Joint quantum measurement using unbalanced array detection

Measuring the Joint Spectral Mode of Photon Pairs Using Intensity Interferometry

The ability to manipulate and measure the time-frequency structure of quantum light is useful for information processing and metrology. Measuring this structure is also important when developing quantum light sources with high modal purity that can interfere with other independent sources. Here, we present and experimentally demonstrate a scheme based on intensity interferometry to measure the joint spectral mode of photon pairs produced by spontaneous parametric down-conversion. We observe correlations in the spectral phase of the photons due to chirp in the pump. We show that our scheme can be combined with stimulated emission tomography to quickly measure their mode using bright classical light. Our scheme does not require phase stability, nonlinearities, or spectral shaping and thus is an experimentally simple way of measuring the modal structure of quantum light.

Direct Tomography of High-Dimensional Density Matrices for General Quantum States of Photons

Quantum-state tomography is the conventional method used to characterize density matrices for general quantum states. However, the data acquisition time generally scales linearly with the dimension of the Hilbert space, hindering the possibility of dynamic monitoring of a high-dimensional quantum system. Here, we demonstrate a direct tomography protocol to measure density matrices of photons in the position basis through the use of a polarization-resolving camera, where the dimension of density matrices can be as large as 580×580 in our experiment. The use of the polarization-resolving camera enables parallel measurements in the position and polarization basis and as a result, the data acquisition time of our protocol does not increase with the dimension of the Hilbert space and is solely determined by the camera exposure time (on the order of 10 ms). Our method is potentially useful for the real-time monitoring of the dynamics of quantum states and paves the way for the development of high-dimensional, time-efficient quantum metrology techniques.

Compressive direct measurement of the quantum wave function

The direct measurement of a complex wave function has been recently realized by using weak values. In this Letter, we introduce a method that exploits sparsity for the compressive measurement of the transverse spatial wave function of photons. The procedure involves weak measurements of random projection operators in the spatial domain followed by postselection in the momentum basis. Using this method, we experimentally measure a 192-dimensional state with a fidelity of 90% using only 25 percent of the total required measurements. Furthermore, we demonstrate the measurement of a 19,200-dimensional state, a task that would require an unfeasibly large acquiring time with the standard direct measurement technique.

Adaptive detection of arbitrarily shaped ultrashort quantum light states

A quantum state of light is the excitation of a particular spatiotemporal mode of the electromagnetic field. A precise control of the mode structure is therefore essential for processing, detecting, and using photonic states in novel quantum technologies. Here we demonstrate an adaptive scheme, combining techniques from the fields of ultrafast coherent control and quantum optics, for probing the arbitrary complex spectrotemporal profile of an ultrashort quantum light pulse. The ability to access the modal structure of a quantum light state could boost the capacity of current quantum information protocols.

Shot noise limited characterization of ultraweak femtosecond pulse trains

Ultrafast science is inherently, due to the lack of fast enough detectors and electronics, based on nonlinear interactions. Typically, however, nonlinear measurements require significant powers and often operate in a limited spectral range. Here we overcome the difficulties of ultraweak ultrafast measurements by precision time-domain localization of spectral components. We utilize this for linear self-referenced characterization of pulse trains having ∼ 1 photon per pulse, a regime in which nonlinear techniques are impractical, at a temporal resolution of ∼ 10 fs. This technique does not only set a new scale of sensitivity in ultrashort pulse characterization, but is also applicable in any spectral range from the near-infrared to the deep UV.

Universal measurement of quantum correlations of radiation

A measurement technique is proposed which, in principle, allows one to observe the general space-time correlation properties of a quantized radiation field. Our method, called balanced homodyne correlation measurement, unifies the advantages of balanced homodyne detection with those of homodyne correlation measurements.

Correlations in atomic systems: diagnosing coherent superpositions

While investigating quantum correlations in atomic systems, we note that single measurements contain information about these correlations. Using a simple model of measurement-analogous to the one used in quantum optics-we show how to extract higher-order correlation functions from individual "photographs" of the atomic sample. As a possible application, we apply the method to detect a subtle phase coherence in mesoscopic superpositions.

Generation of entanglement between frequency bands by nonlinear fiber propagation and spectral pulse shaping

A novel fiber-based scheme for generating quadrature entanglement between a specific pair of frequency bands is proposed. The scheme is based on use of a nonlinear fiber, a spectral pulse shaper, and an adaptive feedback loop. It is numerically predicted that at least -5 dB of the quadrature entanglement will be created by preparation of an appropriate initial phase spectrum. Optimal control of the four-wave mixing terms of the Kerr Hamiltonian is crucial for improvement of the entanglement.