Atmospheric continuous-variable quantum communication

Abstract

We present a quantum communication experiment conducted over a point-topoint free-space link of 1.6 km in urban conditions. We study atmospheric influences on the capability of the link to act as a continuous-variable (CV) quantum channel. Continuous polarization states (that contain the signal encoding as well as a local oscillator (LO) in the same spatial mode) are prepared and sent over the link in a polarization multiplexed setting. Both signal and LO undergo the same atmospheric fluctuations. These are intrinsically autocompensated which removes detrimental influences on the interferometric visibility. At the receiver, we measure the Q-function and interpret the data using the framework of effective entanglement (EE). We compare different state amplitudes and alphabets (two-state and four-state) and determine their optimal working points with respect to the distributed EE. Based on the high entanglement transmission rates achieved, our system indicates the high potential of atmospheric links in the field of CV quantum key distribution. 6 Contributed equally to this work.Quantum communication refers to the distribution of quantum states between two parties via a quantum channel. With regard to this, it is crucial that this quantum channel preserves the quantum properties of the distributed states. The most common channel implementations are optical fibers and free space. The latter offers great flexibility in terms of infrastructure establishment and links to moving objects are also feasible, see e.g. [1]. For a review of representative free-space quantum communication experiments see [2].Quantum key distribution (QKD) [3,4] is probably the most practical branch of quantum communication and concerns the establishment of a secret key jointly between two legitimate parties, Alice and Bob. As the security is based on the laws of quantum mechanics, in principle information theoretic-security can be achieved [4].Free-space QKD over a real atmospheric channel was first demonstrated in 1996 [5]. Since then, a number of prepare-and-measure as well as entanglement based schemes have been implemented in free space; the longest distance so far achieved on Earth is 144 km [6,7]. Nowadays, even quantum communication between earth and space is being conceived [8][9][10][11][12][13][14]. All of the systems so far referred to here have one major aspect in common: they are based on discrete quantum variables and use single photon threshold ('click') detectors, which involves spatial, spectral and/or temporal filtering in order to reduce background noise (see e.g. [15,16] for the first point-to-point demonstrations of free-space QKD in daylight).As already shown by Bennett [17], any two non-orthogonal quantum states suffice to ensure secure key distribution. This paved the way for continuous-variable (CV) protocols (for a review see [18,19]), based on a different approach: performing homodyne measurements on weak coherent states with the help of a bright local oscillator (LO). Generally, homodyne detectors offer immunity to st...

Optimally cloned binary coherent states

“…The transformations invoked by the optimal binary qubit state cloning procedure, see Eq. (12), are applied, and finally the transformed density matrix is mapped back to the infinite dimensional Hilbert space of the coherent-state basis in which we study the characteristics of the clones.…”

“…They are readily produced, loss tolerant, and have proven to be ideal signal carriers in conventional telecommunication [9]. Moreover, coherent states are of outstanding importance in novel applications such as quantum repeaters [10,11], quantum communication [12], or quantum key distribution (QKD) [13,14]. In contrast to qubit states, the underlying Hilbert space of coherent states is infinite dimensional.…”

Quantum measurements of signals from the Alphasat TDP1 laser communication terminal

“…MPL Erlangen implements free space CV quantum communication (see Fig. 1) based on binary phase-shift keying of coherent states and homodyne detection of the optical field quadratures [46][47][48][49][50]. This operating principle is equal to the one of the TESAT/DLR LCTs [51][52][53].…”

High-speed free-space optical continuous-variable quantum key distribution enabled by three-dimensional multiplexing

“…QKD has been recognized as the only approach so far to realize secure communications, guaranteed by the laws of quantum mechanics. QKD implementations can be built via optical fibers [3,4] or free-space optical (FSO) links [5][6][7]. The latter one has attracted increasing attentions because of its great flexibility for infrastructure establishment.…”