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Quantum Information Transfer: From Microwaves to Motion to Light
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lenge,” says Konrad Lehnert of the National Institute of Standards and Technology’s Quantum Physics Divi- sion, and JILA, a joint institute of NIST and the University of Colorado at Boulder. “How do we take the quantum state of a superconducting circuit and map it onto a light field that can run around in an optical fiber? That’s the bottleneck between us having a quantum network or not. We’ve chosen to attack that chal- lenge using micromechanics.”
Tiny Drum Head A colleague, Cindy Regal, has
devised an experimental system in which a tiny “drum head” mechanical oscillator can be used as the interme- diary medium between electrical sig- nals and light, and vice versa. The scientists will soon begin the process of shrinking it drastically and lower- ing the temperature by an order of magnitude. The system is based on a two-
stage process of transduction in which patterns in one form of energy are converted to patterns in another. First, the microwave “pump” or carri- er wave, which contains quantum in- formation in the form of small varia- tions in the carrier waveform, is rout- ed to a resonant circuit that contains a two-plate capacitor, the top plate of which is a thin membrane that is free to vibrate like a drum head. In the current configuration,
the top plate consists of a square membrane of silicon nitride only half a millimeter on a side and a few tens of molecules thick. The microwave signal is imposed on the vibration pattern of the membrane, thus trans- ferring electromagnetic information into mechanical motion. In the second stage, the mem-
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brane’s vibrations are coupled to light by placing the membrane be- tween two facing mirrors in an opti- cal cavity through which a light beam passes. The membrane’s mo- tion changes the frequency of light that resonates in the cavity, thus transferring mechanical changes into frequency changes in the beam of photons.
Weak Coupling “The coupling among motion,
electricity, and light is very, very weak,” Lehnert says. “In order to make it strong enough to do the transduction, we excite the circuit continuously, drive it with a strong microwave tone, and it’s the little changes or fluctuations in that strong drive that are the information we’d like to transfer. Similarly, there’s an intense light field, and it’s the fluctuations in that field that en- code the information.” Over the past four or five years,
Lehnert’s group has developed con- siderable expertise in working with the first stage that links the super- conducting circuit to the vibrating membrane. “We can take informa- tion in the form of quantum states in the microwave regime and convert it really noiselessly into mechanical motion,” Lehnert says. “Cindy’s group meanwhile has been working
on the second stage.” The group re- ports recent research in “Quantum- enabled temporal and spectral mode conversion of microwave signals,” R.W. Andrews et al, Nature Commu- nications, 30 Nov. 2015, DOI: 10.1038/ncomms10021. The most recent result is a
proof-of-principle device with input/ output ports on different sides. “It’s bidirectional,” Lehnert says. “We showed that this device can convert microwave information into light just as well as it turns light back into mi- crowaves. That was kind of a break- through for this effort. But we still have a very long way to go.” For one thing, the researchers
need to make the apparatus about 20 times colder in order to ensure that the small variations that make up the signal are not obscured by the thermal noise of the electrical circuit. They have been operating the system at 4 K (the temperature of liquid he- lium). That’s cold enough to make the metal in the resonant circuit su- perconducting. But it is not cold enough for the circuit to be in its low- est-energy condition, or “ground state,” where thermal noise —electri- cal fluctuation resulting from heat — effectively stops.
More Miniaturization Needed In addition, further miniatur-
ization will be needed. “The frequen- cy difference between the intense pump microwave signal we impose and the little fluctuations has to be close to the mechanical resonance frequency of the capacitor mem- brane,” Lehnert says. “It can’t be too low. But the only way of increasing the frequency is to make the mem- brane smaller so that it will vibrate faster. That means miniaturizing all the other structures as well. But at some point, sliding a membrane be- tween two mirrors, as the mirrors get increasingly close to each other, is going to become very difficult.” Reducing the device dimensions
would also bring the intense laser light field very close to the supercon- ducting circuit, possibly raising the circuit above the critical temperature at which superconductivity disap- pears. “At the current scale, we haven’t observed any deleterious ef- fects of having the light near the cir- cuit,” Lehnert says. “But with the next reduction in temperature factor of 20 or so, it has to work much bet- ter, and be orders of magnitude more sensitive. That’s our biggest techni- cal concern right now.” Tom O’Brian, Chief of NIST’s
Quantum Physics Division, said “Konrad and Cindy and their teams are making remarkable progress on new ways of transforming quantum states between optical, mechanical, and microwave signals. Although the technical challenges of their work are substantial, if successful, Konrad’s and Cindy’s research would allow one of our most promising quantum information technologies — super- conducting circuits — to be combined with the only method of transporting quantum information over long dis- tances.” Web:
www.nist.gov r
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