Setting a disordered password on a photonic memory

An all-optical method of setting a disordered password on different schemes of photonic memory is theoretically studied. While photons are regarded as ideal information carriers, it is imperative to implement such data protection on all-optical storage. However, we wish to address the intrinsic risk of data breaches in existing schemes of photonic memory. We theoretically demonstrate a protocol using spatially disordered laser fields to encrypt data stored on an optical memory, namely, encrypted photonic memory. To address the broadband storage, we also investigate a scheme of disordered echo memory with a high fidelity approaching unity. The proposed method increases the difficulty for the eavesdropper to retrieve the stored photon without the preset password even when the randomized and stored photon state is nearly perfectly cloned. Our results pave ways to significantly reduce the exposure of memories, required for long-distance communication, to eavesdropping and therefore restrict the optimal attack on communication protocols. The present scheme also increases the sensitivity of detecting any eavesdropper and so raises the security level of photonic information technology.

Figure 1

(a) Disordered echo memory is composed of an atomic ensemble interacting with a spatially disordered field (colored surface) and a probe pulse (blue line) to be stored. The green randomly oriented arrows demonstrate the quantum phase evolution of memory atoms at various sections of the disorder. (b) The probability distribution of the Rabi frequency of a disordered field should cover (c), (f) the Fourier spectrum of the incident probe field. (d) Gradient echo memory is made of an atomic ensemble interacting with a linearly gradient field which introduces a spatially ordered phase modulation. (e) The equal probability distribution of the linearly gradient field strength is distinct from that in panel (f).

Figure 2

Panel (a) illustrates broadband disordered echo memory based on (b) a three-level $\mathrm{\Lambda }$-type scheme, and panel (c) depicts narrowband EIT memory based on (d) a four-level $N$-type system. Incident probe pulses (green dashed lines) driving transition $|1\rangle \leftrightarrow |3\rangle$ are encrypted by the disorder in panel (e), and decrypted by different keys in panels (f)–(h). The red solid lines depict the only successful retrieval with the correct key in panel (f). The blue long-dashed lines and gray dotted lines are the attempted retrievals by two wrong keys depicted respectively in panels (g) and (h). In panel (a) the access and encryption of a broadband probe are accomplished by the disorder, inverted at $t=0.22\tau$ for decryption, driving transition $|2\rangle \leftrightarrow |3\rangle$. In panel (c) the brown dashed dotted line illustrates the temporal sequence of control field for EIT storage. Two disordered pulses (orange dashed dotted dotted line), driving transition $|2\rangle \leftrightarrow |4\rangle$, are far detuned respectively with ${\mathrm{\Delta }}_d$ and $-{\mathrm{\Delta }}_d$ for, in turn, encryption and decryption.

Figure 3

The mean fidelity of the disordered echo memory with correlation length $\sigma =L/100$ of ${\mathrm{\Omega }}_c$ at various optical depths $\xi$ and control field strengths ${\mathbf{D}}_{\mathbf{c}}$. The results are obtained by averaging more than 1000 realizations of different disorder.

Figure 4

(a) Disordered encryption key with length $\alpha$ interacting with a memory of size $L$. (b) The decryption key is made of the inversion of an encryption key spatially shifted by $\delta$. The mean $\delta$-dependent normalized storage efficiency over 100 realizations for (c) disordered echo memory and (d) EIT-based memory with $|{\mathrm{\Delta }}_d|=500\mathrm{\Gamma }$. ${\mathbf{D}}_{\mathbf{c}}=1500\mathrm{\Gamma }$ and $\xi =1500$ are used in both panels (c) and (d). Three lines correspond to different correlation lengths of disorder $\sigma =L/100$ (red solid), $\sigma =L/50$ (green dashed), and $\sigma =L/30$ (blue dashed dotted).