With such information already available, in practice one can achieve authenticated and sufficiently secure communication without using QKD, such as by using the Galois/Counter Mode of the Advanced Encryption Standard.
If a third party (usually referred to as Eve, for "eavesdropper") has gained any information about the photons' polarization, this introduces errors in Bob's measurements.
First, the entangled states are perfectly correlated in the sense that if Alice and Bob both measure whether their particles have vertical or horizontal polarizations, they always get the same answer with 100% probability.
[7] DIQKD presents difficulties in creating qubits that are in such high quality entangled states, which makes it a challenge to realize experimentally.
[10] The rate-distance limit, also known as the rate-loss trade off, describes how as distance increases between Alice and Bob, the rate of key generation decreases exponentially.
[11] In traditional QKD protocols, this decay has been eliminated via the addition of physically secured relay nodes, which can be placed along the quantum link with the intention of dividing it up into several low-loss sections.
A European collaboration achieved free space QKD over 144 km between two of the Canary Islands using entangled photons (the Ekert scheme) in 2006,[21] and using BB84 enhanced with decoy states[22][23][24][25][26] in 2007.
Quantum based security against eavesdropping was validated for the deployed system at over 12 km (7.5 mi) range and 10 dB attenuation over fibre optic channel.
A continuous wave laser source was used to generate photons without depolarization effect and timing accuracy employed in the setup was of the order of picoseconds.
A free-space QKD was demonstrated at Space Applications Centre (SAC), Ahmedabad, between two line-of-sight buildings within the campus for video conferencing by quantum-key encrypted signals.
This process also creates two photons, which are then captured and transported using an optical fiber, at which point a Bell-basis measurement is performed and the ions are projected to a highly entangled state.
[6] A separate experiment published in July 2022 demonstrated implementation of DIQKD that also uses a Bell inequality test to ensure that the quantum device is functioning, this time at a much larger distance of about 400m, using an optical fiber 700m long.
The atoms are entangled by electronic excitation, at which point two photons are generated and collected, to be sent to the bell state measurement (BSM) setup.
[7] Since the proposal of Twin Field Quantum Key Distribution in 2018, a myriad of experiments have been performed with the goal of increasing the distance in a QKD system.
[42] In 2013, Battelle Memorial Institute installed a QKD system built by ID Quantique between their main campus in Columbus, Ohio and their manufacturing facility in nearby Dublin.
[46] Id Quantique has successfully completed the longest running project for testing Quantum Key Distribution (QKD) in a field environment.
The main goal of the SwissQuantum network project installed in the Geneva metropolitan area in March 2009, was to validate the reliability and robustness of QKD in continuous operation over a long time period in a field environment.
As in the classical case, Alice and Bob cannot authenticate each other and establish a secure connection without some means of verifying each other's identities (such as an initial shared secret).
The most promising solution is the decoy states[22][23][24][25][26] in which Alice randomly sends some of her laser pulses with a lower average photon number.
This idea has been implemented successfully first at the University of Toronto,[69][70] and in several follow-up QKD experiments,[71] allowing for high key rates secure against all known attacks.
In a recent research study it has been shown that Eve discerns Bob's secret basis choice with higher than 90% probability, breaching the security of the system.
Later on, the phase-remapping attack was also demonstrated on a specially configured, research oriented open QKD system (made and provided by the Swiss company Id Quantique under their Quantum Hacking program).
His seminal paper titled "Conjugate Coding" was rejected by IEEE Information Theory but was eventually published in 1983 in SIGACT News (15:1 pp.
In this paper he showed how to store or transmit two messages by encoding them in two "conjugate observables", such as linear and circular polarization of light, so that either, but not both, of which may be received and decoded.
A decade later, building upon this work, Charles H. Bennett, of the IBM Thomas J. Watson Research Center, and Gilles Brassard, of the University of Montreal, proposed a method for secure communication based on Wiesner's "conjugate observables".
This has the advantage of not being intrinsically distance limited, and despite long travel times the transfer rate can be high due to the availability of large capacity portable storage devices.
QKD (quantum key distribution) systems also have the advantage of being automatic, with greater reliability and lower operating costs than a secure human courier network.
An Industry Specification Group (ISG) of the European Telecommunications Standards Institute (ETSI) has been set up to address standardisation issues in quantum cryptography.
[92] European Metrology Institutes, in the context of dedicated projects,[93][94] are developing measurements required to characterise components of QKD systems.
This recognises Toshiba's pioneering QKD[95] technology developed over two decades of research, protecting communication infrastructure from present and future cyber-threats, and commercialising UK-manufactured products which pave the road to the quantum internet.