New Tool for Sculpting Single Photons
Future quantum technologies will require practical techniques for adjusting the frequencies and bandwidths of individual photons to optimize them for various purposes without losing the delicate quantum data that they carry. Now researchers have improved on previous technology and have shown how both properties can be tuned over a wide range inside a short length of standard optical fiber [1]. They expect that this technique will be more practical and effective than current alternatives and will find wide use in interfacing devices in future quantum computing and communications networks.
Photons are likely to provide the means for transmitting information within future quantum networks, but frequent changes to their properties will be required in order for them to carry out a diversity of tasks. For example, a trapped-ion quantum memory emits or absorbs photons at a specific visible wavelength with an extremely narrow bandwidth, which means that a photon with which it interacts must be produced as a relatively long light pulse. In contrast, a high-speed fiber-optic channel works best with infrared photons having much broader bandwidths, which require short light pulses.
Techniques to bridge these gaps, such as frequency conversion in crystals, typically involve large signal loss, because only a fraction of incoming photons become transformed, says Frédéric Bouchard of the National Research Council of Canada. These methods also introduce noise, which can destroy a photon’s quantum state.
Researchers have been developing an alternative, all-optical technique, called cross-phase modulation (XPM), that uses an intense “pump” laser pulse to create a short-lived change in the refractive index of an optical fiber [2]. A photon that travels in close proximity to this pulse will experience a time-dependent index of refraction, so its propagation speed will vary, which will cause compression or stretching of the photon’s waves. However, previous implementations of this technique required special photonic crystal fibers and achieved frequency changes too small to be of practical use in realistic quantum networks. In addition, these methods could not alter the bandwidth and the frequency independently.
Now Bouchard and colleagues have shown how to significantly extend XPM’s capabilities, demonstrating frequency shifts 10 times larger than previously achieved. Their technique also uses a much shorter segment of optical fiber and works with easily available single-mode fibers. Moreover, by including two consecutive XPM stages—one for shifting frequency and the other for adjusting bandwidth—they have far more flexibility in specifying a photon’s properties. The new design also reduces noise enough to preserve the quantum information stored in a photon.
To demonstrate the method, Bouchard and colleagues used a 15-cm-long piece of optical fiber. They sent a photon of wavelength 1550 nm, a standard telecom wavelength, through this fiber and timed it to arrive during the early or late portion of an intense 1035-nm pump pulse lasting 180 femtoseconds. A photon coinciding with the leading edge of the pump pulse was shifted down in frequency because the increase in refractive index slowed and stretched the photon’s waves. A photon at the trailing edge was shifted up in frequency by the opposite effect.
Two pulses with different wavelengths typically travel at slightly different speeds through a fiber, limiting their overlap time. However, the combination of fiber properties and the chosen wavelengths and pulse durations allowed the pump pulse and the photon to propagate together for longer than was possible in previous experiments, Bouchard says. As a result, the researchers achieved an absolute frequency increase as high as 6.42 THz and a decrease of 5.82 THz, roughly 10 times larger than previous experiments had achieved.
To control the photon bandwidth, the researchers added a second XPM stage with separate pump pulses. Here the photon must experience the refractive index increasing and then decreasing with time, or the reverse process. A photon that coincides with the peak amplitude of the pump pulse has its bandwidth reduced; if the photon appears in the valley between two consecutive, overlapping pump pulses, the bandwidth increases. Bouchard and colleagues showed that they could alter a photon’s bandwidth by factors ranging from 0.76 to 7.1. The team also measured a quantity called the second-order correlation function to confirm that the light remained a pure single photon and that its quantum information remained intact.
The new method “constitutes an ingenious way to change the frequency and bandwidth of a single photon,” says quantum optics specialist Robert Boyd of the University of Rochester in New York. “It could prove useful for many applications within quantum communication and other quantum technologies.”
–Mark Buchanan
Mark Buchanan is a freelance science writer who splits his time between Abergavenny, UK, and Notre Dame de Courson, France.
References
- K. L. Fenwick et al., “Broadband spectral manipulation of single photons using cross-phase modulation,” Phys. Rev. Lett. 136, 090803 (2026).
- N. Matsuda, “Deterministic reshaping of single-photon spectra using cross-phase modulation,” Sci. Adv. 2, e1501223 (2016).