Quantum-optical state engineering

About a century ago, Max Planck introduced quantization of light to explain the spectrum of blackbody radiation. The first experimental confirmation of non-classical features of the light field came almost 60 years later with measurements of photon correlations, and over 20 more years passed before we learned to controllably generate first nonclassical states of light, such as single photons and squeezed vacuum. After these observations, over the past two and a half decades, the field of quantum optics grew rapidly. Optics has become a playground for testing fundamental concepts of quantum mechanics, such as entanglement, measurement, nonlocality and decoherence. With the beginning of the present century, quantum optics has given rise to applications in quantum information technology, such as quantum communication, metrology and computation.

Still, our ability to generate quantum states of light is strongly limited. Until recently, we could only produce very basic states: single photons, entangled photon pairs, squeezed and quadrature entangled states. However, the past few years have seen a technology boom, resulting in a plethora of new quantum optical states produced and measured: single- and dual-rail optical qubits, displaced Fock states, photon-added states, "Schrödinger cats", and many others. Yet the "holy grail" – an ability to synthesize any arbitrary state of the electromagnetic field is not yet achieved. This is a primary vision of the present project. At the same time, we are working on various other tasks in continuous-variable quantum-information processing, such as entanglement purification, simulation of optical nonlinearity as well as heralded linear-optical quantum computation.

How do we generate quantum states of light? We begin with parametric down-conversion – a nonlinear optical effect that can produce squeezed light (deterministically) or pairs of single photons (spontaneously). This state is our "blank", which we process further to shape the state we need. This processing is done by conditional measurements performed in a clever way. For example, in order to obtain a single photon from a spontaneously generated photon pair, we detect one photon in a pair, and then we know the other photon has been generated as well. Increasingly complex optical states can be engineered by more complicated measurements, such as, for example, in our recent work on preparing arbitrary superpositions of 0-, 1-, and 2-photon states for the first time.

In addition to employing parametric down-conversion in crystals, we are developing a novel photon source based on four-wave mixing in hot atomic vapors. This technique features a very high photon production rate combined with very low losses and excess noise. Its further advantage is the narrowband character of the produced photons, permitting compatibility with our project on light-atom interface, particularly, the storage of light in atomic ensembles.

Research on quantum engineering requires reliable means of measuring the states we prepared. These means are provided by homodyne tomography. In our quest for fast and precise state measurement, we developed new homodyne detectors that can also be successfully used in continuous-variable quantum cryptography. With these detectors, improvement of secure key transmission rates by a factor of 50-100 compared with existing systems is possible. This project is underway in collaboration with the University of Toronto. Once it is sufficiently advanced, commercialization of such systems is possible, opening up a new niche in the quantum cryptography market.

Our review paper on quantum state engineering and continuous-variable tomography