Broadband amplitude squeezing in a periodically poled KTiOPO_4 waveguide

Abstract

We generated −2.2 dB of broadband amplitude squeezing at 1064 nm in a periodically poled KTiOPO 4 (PP-KTP) waveguide by coupling of the fundamental and second-harmonic cw fields. This is the largest amount of squeezing obtained to date in a KTP waveguide, limited by propagation losses. This result paves the way for further improvements by use of lower-loss buried ion-exchanged waveguides. © 2009 Optical Society of America OCIS codes: 270.6570, 270.1670 The experimental implementation of continuousvariable (cv) quantum information [1], an ambitious and exciting endeavor requires the creation of strongly squeezed light. Squeezed light has been produced using a number of methods, but the most successful experiments to date (ranging from −9 to −10 dB of squeezing) have used optical parametric oscillators, which feature a second-order nonlinear material placed in a resonant optical cavity [2][3][4]. In such systems, intracavity losses are amplified by the resonator buildup and thus present a serious hindrance to increasing the squeezing level. It would therefore be beneficial to suppress the optical cavity by use of nonlinear optical waveguides in which the transverse field confinement yields an increase of the nonlinear efficiency that can make up for the buildup of a reasonably high finesse cavity [5,6]. If need be, some cavity modes can still be exquisitely well defined by seeding the nonlinear waveguide with an optical frequency comb [7,8]. Waveguides are ideally suited for applications, such as integrated circuits, owing to their small size [9] and could also help alleviate gain-induced diffraction, which has been seen with traveling waves in bulk crystals [10]. Moreover, removing the optical cavity yields an increase of the squeezing bandwidth by several orders of magnitude [11], which is of interest for fast quantum processing. Finally, comb-seeded waveguides are of great interest for a recently proposed method to generate massively scalable cv entanglement [12]. Over a decade ago, several experiments tried to exploit the increase in nonlinear efficiency that waveguides provide in an attempt to obtain large amounts of traveling-wave squeezing with pulsed inputs [13][14][15][16]. However, owing to propagation losses in the waveguide, these works achieved a maximum of −1.5 dB of squeezing. Recently, advances in waveguide fabrication techniques have allowed for better than −4 dB of pulsed traveling-wave squeezing in MgO-doped periodically poled LiNbO 3 (PPLN) waveguides [17]. Undoped PPLN waveguides were used to obtain squeezing and entanglement with a cw input [11]. Other media for nonlinear optical waveguides include quasi-phase-matched KTP [13], quasi-phase-matched LiTaO 3 (LT) [14], and periodically poled stoichiometric LT [18]. Although the first measurement of squeezed light from an optical waveguide was made in KTP [13], squeezing in KTP waveguides has not been explored in recent years. There are, however, a number of reasons to do so. High-squeezing experiments have been carried out in bulk KTP and PPKT...

30 years of squeezed light generation

“…By confining the pump field in the nonlinear crystal over a long distance by means of a crystalline waveguide structure (thereby circumventing diverging beams due to diffraction), it is possible to increase the effective nonlinearity even further. Using such systems single-pass pulsed squeezing [48][49][50] and even single-pass continuous wave squeezing has been observed [51,52].…”

Fabrication limits of waveguides in nonlinear crystals and their impact on quantum optics applications

“…Nonlinear optical processes enable complex manipulation of light and have been exploited extensively both in the classical and quantum regime for a wide variety of purposes, e.g. classical single-and multiple-channel frequency conversion [1,2], optical parametric amplification [3], generation of squeezed states and entangled photons [4][5][6], frequency conversion for single-photon detection [7][8][9] and to interface single photons with quantum memories [10][11][12]. Realizing nonlinear processes in integrated waveguides is fundamental in bringing quantum protocols and devices closer to every-day life [13].…”

Towards a quantum interface between telecommunication and UV wavelengths: design and classical performance

“…Potassium titanyl phosphate (KTP) is a material with a high photorefractive damage resistance. It allows not only the fabrication of short poling periods for QPM, but also the fabrication of high quality waveguides by rubidium ion exchange [25]. An efficient QFC between a telecommunication and a red wavelength has been realized in rubidium doped periodically poled KTP (Rb:PPKTP) waveguide [26].…”