Measuring photon antibunching from continuous variable sideband squeezing

Generation of Long-Term Stable Squeezed Vacuum States Using Dither-Locking Technique

We report the generation of long-term stable squeezed vacuum states at 1064 nm using a degenerate optical parametric amplifier (DOPA) with a periodically poled KTiOPO4 crystal (PPKTP). The OPA is pumped by a 532 nm light produced by frequency doubling the fundamental light with a bow-tie enhancement second harmonic generator (SHG). When the DOPA and relative phases are locked using a dither-locking method, the squeezed vacuum states are stably measured over 2 h at 11 MHz. The highly compact and simple squeezed light source is suitable for applications in quantum optics experiments.

High-order continuous-variable coherence of phase-dependent squeezed state

We study continuous variable coherence of phase-dependent squeezed state based on an extended Hanbury Brown-Twiss scheme. High-order coherence is continuously varied by adjusting squeezing parameter r, displacement α, and squeezing phase θ. We also analyze effects of background noise γ and detection efficiency η on the measurements. As the squeezing phase shifts from 0 to π, the photon statistics of the squeezed state continuously change from the anti-bunching (g⁽ⁿ⁾ < 1) to super-bunching (g⁽ⁿ⁾ > n!) which shows a transition from particle nature to wave nature. The experiment feasibility is also examined. It provides a practical method to generate phase-dependent squeezed states with high-order continuous-variable coherence by tuning squeezing phase θ. The controllable coherence source can be applied to sensitivity improvement in gravitational wave detection and quantum imaging.

Observation of the Unconventional Photon Blockade in the Microwave Domain

We have observed the unconventional photon blockade effect for microwave photons using two coupled superconducting resonators. As opposed to the conventional blockade, only weakly nonlinear resonators are required. The blockade is revealed through measurements of the second order correlation function g^{(2)}(t) of the microwave field inside one of the two resonators. The lowest measured value of g^{(2)}(0) is 0.4 for a resonator population of approximately 10^{-2} photons. The time evolution of g^{(2)}(t) exhibits an oscillatory behavior, which is characteristic of the unconventional photon blockade.

Finite-time quantum entanglement in propagating squeezed microwaves

Two-mode squeezing is a fascinating example of quantum entanglement manifested in cross-correlations of non-commuting observables between two subsystems. At the same time, these subsystems themselves may contain no quantum signatures in their self-correlations. These properties make two-mode squeezed (TMS) states an ideal resource for applications in quantum communication. Here, we generate propagating microwave TMS states by a beam splitter distributing single mode squeezing emitted from distinct Josephson parametric amplifiers along two output paths. We experimentally study the fundamental dephasing process of quantum cross-correlations in continuous-variable propagating TMS microwave states and accurately describe it with a theory model. In this way, we gain the insight into finite-time entanglement limits and predict high fidelities for benchmark quantum communication protocols such as remote state preparation and quantum teleportation.

Periodic squeezing in a polariton Josephson junction

The use of a Kerr nonlinearity to generate squeezed light is a well-known way to surpass the quantum noise limit along a given field quadrature. Nevertheless, in the most common regime of weak nonlinearity, a single Kerr resonator is unable to provide the proper interrelation between the field amplitude and squeezing required to induce a sizable deviation from Poissonian statistics. We demonstrate experimentally that weakly coupled bosonic modes allow exploration of the interplay between squeezing and displacement, which can give rise to strong deviations from the Poissonian statistics. In particular, we report on the periodic bunching in a Josephson junction formed by two coupled exciton-polariton modes. Quantum modeling traces the bunching back to the presence of quadrature squeezing. Our results, linking the light statistics to squeezing, are a precursor to the study of nonclassical features in semiconductor microcavities and other weakly nonlinear bosonic systems.

Here, dynamical light squeezing is observed in the emission from two coupled weakly nonlinear populations of exciton-polaritons through ultrafast time-resolved second-order correlation measurements. Full quantum simulations link the squeezing to the periodic bunching observed in the light statistics.

Direct sampling of electric-field vacuum fluctuations

The ground state of quantum systems is characterized by zero-point motion. This motion, in the form of vacuum fluctuations, is generally considered to be an elusive phenomenon that manifests itself only indirectly. Here, we report direct detection of the vacuum fluctuations of electromagnetic radiation in free space. The ground-state electric-field variance is inversely proportional to the four-dimensional space-time volume, which we sampled electro-optically with tightly focused laser pulses lasting a few femtoseconds. Subcycle temporal readout and nonlinear coupling far from resonance provide signals from purely virtual photons without amplification. Our findings enable an extreme time-domain approach to quantum physics, with nondestructive access to the quantum state of light. Operating at multiterahertz frequencies, such techniques might also allow time-resolved studies of intrinsic fluctuations of elementary excitations in condensed matter.

Photon number distributions from a diode laser

We use balanced homodyne detection to characterize light from a diode laser as it crosses the threshold. We measure the single-time second-order correlation function g(2), and also extract the photon number distribution. Just above the laser threshold, we find that the measured g(2) exceeds the prediction of the semiclassical single-mode laser model. From the reconstructed photon number distributions, we conclude that this excess is due to emission from nonlasing modes. For higher pumping current, the light noise increases due to a different mechanism, possibly mode competition or mode partition noise.

Generation of nonclassical microwave states using an artificial atom in 1D open space

We have embedded an artificial atom, a superconducting transmon qubit, in a 1D open space and investigated the scattering properties of an incident microwave coherent state. By studying the statistics of the reflected and transmitted fields, we demonstrate that the scattered states can be nonclassical. In particular, by measuring the second-order correlation function, g((2)), we show photon antibunching in the reflected field and superbunching in the transmitted field. We also compare the elastically and inelastically scattered fields using both phase-sensitive and phase-insensitive measurements.

Photon extrabunching in ultrabright twin beams measured by two-photon counting in a semiconductor

For many years twin beams originating from parametric down-converted light beams have aroused great interest and attention in the photonics community. One particular aspect of the twin beams is their peculiar intensity correlation functions, which are related to the coincidence rate of photon pairs. Here we take advantage of the huge bandwidth offered by two-photon absorption in a semiconductor to quantitatively determine correlation functions of twin beams generated by spontaneous parametric down-conversion. Compared with classical incoherent sources, photon extrabunching is unambiguously and precisely measured, originating from exact coincidence between down-converted pairs of photons, travelling in unison. These results strongly establish that two-photon counting in semiconductors is a powerful tool for the absolute measurement of light beam photon correlations at ultrashort timescales.

Giant Kerr nonlinearities in circuit quantum electrodynamics

The very small size of optical nonlinearities places strict restrictions on the types of novel physics one can explore. This work describes how a single artificial multilevel Cooper pair box molecule, interacting with a superconducting microwave coplanar resonator, when suitably driven, can generate extremely large optical nonlinearities at microwave frequencies, with no associated absorption. We describe how the giant self-Kerr effect can be detected by measuring the second-order correlation function and quadrature squeezing spectrum.

Electronic noise-free measurements of squeezed light

We study the implementation of a correlation measurement technique for the characterization of squeezed light. We show that the sign of the covariance coefficient revealed from the time-resolved correlation data allow us to distinguish among squeezed, coherent, and thermal states. In contrast to the traditional method of characterizing squeezed light, involving measurement of the variation of the difference photocurrent, the correlation measurement method allows one to eliminate the contribution of the electronic noise, which becomes a crucial issue in experiments with dim sources of squeezed light.

Violation of the Cauchy-Schwarz inequality in the macroscopic regime

We have observed a violation of the Cauchy-Schwarz inequality in the macroscopic regime by more than 8 standard deviations. The violation has been obtained while filtering out only the low-frequency noise of the quantum-correlated beams that results from the technical noise of the laser used to generate them. We use bright intensity-difference squeezed beams produced by four-wave mixing as the source of the correlated fields. We also demonstrate that squeezing does not necessarily imply a violation of the Cauchy-Schwarz inequality.