Generation and manipulation of the quantum state of a single photon is at the heart of many quantum information protocols. There has been growing interest in using phase modulators as quantum optics devices that preserve coherence. In this Rapid Communication, we have used an electro-optic phase modulator to shape the state vector of single photons emitted by a quantum dot to generate new frequency components (modes) and explicitly demonstrate that the phase modulation process agrees with the theoretical prediction at a single-photon level. Through two-photon interference measurements we show that for an output consisting of three modes (the original mode and two sidebands), the indistinguishability of the mode engineered photon, measured through the second-order intensity correlation [g 2 (0)] is preserved. This work demonstrates a robust means to generate a photonic qubit or more complex state (e.g., a qutrit) for quantum communication applications by encoding information in the sidebands without the loss of coherence. DOI: 10.1103/PhysRevA.98.011802 Quantum communication and computing protocols often require a flexible and customizable single-photon source that can form a link between distant nodes [1][2][3]. Swapping entanglement between these nodes can be achieved utilizing the two-photon interference [Hong-Ou-Mandel (HOM)] measurements [4][5][6], where the optimal interference to assure indistinguishability requires that the spatial, temporal, polarization, and spectral modes of the input photon wave functions must be identical [6,7]. Thus manipulation of the photonic degrees of freedom while maintaining coherence is very important for many quantum information applications.Single photons also function for cryptographic key distributions to enable transfer of information between two remote parties [2,8], where quantum information is encoded in the various degrees of freedom of a single photon. Polarization qubits are typical, but are prone to decoherence when transmitted through a fiber [9][10][11]. Frequency qubits [12], on the other hand, are known to be robust against any mechanically or environmentally induced fluctuation in a fiber [13][14][15]. Frequency qubits can be generated through phase modulation of a single photon [14,16], where the information is encoded in the relative amplitude between the sidebands. Recently, Lukens and Lougovski proposed a universal linearoptical quantum computing (LOQC) platform using frequency components generated from an electro-optic modulators [14]. Similarly, there has been proof-of-concept demonstrations of the 1984 protocol of Bennett and Brassard (BB84) using phase-modulated weak coherent sources [13,17]. A quantum analysis of phase modulation was first discussed by Louisell, Yariv, and Siegman [18]. Frequency conversion is described as a two-mode coupling process with sinusoidal coupling between the unmodulated and the new frequency component. The coupling between the two modes is generated through a periodic perturbation of the refractive index of the medium ...