Squeezing-enhanced quantum key distribution over atmospheric channels

Abstract

We propose the Gaussian continuous-variable quantum key distribution using squeezed states in the composite channels including atmospheric propagation with transmittance fluctuations. We show that adjustments of signal modulation and use of optimal feasible squeezing can be sufficient to significantly overcome the coherent-state protocol and drastically improve the performance of quantum key distribution in atmospheric channels, also in the presence of additional attenuating and noisy channels. Furthermore, we consider examples of atmospheric links of different lengths, and show that optimization of both squeezing and modulation is crucial for reduction of protocol downtime and increase of secure atmospheric channel distance. Our results demonstrate unexpected advantage of fragile squeezed states of light in the free-space quantum key distribution applicable in daylight and stable against atmospheric turbulence.Quantum key distribution (QKD) [1-5] is one of the major practical applications of quantum information theory, which provides trusted parties (Alice and Bob) with the methods (protocols) for provably secure distribution of secret cryptographic keys so that security of the key can be verified using fundamental principles of quantum physics. One of the main requirements of QKD is the availability of a dedicated quantum channel capable of transmitting coherent quantum signals between the sending and receiving stations. In the case of fiber-optical channels, being the typical media for QKD implementations, this means a dedicated optical fiber, possibly with co-existing classical or quantum signals. However, the dedicated fiber-optical infrastructure can be unavailable, e.g., in the case of movable stations, necessity of quick channel deployment or in hostile environments. Moreover, the extra-long-distance inter-continental quantum communication over satellites relies on the free-space channels [6]. Therefore, the free-space channels are an important physical medium for QKD implementations.The main issue faced by the discrete-variable (DV) QKD protocols, based on single-photon states or weak coherent pulses and the direct photon counting, is the sensitivity of the detectors to the background light, which adds noise to the measured data. This renders standard DV QKD protocols practically unusable in the daylight conditions unless spectral filtering is applied, which adds unwanted additional loss and complexity to the set-up. At the same time, applicability and efficiency are crucial for QKD as they directly affect the secret communication, based on the quantum-secure keys. Alternatively, continuous-variable (CV) QKD protocols [7-10], based on the multiphoton coherent [11][12][13][14][15] or squeezed states [16] and homodyne quadrature detection using off-the-shelf equipment, can overcome this limitation. Indeed, a homodyne detector, which matches a signal to a narrow-band local oscillator (LO) beam, being the phase reference for the measurement, can intrinsically filter out the background radiation and make CV Q...

Feasibility of quantum key distribution with macroscopically bright coherent light

“…These tests, along with the numerical modeling, confirm the feasibility of quantum key distribution with macroscopically bright (intense and multimode) coherent states, which can be now fully implemented in real optical channels. Indeed, we show that key rates of about 0.25 bits per channel should be achievable with the states containing 10 4 photons at attenuation of 50%, which corresponds to a few kilometers long atmospheric link [33][34][35] (or 15 kilometers of a telecom fiber) and at local oscillator brightness of 10 6 photons, so that the key rate is only 15% reduced compared to the standard quantum key distribution with low-energy signals. In addition to increasing the LO brightness, the trusted parties can suppress the noise, concerned with the mode mismatch, by increasing the number of matched modes, which shows the potential of multiplexed continuous-variable quantum key distribution even in the case of join measurement of the multiple signal modes.…”

“…The results are given in Fig. 5 versus the LO brightness, as set in the experiment (left panel) and versus total signal beam brightness at the maximum reached LO brightness of 10 6 photons (right) at T = 0.5, which would correspond to a few kilometers long free-space channel [33][34][35] (or cca. 15 kilometers of the telecom fiber with attenuation of −0.2 dB/km).…”

Applicability of Squeezed- and Coherent-State Continuous-Variable Quantum Key Distribution over Satellite Links

“…In this regime, security can be established for every LEO satellite altitude and feasible squeezing (SNU) provides a noticeable rate gain. Note that under the individual attacks, the higher levels of squeezing always translate into a higher secure key rate, which is not the case for collective attacks where the transmittance fluctuations limit the applicable values of squeezing [ 66 ] and require active squeezing optimization based on the estimated atmospheric transmittance distribution. The optimization of the latter is a crucial step required for the establishing of secure key rate based on the data generated from a single pass of the satellite.…”

Fading channel estimation for free-space continuous-variable secure quantum communication

“…Therefore, if the channel is heavily fluctuating, then Var ( √ T ) and the respective effective excess noise will be large, which results in a low key rate or could even lead to a security break [35]. This can be partially compensated by the use of squeezed states [7] or by stabilizing the channel [36].…”