTOCOL (Bennet Brassard 1984)Now Bob sends to Alice on a public channel (e.g.Internet) his polarization sequence (but NOT the result of his measures) Alice selects the positions in the sequence thatBob sent correctly and sends them back to Bob on the public channel
THE BB84 PROTOCOL (Bennet Brassard 1984) Both Alice and Bob share now an identicalsequence of bits, i.e. they possess a shared key that is definitely secret.
BB84 – THE INTRUSION In this kind of transaction an intrinsic error rate exists, that can be minimized by means of error correction and privacy amplification techniques If an eardropper E interposes to intercept thesequence of bits, for the quantum physics laws he corrupts the sequence and sends back to Bob a sequence with a much higher error rate This reveals immediately the presence of the intruder and the transaction can be stopped without damage
OUR SYSTEM Our system is based on two custom cards: the transmitter and the receiver. TRANSMITTER It is an electronic circuit that drives four high-performances LEDs The LEDS are endowed with polarizing filters and their intensity is suitably attenuated. Random logical signals are generated that turn on the four LEDs in sequence
OUR SYSTEM RECEIVER The receiving circuit must re-establish a sequence of data starting from the received photons. Four high-sensitivity photodiodes turn the photons (passed through four polarizing filters) into electrical signals, then into bits. This is made possible by a logic state analyzer that detects the voltage peaks coming from the photodiodes.
THE FIRMWAREA C-written software drives the whole process on two separated PCs. In the first PC the software, using theBlumBlumShub pseudorandom number generator,generates the sequence of bits and synchronizes it This is acquired by the transmitter through the parallel port.
THE FIRMWARE On the second PC the software reads the signals reconstructed by the logic state analyzer and syncronizes themWe also simulated the comparison on publicchannel between sequences generated by transmitter and receiver At the end of simulation we obtain the secure key.
FUTURE DEVELOPMENTSAt the moment our system is a prototype on optical benchIn the future it can be adapted to work onoptical fibers or directly on ATM terminals.The system performances are improvablewith more effective components and with more powerful software algorithms
FUTURE DEVELOPMENTS We are acquiring avalanche photodiodesthat will ensure single-photon performances The software random number generator will be substituted by a portable andaffordable hardware generator (IdQuantique o custom) Robust algorithms of error correction and privacy amplification will be developed.