Spectral compression of single photons

Journal name:
Nature Photonics
Year published:
Published online


Photons are critical to quantum technologies because they can be used for virtually all quantum information tasks, for example, in quantum metrology1, as the information carrier in photonic quantum computation2, 3, as a mediator in hybrid systems4, and to establish long-distance networks5. The physical characteristics of photons in these applications differ drastically; spectral bandwidths span 12 orders of magnitude from 50 THz (ref. 6) for quantum-optical coherence tomography7 to 50 Hz for certain quantum memories8. Combining these technologies requires coherent interfaces that reversibly map centre frequencies and bandwidths of photons to avoid excessive loss. Here, we demonstrate bandwidth compression of single photons by a factor of 40 as well as tunability over a range 70 times that bandwidth via sum-frequency generation with chirped laser pulses. This constitutes a time-to-frequency interface for light capable of converting time-bin to colour entanglement9, and enables ultrafast timing measurements. It is a step towards arbitrary waveform generation10 for single and entangled photons.

At a glance


  1. Single-photon bandwidth compression scheme.
    Figure 1: Single-photon bandwidth compression scheme.

    a, A broad-bandwidth single photon (P) with a linear frequency chirp is converted into a narrowband photon of higher frequency via SFG with a strong laser pulse (L) of opposite chirp, in a nonlinear crystal (NL). b, Experimental set-up. Photon P is generated via SPDC in a β-barium-borate (BBO) crystal (type I, 1 mm) and sent through 34 m of optical fibre (SMF) to introduce a linear chirp via group velocity dispersion33. The strong laser pulse is antichirped after a double pass between two diffraction gratings (DG, 1,200 lines/mm)33. The spectrally narrowed photon is generated in a 1-mm-thick bismuth-borate (BiBO) crystal and detected with a photomultiplier tube (PMT) or sent to a spectrometer. For alignment purposes, the single photons can be substituted by a weak coherent state W, split off from the laser beam using a half-wave plate and a polarizing beamsplitter.

  2. Single-photon spectra versus wavelength (top) and relative frequency (bottom).
    Figure 2: Single-photon spectra versus wavelength (top) and relative frequency (bottom).

    The signal photons at the source (shown in red) have an initial bandwidth of 1,740 GHz centred at 811 nm after transmission through an interference filter. Once the quadratic phase is applied and the photons are upconverted, the photon bandwidth reduces to 74 ± 4 GHz centred at 399.70 nm (blue curve). The spectra are shown as normalized spectral intensities and, for the upconverted signal case, correspond to the average of six consecutive scans of 20 min acquisition time. We subtracted background counts determined by a supplementary scan with the signal photon path blocked. The blue dashed curve shows the theoretical spectrum of photons upconverted without our chirping technique, but with otherwise identical conditions.

  3. Wavelength tunability.
    Figure 3: Wavelength tunability.

    The centre wavelength of the upconverted light can be tuned by controlling the relative delay between the input pulses at the nonlinear crystal. The blue circles represent the centre wavelength of the upconverted single photon, spanning a range of 3 nm. The upconverted light from the weak coherent state behaves in the same way and is plotted with black squares. The lines are linear fits yielding slopes of −0.0640 ± 0.0005 nm ps−1 and −0.0641 ± 0.0001 nm ps−1 for the single photons and weak pulses, respectively. The vertical offset between the two curves (~0.2 nm) arises from a slight difference in the delay between the weak coherent state and the single photons at the crystal and a difference in their centre wavelength. Error bars are smaller than the data points.

  4. Temporal correlations with the idler photon.
    Figure 4: Temporal correlations with the idler photon.

    a, The signal and idler from SPDC are produced in pairs, strongly correlated in time with a total measured coincidence rate of 160,000 s−1 around zero delay. b, The upconverted single photon maintains the strong timing correlation expected from individual photon pairs, and a coincidence rate of 50 s−1 is detected. c, If the weak coherent state is upconverted instead, the histogram shows equal height peaks, as expected for pulsed, but uncorrelated events. For each histogram, the optical path length difference with the idler is accounted for in post-processing and the abscissa is a variable electronic delay. An additional electronic delay box, with an observed asymmetric temporal jitter, was used on the idler side only, causing the asymmetry in b and c. Error bars represent 1 s.d., which are too small to be observed at this scale.


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Author information


  1. Institute for Quantum Computing and Department of Physics & Astronomy, University of Waterloo, Waterloo, N2L 3G1 Canada

    • J. Lavoie,
    • J. M. Donohue,
    • L. G. Wright &
    • K. J. Resch
  2. Centre for Engineered Quantum Systems and Centre for Quantum Computer and Communication Technology, School of Mathematics and Physics, University of Queensland, Brisbane 4072, Australia

    • A. Fedrizzi


A.F. and K.J.R. conceived the idea for the study. K.J.R., A.F. and J.L. designed the experiment. J.L. performed the experiments and analysed the data. J.M.D. and L.G.W. contributed to building the experimental setup and in taking data. All authors contributed to writing the manuscript. A.F. is supported by an ARC Discovery Early Career Researcher Award DE130100240.

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