Final Report Summary - PRECISION DC LIGHT (Precision measurement beyond the classical limit with novel sources of broadband squeezed light)
Precision Measurement with Broadband Down Converted Light
Scientific overview
The measurement of optical phase is fundamental in experimental physics. While classically, the phase of a wave is a fully defined quantity, the quantum nature of light poses limits. With coherent laser light, quantum shot noise limits the phase resolution, however, the ultimate quantum limit is the Heisenberg limit, where precision scales linearly with the number of photons, holding promise for a dramatic improvement in resolution. Heisenberg precision however, requires use of non-classical (phase squeezed) states of light with inherent quantum correlations, which are very sensitive to loss and require ideal photo-detectors. These limitations are the major obstacles for demonstrating sub-shot noise limited precision.
This research aimed to generate correlated quantum states and to develop new concepts to detect these quantum correlations. To attack these objectives from a fresh angle we exploited ultra broadband correlated fields. We harnessed broadband parametric down-conversion (PDC) or four-waves mixing (FWM) as sources of quantum entanglement / squeezing and broadband sum-frequency generation (SFG) as a physical correlation detector with superb properties. With these novel tools we explored new avenues to super resolved phase measurement.
Spontaneous PDC is a key method for generation of time-energy entangled photon pairs in quantum optics, producing an ultrahigh flux of entangled photon pairs (up to 10^14 pairs/second), as we demonstrated. Since the correlation is too tight for standard detection, the unique properties of SFG are important. Because of the well defined sum energy (the pump frequency), all the broadband information about the quantum correlation is coherently transferred by SFG back into one frequency. The non-classical correlation is manifested through a linear dependence of the non-linear SFG generation on input flux. Consequently, the SFG amplitude at the pump frequency serves as an ideal, ultrafast physical detector for two-photon correlation with superb noise rejection. As opposed to correlation detection with standard detectors SFG detection is robust to detection efficiency, so quantum squeezing can be detected and utilized even with low SFG conversion efficiencies.
In an oscillator cavity or when the pump intensity is very high, PDC becomes stimulated, generating intense fields, adequately described by classical equations. Classically, the signal and the idler fields are incoherent white noises, yet the amplitudes of twin frequencies are complex conjugated. A corner stone of this research was to develop a low-threshold OPO that emits high power broadband PDC. Since mode competition in an oscillator cavity normally causes significant narrowing of the emitted spectrum, the design of such an OPO cavity is non-trivial and must incorporate some mechanism to suppress mode competition, similar to mode locking of pulsed lasers. Such a source of high power, broadband PDC is attractive for applications, such as optical spread spectrum communication.
Project objectives
In light of the above, the major objectives of the project are:
1. To demonstrate sub-shot noise phase measurement using PDC as the source for broadband quantum squeezing and SFG as the ultrafast detector of this squeezing.
2. To implement a high power broadband OPO oscillator, characterize and optimize it’s performance and consider it for applications, such as spread spectrum optical communication.
Results
During the four years term of this research, the above objectives were achieved to a large extent as detailed in the periodic reports:
1. A source of an ultrahigh flux of entangled photon pairs
2. Generation of quantum correlated fields by broadband FWM
3. Theoretical analysis of SFG as an ultrafast detector of squeezing
4. Detailed simulation of OPO source for high power broadband PDC – A quantum two-photon frequency comb source
5. Observation of the nonclassical nature of broadband bi-photons at ultrafast speed.
6. Observation of the quantum-to-classical transition in photon correlation of broadband FWM
Contact
Dr. Avi Pe'er,
Physics department and BINA center for nanotechnology,
Bar Ilan University, Ramat Gan 52900 ISRAEL
Avi.peer@biu.ac.il