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Physical Review Letters

Adaptive Detection of Arbitrarily Shaped Ultrashort Quantum Light States

C. Polycarpou, K. N. Cassemiro, G. Venturi, Alessandro Zavatta, Marco Bellini

Published Aug 3, 2012• 70 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.109.053602

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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.
A photon is the single excitation of a particular spatiotemporal mode of the electromagnetic field. A precise knowledge of the mode structure is therefore essential for its processing and detection, as well as for applying generic quantum light states to novel technologies. Here we demonstrate an adaptive scheme for reconstructing the arbitrary amplitude and phase spectro-temporal profile of an ultrashort single-photon pulse. The method combines techniques from the fields of ultrafast coherent control and quantum optics to map the mode of a fragile quantum state onto that of an intense coherent field. In addition, we show that the possibility of generating and detecting quantum states in multiple spectro-temporal modes may serve as a basis for encoding qubits (and qudits) into single, broadband, ultrashort, photons. Providing access to a much larger Hilbert space, this scheme may boost the capacity of current quantum information protocols.
The ability to prepare, control, and detect high-dimensional photonic quantum states is a key capability in modern quantum science. Spatially structured single- and bi-photon states provide access to large Hilbert spaces, enabling both fundamental studies and practical advances across quantum technologies. Precise control of these states is essential to characterising quantum light sources and to using spatial degrees of freedom for information encoding, with applications ranging from quantum imaging and simulation to high-capacity, noise-resilient quantum communication. In this talk, I will review recent experimental and theoretical progress in structured-light quantum information processing. I will discuss high-dimensional QKD with structured photons, strategies based on adaptive optics and tailored measurements to counter channel distortions, and protocols that exploit spatial mode structure. I will also describe how structured photonic states can be used to simulate linear and cyclic quantum systems, illustrating their potential as scalable platforms for high-dimensional quantum information processing.
The ability to control and exploit quantum coherence and entanglement drives research across many fields ranging from ultra-cold quantum gases to spin systems in condensed matter. Transcending different physical systems, optical approaches have proven themselves to be particularly powerful, since they profit from the established toolbox of quantum optical techniques, are state-selective, contact-less and can be extremely fast. Here, we demonstrate how a precisely timed sequence of monochromatic ultrafast (~ 2-5 ps) optical pulses, with a well defined polarisation can be used to prepare arbitrary superpositions of exciton spin states in a semiconductor quantum dot, achieve ultrafast control of the spin-wavefunction without an applied magnetic field and make high fidelity read-out the quantum state in an arbitrary basis simply by detecting a strong (~ 2-10 pA) electric current flowing in an external circuit. The results obtained show that the combined quantum state preparation, control and read-out can be performed with a near-unity (≥97%) fidelity.
Summary form only given. Many realized or potential applications of optically driven ultrafast quantum dynamics require specially shaped quantum states, with the ability to detect these shapes and modify them using active feedback. We have developed methods to sculpt wave packets using shaped ultrafast optical pulses. We can also view these wave packets using quantum interference methods that have been adapted from holographic techniques used to characterize classical light. These quantum state holograms have been used as feedback to sculpt specific target shapes.
Temporal-spectral modes of light provide a fundamental window into the nature of quantum systems and offer a robust means for information encoding. Methods to precisely characterize the temporal-spectral state of light at the single-photon level thus play a central role in understanding single-photon sources and their applications in emerging optical quantum technologies. Here we demonstrate an optical reference-free method, which melds techniques from ultrafast metrology and single-photon spectral detection, to characterize the pulse-mode structure of heralded single photons.