Two-photon Quantum State Research Articles

Experimental implementation of the multifunctional compact two-photon state analyzer

We report on experimental implementation of a multifunctional two-photon state analyzer. The device aims to be compact and able to provide several important characteristics about any two-photon quantum state. It operates in two modes: first mode is the two-photon interference analysis giving the information about spectral properties of the photons and the degree of mutual indistinguishability. The second mode provides polarization analysis and complete two-photon state tomography. Density matrix estimated from the tomography data reveals namely the quantum state purity or negativity. This device was tested on the photon pairs generated by the Kwiat source.

Two-photon transport in a waveguide coupled to a cavity in a two-level system

We consider a system where a waveguide is coupled to a cavity embedded with a two-level system (TLS), and study the effects when a two-photon quantum state is injected into the waveguide. The wave function of two outgoing photons is exactly solved using the Lehmann-Symanzik-Zimmermann (LSZ) reduction formalism. Our results explicitly exhibit the photon blockade effects in the strong atom-cavity coupling regime. The quantum statistical characters of the outgoing photons, including the photon bunching and anti-bunching behaviors, are also investigated in both the strong and weak coupling regimes. These results agree with the observations of recent experiments.

Quantum-state engineering using nonlinear optical Sagnac loops

The Kerr nonlinearity of an optical-fibre Sagnac loop can be utilized to engineer a variety of two-photon quantum states. These include correlated, identical photon pairs as well as degenerate, maximally entangled states—both of which are used in quantum information processing. In fact, their underlying principle—the reverse Hong–Ou–Mandel effect—can also be applied to free-space, down-conversion-based analogues of either identical or entangled photon-pair sources. Due to their simple structure, such versatile devices are expected to find widespread applications in quantum-state engineering.

Joint Temporal Density Measurements for Two-Photon State Characterization

We demonstrate a technique for characterizing two-photon quantum states based on joint temporal correlation measurements using time-resolved single-photon detection by femtosecond up-conversion. We measure for the first time the joint temporal density of a two-photon entangled state, showing clearly the time anticorrelation of the coincident-frequency entangled photon pair generated by ultrafast spontaneous parametric down-conversion under extended phase-matching conditions. The new technique enables us to manipulate the frequency entanglement by varying the down-conversion pump bandwidth to produce a nearly unentangled two-photon state that is expected to yield a heralded single-photon state with a purity of 0.88. The time-domain correlation technique complements existing frequency-domain measurement methods for a more complete characterization of photonic entanglement.

Experimental verification of differences between classical and quantum polarization properties

We have carried out polarization measurements on a two-photon quantum state generated by spontaneous parametric down conversion. Our measurements show that the state is unpolarized in the classical theory, but they also show that the state is not invariant under geometric rotation. Therefore, it is not unpolarized in the quantum theory. This is another example of a rather simple experiment that clearly shows the effects of quantum interference. It also confirms the theoretical investigation by Klyshko [Phys. Lett. A 163 (1992) 349]. The experiment is explained theoretically and a systematic treatment of polarization in quantum theory is outlined.

Ladder operator formalisms and generally deformed oscillator algebraic structures of quantum states in Fock space

We show that various kinds of one-photon quantum states studied in the field of quantum optics admit ladder operator formalisms and have the generally deformed oscillator (GDO) algebraic structure. The two-photon case is also considered. We obtain the ladder operator formalisms of two general states defined in the even/odd Fock space. The two-photon states may also have a GDO algebraic structure. Some interesting examples of one- and two-photon quantum states are given.

Tunable Bell-inequality violations by non-maximally-violating states in type-II parametric down-conversion.

We use a tunable quantum interference in type-II parametric down-conversion to construct two-photon quantum states that exhibit less-than-maximal, and tunable, violations of Bell-type inequalities. These states, no longer singlet-state analogs, possess an interference term of controllable magnitude that we adjust by tuning the measurement bandwidth. We show violations of two Bell-type inequalities in polarization variables, one more general than the other. We use these tunable violations to probe the threshold degree of interference that must be present to generate a violation, using an appropriate quantitative figure of merit.