Quantum Key Distribution with Higher-Order Alphabets Using Spatially Encoded Qudits

We present a proof of principle demonstration of a quantum key distribution scheme in higher-order $d$-dimensional alphabets using spatial degrees of freedom of photons. Our implementation allows for the transmission of 4.56 bits per sifted photon, while providing improved security: an intercept-resend attack on all photons would induce an average error rate of 0.47. Using our system, it should be possible to send more than a byte of information per sifted photon.

Figure 1

Illustration of QKD using imaging ($I$) and Fourier ($F$) optical systems.

Figure 2

Experimental setup.

Figure 3

Intensity distributions at Bob’s detection plane for the four lens configurations $II$, $IF$, $FI$, and $FF$ for the case when Alice sends the character 7. Here lighter squares correspond to a larger number of photocounts.

Figure 4

Normalized counts for Bob’s Fourier ($F$) detection system when Alice uses Fourier (left) and image (right) encoding.

Figure 5

Normalized counts for Bob’s image ($I$) detection system when Alice uses image (left) and Fourier (right) encoding.