We present an all-optical control device in which coupling a weak control optical field into a high-$Q$ lithium niobate whispering-gallery-mode microcavity decouples it from a signal field due to nonlinear optical interactions. This results in switching and modulation of the signal with low-power control pulses. In the quantum limit, the underlying nonlinear-optical process corresponds to the quantum Zeno blockade. Its ``interaction-free'' nature effectively alleviates loss and decoherence for the signal waves. This work therefore presents experimental progress towards acquiring large phase shifts with few photons or even at the single-photon level.
We present an all-optical control device in which coupling a weak control optical field into a high-Q lithium niobate whispering-gallery-mode microcavity decouples it from a signal field due to nonlinear optical interactions. This results in switching and modulation of the signal with low-power control pulses. In the quantum limit, the underlying nonlinear-optical process corresponds to the quantum Zeno blockade. Its ``interaction-free'' nature effectively alleviates loss and decoherence for the signal waves. This work therefore presents experimental progress towards acquiring large phase shifts with few photons or even at the single-photon level.
Quantum Zeno blockade allows all-optical switching in a counterintuitive “interaction-free” manner. Using a lithium-niobate microdisk cavity nanofabricated on chip, we have observed phase matched sum-frequency generation and interaction-free switching between optical pulses in two tightly confined whispering-gallery modes. We have also verified that our device is suitable for nonlinear operations in a single-photon regime. Our results point to a scalable, chip-integrated platform for nonlinear optics extendable to the quantum regime.
Over the last two decades, the emergence of quantum information science has uncovered many practical applications in areas such as communications, imaging, and sensing where harnessing quantum features of Nature provides tremendous benefits over existing methods exploiting classical physical phenomena. In this effort, one of the frontiers of research has been to identify and utilize quantum phenomena that are not susceptible to environmental and parasitic noise processes. Quantum photonics has been at the forefront of these studies because it allows room-temperature access to its inherently quantum-mechanical features, and allows leveraging the mature telecommunication industry. Accompanying the weak environmental influence, however, are also weak optical nonlinearities. Efficient nonlinear optical interactions are indispensible for many of the existing protocols for quantum optical computation and communication, e.g. high-fidelity entangling quantum logic gates rely on large nonlinear responses at the one- or few-photon-level. While this has been addressed to a great extent by interfacing photons with single quantum emitters and cold atomic gases, scalability has remained elusive. In this work, we identify the macroscopic second-order nonlinear polarization as a robust platform to address this challenge, and utilize the recent advances in the burgeoning field of optical microcavities to enhance this nonlinear response. In particular, we show theoretically that by using the quantum Zeno effect, low-noise, single-photon-level optical nonlinearities can be realized in lithium niobate whispering-gallery-mode microcavities, and present experimental progress toward this goal. Using the measured strength of the second-order nonlinear response in lithium niobate, we modeled the nonlinear system in the strong coupling regime using the Schrödinger picture framework and theoretically demonstrated that the single-photon-level operation can be observed for cavity lifetimes in excess of 500 ns for all the three waves in the interaction, provided a cavity of radius R < 10 μm is fabricated. Experimentally, we showed that the absorption-limited quality ( Q) factors for lithium niobate, Qintrinsic ≈ 108, can be achieved using diamond-turning methods for disk radii, R > 100 μm, whereas for the smaller disks, additional rigorous polishing may be required. We also fabricated resonators as small as R ∼ 40 μm via this method. In a millimeter-sized resonator, we experimentally demonstrated triply resonant sum-frequency generation, which allowed for an observation of the classical manifestation of the quantum Zeno effect, wherein line-splitting occurs due to the high efficiency intracavity frequency conversion. For the sub-100 μm resonators, we present phase-matching calculations and dispersion-management techniques using analytical approximations and rigorous finite-element-method simulations. Experimentally, Q -factor measurements are shown, and we identify the specific short-comings of the fabrication procedure that may have led to the lower, surface-roughness-limited Q-factors. Finally, we identify pathways toward achieving the single-photon-level nonlinear optics using off-resonant nonlinear optics, which requires the simultaneous realization of phase-matching, large cavity lifetimes, and small mode volumes. We believe this would be feasible in the near future as more advanced fabrication and processing methods are developed for crystalline materials and novel nonlinear crystals are synthesized.
When overlapping in an optical medium with nonlinear susceptibility, light waves can interact with each other, changing their phases, wavelengths, shapes, and so on. Such nonlinear effects, discovered over a half century ago, have given rise to a breadth of important applications. Applying to quantum-mechanical signals, however, they face fundamental challenges arising from the multimode nature of the interacting electromagnetic fields, such as phase noises and Raman scattering. Quantum Zeno blockade allows strong interaction of light waves without them physically overlapping, thus providing a viable solution for those challenges, as indicated in recent bulk-optics experiments. Here, we report on the observation of quantum Zeno blockade on chip, where a light wave is modulated by another in a distinct "interaction-free" manner. For quantum applications, we also verify its operations on a single-photon level. Our results promise a scalable platform for overcoming several grand challenges faced by nonlinear optics and quantum information processing, enabling, e.g., manipulation and interaction of quantum signals without decoherence.
Abstract : All-optical switching is crucial for efficient optical communications. The quantum Zeno effect can be used to induce or inhibit optical switching through a variety of processes, all of which share the common characteristic that the evolution of one quantum state into another be inhibited or induced based on the presence or absence of a controllable and continuous quantum measurement. By utilizing this effect, it is the goal of this project to design and construct ultra-low power dissipation, ultra-low volume, fast all-optical switches. In particular, a fundamental limit in existing all-optical switching devices is due to the photon loss resulting from signal-pump coupling. When extended to the quantum domain, this photon loss additionally causes the quantum states of photonic signals to collapse (i.e., decohere). By using the quantum Zeno effect (QZE), it is possible to realize interaction-free switching that eliminates direct signal-pump coupling i.e., QZE for all-optical switching.
We propose an interaction-free scheme for all-optical switching which does not rely on the physical coupling between signal and control waves. The interaction-free nature of the scheme allows it to overcome the fundamental photon-loss limit imposed by the signal-pump coupling. The same phenomenon protects photonic-signal states from decoherence, making devices based on this scheme suitable for quantum applications. Focusing on $\ensuremath{\chi}{}^{(2)}$ waveguides, we provide device designs for traveling-wave and Fabry-Perot switches. In both designs, the performance is optimal when the signal switching is induced by coherent dynamical evolution. In contrast, when the switching is induced by a rapid dissipation channel, it is less efficient.
Realizing optical-nonlinear effects at a single-photon level is a highly desirable but also extremely challenging task, because of both fundamental and practical difficulties. We present an avenue to surmounting these difficulties by exploiting quantum Zeno blockade in nonlinear optical systems. Considering specifically a lithium-niobate microresonator, we find that a deterministic phase gate can be realized between single photons with near-unity fidelity. Supported by established techniques for fabricating and operating such devices, our approach can provide an enabling tool for all-optical applications in both classical and quantum domains.