The Jerusalem Post
Researchers at Bar-Ilan University have devised a method to overcome a “speed limit” on quantum information processing by nearly a million times what it is now, while ensuring absolute data security. Their findings have just been published in the journal Nature Communications.
Quantum communication is one of the most advanced branches of the “second quantum revolution,” in which any attempt at eavesdropping can be detected by using the fundamental principle of quantum mechanics. Because any measurement affects the quantity being measured, the mere existence of an eavesdropper can be detected by identifying traces that are left behind by the measurements of the communication channel.
The major drawback of quantum communication today is the slow speed of data transfer, which is limited by the speed at which the parties can perform quantum measurements.
Homodyne detection is a cornerstone of quantum optics, acting as a fundamental tool for processing quantum information. But the standard homodyne method suffers from limitations if there is no strong bandwidth.
Quantum optical phenomena, exploited for quantum communication, can easily span a bandwidth of many terahertz, or THz, a unit of electromagnetic wave frequency equal to one trillion hertz. But the standard processing methods of this information are inherently limited to the electronically accessible megahertz-to-gigahertz or MHz-to-GHz range. That leaves a dramatic gap between the relevant optical phenomena used for carrying the quantum information and the capability to measure it. Thus, the rate at which quantum information can be processed is very limited.
In their work, the Bar-Ilan researchers replaced the electrical nonlinearity that serves as the heart of homodyne detection, which transforms the optical quantum information into a classical electrical signal, with a direct optical nonlinearity, transforming the quantum information into a classical optical signal. Thus, the output signal of the measurement remains in the optical regime and preserves the enormous bandwidth optical phenomena offers.
“What we’ve done is to offer a direct optical measurement that conserves the information bandwidth, instead of an electrical measurement that compromises the bandwidth of the quantum optical information,” said Dr. Yaakov Shaked, who conducted the research during his PhD studies in the quantum optics labs of Prof. Avi Pe’er.
To demonstrate this idea, the researchers perform a simultaneous measurement of an ultra-broadband quantum optical state, spanning 55 THz, presenting non-classical behavior across the entire spectrum. Such a measurement, using standard method, would be practically impossible.
This new form of quantum measurement is relevant also to other branches of the “second quantum revolution,” such as quantum computing with super powers, quantum sensing with super sensitivity and quantum imaging with super resolution.