Nonlocal subpicosecond delay metrology using spectral quantum interference

Abstract

Timing and positioning measurements are key requisites for essential quantum network operations such as Bell state measurement. Conventional time-of-flight measurements using single photon detectors are often limited by detection timing jitter. In this work, we demonstrate a nonlocal scheme to measure changes in relative link latencies with subpicosecond resolution by using tight timing correlation of broadband timeenergy entangled photons. Our sensing scheme relies on spectral interference achieved via phase modulation, followed by filtering and biphoton coincidence measurements, and is resilient to microsecond-scale mismatch between the optical link traversed by the biphotons. Our experiments demonstrate precision of ±0.04 ps in measurements of nonlocal delay changes and ±0.7 • in measurements of radio-frequency phase changes. Furthermore, we complement our technique with time-tag information from single photon detectors in the same setup to present unambiguous sensing of delay changes. The proposed technique can be implemented using off-the-shelf telecom equipment thus rendering it adaptable to practical quantum network infrastructure.

Complete frequency-bin Bell basis synthesizer

“…The negatively correlated | Ψ state is affected only by the difference in the delays traversed by the two photons while the positively correlated | Φ state is affected only by the (common-mode) sum of the delays traversed by the two photons and equivalently the phase of the RF waveforms modulating the biphotons. Specific to our experiment where the physical paths are such that τ S = τ I , we impart differential-mode and common-mode delays on a given state by applying phases using a pulse shaper, making use of the equivalence between group delay and linear spectral phase [44,50].…”

“…Quantum delay sensing.-The joint temporal correlation of time-energy entangled biphotons can be utilized for delay metrology with potential quantum advantages [22,42]. Negatively correlated entangled states (such as |Ψ ± ) can probe changes to the difference in the delays traversed by the photons (differential-mode delay) via nonlocal measurements only [43,44]. Entangled photons with positive frequency correlations (such as |Φ ± ) can offer enhancement in delay sensitivity beyond the shot noise limit [22,45,46], by responding to changes in the sum of the signal and idler delays (common-mode delay).…”

“…Combined, these complementary capabilities suggest that frequency-bin Bell states could be employed for distributed sensing applications [47,48] and monitoring delays and latencies in quantum networks. We highlight this potential using the demonstrated frequency-bin Bell states by examining their sensitivity to common-mode and differential-mode phase-fully equivalent to temporal delay through the general Fourier relationship between linear spectral phase and group delay [44,49,50]. For the special case of two-dimensional systems, any relative phase shift is trivially equal to a linear phase; thus any phase operation on a frequency-bin qubit can be mapped to a delay [21].…”