Measurement induced nonlinearity is a powerful tool in quantum optics to prepare non-classical states of light. We apply this concept to prepare a vibrational mode in a non-classical state with sub-Poissonian statistics in a diamond crystal at room temperature, using a scheme that is applicable to any Raman active mode. Upon detection of a single Stokes-shifted photon under pulsed excitation, quantum mechanics predicts that the wave-function describing the correlated vibrational mode collapses into its first excited energy eigenstate (Fock state $n=1$). We prove this by mapping the vibrational state onto an anti-Stokes photon, on which we perform a Hanbury-Brown and Twiss intensity interference measurement. Our data reveals deep sub-Poissonian statistics conditioned on the success of the Stokes measurement, and provides the first demonstration of this effect with a mechanical vibration in ambient conditions. Our work is also a proof-of-principle demonstration of a broadly tunable heralded single photon source with integrated short-lived quantum memory.
Measurement induced nonlinearity is a powerful tool in quantum optics to prepare non-classical states of light. We apply this concept to prepare a vibrational mode in a non-classical state with sub-Poissonian statistics in a diamond crystal at room temperature, using a scheme that is applicable to any Raman active mode. Upon detection of a single Stokes-shifted photon under pulsed excitation, quantum mechanics predicts that the wave-function describing the correlated vibrational mode collapses into its first excited energy eigenstate (Fock state n=1). We prove this by mapping the vibrational state onto an anti-Stokes photon, on which we perform a Hanbury-Brown and Twiss intensity interference measurement. Our data reveals deep sub-Poissonian statistics conditioned on the success of the Stokes measurement, and provides the first demonstration of this effect with a mechanical vibration in ambient conditions. Our work is also a proof-of-principle demonstration of a broadly tunable heralded single photon source with integrated short-lived quantum memory.
A single quantum of excitation of a mechanical oscillator is a textbook example of the principles of quantum physics. Mechanical oscillators, despite their pervasive presence in nature and modern technology, do not generically exist in an excited Fock state. In the past few years, careful isolation of GHz-frequency nano-scale oscillators has allowed experimenters to prepare such states at milli-Kelvin temperatures. These developments illustrate the tension between the basic predictions of quantum mechanics that should apply to all mechanical oscillators existing even at ambient conditions, and the complex experiments in extreme conditions required to observe those predictions. We resolve the tension by creating a single Fock state of a vibration mode of a crystal at room temperature using a technique that can be applied to any Raman-active system. After exciting a bulk diamond with a femtosecond laser pulse and detecting a Stokes-shifted photon, the 40~THz Raman-active internal vibrational mode is prepared in the Fock state ∣1> with 98.5% probability. The vibrational state is read out by a subsequent pulse, which when subjected to a Hanbury-Brown-Twiss intensity correlation measurement reveals the sub-Poisson number statistics of the vibrational mode. By controlling the delay between the two pulses we are able to witness the decay of the vibrational Fock state over its 3.9 ps lifetime at room temperature. Our technique is agnostic to specific selection rules, and should thus be applicable to any Raman-active medium, opening a new generic approach to the experimental study of quantum effects related to vibrational degrees of freedom in molecules and solid-state systems.
With the development of quantum optics, photon correlations acquired a prominent role as a tool to test our understanding of physics, and played a key role in verifying the validity of quantum mechanics. The spatial and temporal correlations in a light field also reveal information about its origin, and allow us to probe the nature of the physical systems interacting with it. Additionally, with the advent of quantum technologies, they have acquired technological relevance, as they are expected to play an important role in quantum communication and quantum information processing. This thesis develops techniques that combine spontaneous Raman scattering with Time Correlated Single Photon Counting, and uses them to study the quantum mechanical nature of high frequency vibrations in crystals and molecules. We demonstrate photon bunching in the Stokes and anti-Stokes fields scattered from two ultrafast laser pulses, and use their cross-correlation to measure the 3.9 ps decay time of the optical phonon in diamond. We then employ this method to measure molecular vibrations in CS2, where we are able to excite the respective vibrational modes of the two isotopic species present in the sample in a coherent superposition, and observe quantum beating between the two signals. Stokes scattering, when combined with a projective measurement, leads to a well defined quantum state. We demonstrate this by measuring the second order correlation function of the anti-Stokes field conditional on detecting one or more photons in the Stokes field, which allows us to observe a phonon modeâ s transition form a thermal state into the first excited Fock state, and measure its decay over the characteristic phonon lifetime. Finally, we use this technique to prepare a highly entangled photon-phonon state, which violates a Bell-type inequality. We measure S = 2.360 ± 0.025, violating the CHSH inequality, compatible with the non-locality of the state. The techniques we developed open the door to the study of a broad range of physical systems, where spectroscopic information is obtained with the preparation of specific quantum states. They also hold potential for future technological use, and promote vibrational Raman scattering to a resource in nonlinear quantum optics -- where it used to be considered as a source of noise instead.
Spontaneous vibrational Raman scattering is a ubiquitous form of light-matter interaction whose description necessitates quantization of the electromagnetic field. It is usually considered as an incoherent process because the scattered field lacks any predictable phase relationship with the incoming field. When probing an ensemble of molecules, the question therefore arises: What quantum state should be used to describe the molecular ensemble following spontaneous Stokes scattering? We experimentally address this question by measuring time-resolved Stokes-anti-Stokes two-photon coincidences on a molecular liquid consisting of several sub-ensembles with slightly different vibrational frequencies. When spontaneously scattered Stokes photons and subsequent anti-Stokes photons are detected into a single spatiotemporal mode, the observed dynamics is inconsistent with a statistical mixture of individually excited molecules. Instead, we show that the data are reproduced if Stokes-anti-Stokes correlations are mediated by a collective vibrational quantum, i.e. a coherent superposition of all molecules interacting with light. Our results demonstrate that the degree of coherence in the vibrational state of the liquid is not an intrinsic property of the material system, but rather depends on the optical excitation and detection geometry.
We propose and demonstrate a versatile technique to measure the lifetime of the one-phonon Fock state using two-color pump-probe Raman scattering and spectrally resolved, time-correlated photon counting. Following pulsed laser excitation, the n=1 phonon Fock state is probabilistically prepared by projective measurement of a single Stokes photon. The detection of an anti-Stokes photon generated by a second, time-delayed laser pulse probes the phonon population with subpicosecond time resolution. We observe strongly nonclassical Stokes-anti-Stokes correlations, whose decay maps the single phonon dynamics. Our scheme can be applied to any Raman-active vibrational mode. It can be modified to measure the lifetime of n≥1 Fock states or the phonon quantum coherences through the preparation and detection of two-mode entangled vibrational states.
This thesis presents demonstrations of the storage and manipulation of single photons in \na room-temperature diamond quantum memory using a Raman memory protocol. We \nreport on results from four experiments. \n \nIn the first we demonstrate single photon storage and, upon retrieval, verify the quantum \nnature of the light with a Hanbury Brown Twiss measurement of g^(2)(0) = 0.65±0.07. \nA measurement of g^(2)(0) < 1 is indicative of quantum light. This is the first demonstration \nof single photon storage where the bandwidth of the stored light is greater than 1 THz. The \ndiamond memory stores light for over 13 times the duration of the input wavepacket. In \nthe second experiment, we report the storage and retrieval of polarization-encoded qubits \nand demonstrate qubit storage above a classical bound. We also verify that entanglement \nbetween the input photon and an auxiliary persists through storage and retrieval. \nWe then turn to additional uses of a Raman quantum memory. We demonstrate that \na photon stored in the diamond memory can, upon retrieval, have its frequency and bandwidth \nconverted. We report frequency conversion over a range of 4.2 times the bandwidth of \nthe input photon (4.1 nm, 2.3 THz), and bandwidth modulation between 0.5 to 1.9 times \nthe bandwidth of the input. We verify that the output light from storage and spectral \nmanipulation is still non-classical in nature. \n \nFinally, we demonstrate both single- and two-photon quantum interference mediated \nby the diamond memory, where the memory acts as a beamsplitter between photon and \noptical phonon modes in the diamond lattice. In a first experiment, a single photon is split \ninto two time-bins. The first time-bin is stored in the memory, then recalled and made to \ninterfere with the second time-bin producing fringes. In a second experiment, a photon \nfrom a weak coherent state is stored in the memory and, upon retrieval, undergoes Hong- \nOu-Mandel interference with a second photon. We measure Hong-Ou-Mandel interference \nwith a visibility of 59% giving a signature of non-classical interference (> 50%). \nThis collection of experiments establishes the diamond memory as a prime candidate for \ncertain quantum communication and processing applications. These results demonstrate \nthe potential for the diamond memory to be an integrated platform for photon storage, \nspectral conversion, and information processing.
We present a new experiment, in which we measure quantum correlations between single photons that are mediated by the exchange of a single phonon. We create and annihilate a single optical phonon in bulk diamond using two ultrashort laser pulses at two different wavelengths, generated by a Ti:Sapph oscillator and a frequency-doubled optical parametric oscillator (APE Berlin). During Stokes Raman scattering, the first pulse creates a photon-phonon pair, while the second pulse convert the same phonon into an anti-Stokes photon. Using spectral filtering and photon counting, we measure the cross-correlations between the Stokes and anti-Stokes photons with a few hundred femtoseconds time resolution. As expected, the non-classical correlation (g(2) much larger than the classical bound) decays within a few picosecond, following to the dynamics of the phonon mode. Our results demonstrate a new source of broadly tunable quantum correlated photons, and can be extended to provide a new way of measuring non-classical dynamics in nanoscale systems — down to individual nanostructures.