Preserving photon qubits in an unknown quantum state with Knill dynamical decoupling: Towards an all optical quantum memory

The implementation of polarization-based quantum communication is limited by signal loss and decoherence caused by the birefringence of a single-mode fiber. We investigate the Knill dynamical decoupling scheme, implemented using half-wave plates, to minimize decoherence and show that a fidelity greater than $99\%$ can be achieved in the absence of rotation error and a fidelity greater than $96\%$ can be achieved in the presence of rotation error. Such a scheme can be used to preserve any quantum state with high fidelity and has potential application for constructing all optical quantum delay lines, quantum memory, and quantum repeaters.

Figure 1

Model of a single-mode optical fiber. (a) The fiber is modeled as concatenated segments of fiber of length $\Delta L$ with constant birefringence $\Delta n$. (b) Numerically reproduced birefringence in a single-mode fiber, where the scale parameter for Rayleigh distribution is $\sigma \beta =12.6$ deg/m [26].

Figure 2

A 20-pulse sequence created by combining 5-pulse blocks shifted in phase by $\pi /2$. The cyclic repetition of this 20-pulse sequence is referred to as KDD [22]. (a) Single-mode fiber of length $L$ divided into blocks of length $20\tau$. (b) Each block is then divided into four parts of length $5\tau$. (c) In each part a KDD block is implemented by introducing five half-wave plates at specified distances. (d) CPMG pulse implemented using half-wave plates.

Figure 3

Fidelity of DD sequences in a 500-m single-mode fiber with perfect pulses. (a) CPMG pulse sequence. (b) KDD pulse sequence.

Figure 4

Fidelity of DD sequences in a 500-m single-mode fiber with a $0.5\%$ pulse error. (a) CPMG pulse sequence. (b) KDD pulse sequence.

Figure 5

Fidelity of DD sequences in a 500-m single-mode fiber with a $1\%$ pulse error. (a) CPMG pulse sequence. (b) KDD pulse sequence.

Figure 6

Fidelity of DD sequences in a 1 km single-mode fiber with a $0.5\%$ pulse error. (a) CPMG pulse sequence. (b) KDD pulse sequence.