All-Optical Storage of Phase-Sensitive Quantum States of Light

We experimentally demonstrate storage and on-demand release of phase-sensitive, photon-number superposition states of the form $\alpha |0⟩+\beta e^{i\theta }|1⟩$ for an optical quantized oscillator mode. For this purpose, we newly developed a phase-probing mechanism compatible with a storage system composed of two concatenated optical cavities, which was previously employed for storage of phase-insensitive single-photon states [Phys. Rev. X 3, 041028 (2013)]. This is the first demonstration of all-optically storing highly nonclassical and phase-sensitive quantum states of light. The strong nonclassicality of the states after storage becomes manifest as a negative region in the corresponding Wigner function shifted away from the origin in phase space. This negativity is otherwise, without the phase information of the memory system, unobtainable. While our scheme includes the possibility of optical storage, on-demand release and synchronization of arbitrary single-rail qubit states, it is not limited to such states. In fact, our technique is extendible to more general phase-sensitive states such as multiphoton superposition or entangled states, and thus it represents a significant step toward advanced optical quantum information processing, where highly nonclassical states are utilized as resources.

Figure 1

Conceptual diagram of creation, storage, and on-demand release of phase-sensitive quantum states. The memory and shutter are implemented by concatenated cavities.

Figure 2

Experimental setup. The light source is a continuous-wave (cw) Ti:sapphire laser operating at 860 nm. Optical frequencies of beams are distinguished by colors: Red (gray), orange (light gray), and blue (dark gray) stand for signal, idler, and pump frequencies, respectively, while small detuning is not indicated by colors. Black lines stand for electronic signal lines. Here, SHG denotes a second harmonic generator, PPKTP a periodically poled ${\mathrm{KTiOPO}}_4$ crystal.

Figure 3

Quadrature distributions of $(|0⟩+|1⟩)/\sqrt{2}$ for various measurement phases. (a) Experimental quadrature distribution obtained by homodyne measurements, when the created states are immediately released. (b) Theoretical quadrature distribution for the ideal pure state.

Figure 4

Experimental results of storing $(|0⟩+|1⟩)/\sqrt{2}$ with various storage time. From left to right, the storage time varies from 0 to 400 ns in steps of 100 ns. First row: quadrature distributions obtained by homodyne measurements. Second row: absolute values of reconstructed density matrix elements. Red elements represent the subspace spanned by $|0⟩$ and $|1⟩$, and light blue elements represent the multiphoton components. Third row: Wigner functions $W(x,p)$ with $\hslash =1$ seen from the top, corresponding to the reconstructed density matrices. Fourth row: Wigner functions $W(x,p)$ seen from the side. Minimum values are $W(-0.84,0.09)=-0.024$ in 0 ns, $W(-0.88,-0.23)=-0.015$ in 100 ns, and $W(-0.84,-0.37)=-0.004$ in 200 ns, where statistical errors of the Wigner values are estimated as $\pm 0.001$.