Photon Qubit is Made of Two Colors

Abstract

T he discovery of the photon, the quantum particle of light, played a key role in the development of quantum physics. Today, photons are among the most advanced building blocks for quantum technologies, such as quantum computing [1], secure communication [2], and precision measurement [3]. These applications typically rely on quantum control of a photon's polarization or its spatial mode. Surprisingly, the most manifest property of light-its color or frequency-is difficult to manipulate on the quantum level. An experiment now demonstrates a toolbox for creating, manipulating, and detecting single photons in a quantum superposition of two discrete frequencies [4]. The approach requires an interaction between different frequency components of light, which Stéphane Clemmen from Cornell University, New York, and colleagues have achieved by making use of nonlinear processes in optical fibers. Such photonic quantum bits (qubits) could be useful for connecting quantum systems operating at different frequencies in a quantum network.According to quantum physics, monochromatic light of frequency ν, such as the light emitted by a laser, is composed of photons of energy E = hν, where h is the Planck constant. Polychromatic light, such as the light emitted by the Sun, contains photons of many different frequencies. However, each individual photon usually has a well-defined frequency and energy. Interestingly, the superposition principle of quantum physics allows for yet another version of polychromatic light: a single photon in a superposition of two discrete frequencies ν A and ν B . In this case, neither the frequency nor the energy of the photon is well defined. In some sense, such a "bichromatic" photon can be thought of as having two different colors at the same time, only one of which would be revealed if the photon were measured by a spectrometer or detected by eye.However, the creation and manipulation of bichromatic photons turns out to be challenging. The difficulty is that such processes require an interaction between photons of different frequencies, and in most media, light beams do not interact. The situation changes if light propagates in a non- * Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, SwitzerlandFigure 1: The illustration shows the conversion of a photon of one frequency, or color, into a photon that is in a superposition of two colors, and the subsequent verification of the photon's coherence with Ramsey spectroscopy. (Bottom) A single photon of frequency ν A (red) is sent through a nonlinear optical fiber, converting it to a superposition of ν A and another frequency ν B (blue). An adjustable delay changes the relative phase of the two frequency components in the superposition before they are mixed again. The resulting state is analyzed by separating the two frequency components in a demultiplexer and detecting the single photon, thereby projecting it onto one of its two colors. (Top) A vector on the Bloch sphere with polar angle θ and azimuthal angle φ represents ...

Single-Photon Interference due to Motion in an Atomic Collective Excitation

“…The single excitation is emitted as a single photon with two frequencies. At present there is much interest in these 'bi-chromatic' photons as they could be used to entangle spatially separated quantum memories or perform spectroscopy with small numbers of photons [23,24]. Combining the application of a large magnetic field and strong laser-dressing in a velocity selective ladder-type excitation, we demonstrate excellent control over the state preparation.…”

Frequency domain Stern–Gerlach effect for photonic qubits and qutrits

“…These generalizations can be done with efficient scalability, as the effect still requires only a single optical element, in contrast to what previous χ 2 realizations [2,20] can offer. Since frequency-domain qudits are recently gaining attention as possible candidates for carrying quantum information [2][3][4][5][6][7][8][9][10]20], we are hopeful that this work will help promote such future advancements.…”

“…Entangled photon pairs and squeezing are the most common examples [1], though much attention has also been given to unitary operations on frequency-domain states [2][3][4][5][6][7][8][9][10]. Sum-frequency generation (SFG) [2], four-wave mixing [3][4][5][6][7], and electro-optic modulation [8][9][10] were employed for the dynamic coupling between different frequencies, allowing for high-fidelity spectral-to-spectral (i.e., where both the input and output are in the same spatial mode) quantum gates [10]. The recent interest in these photonic states as quantum information carriers is motivated mainly by their potentially high dimensionality and the ability to transmit them over long distances in optical fibers and in free space.…”