When a long-awaited quantum information network finally arrives, in whatever form, it will incorporate two essential technologies: a method of generating and manipulating quantum bits* (qubits); and a method of moving those qubits from one network node to another one far away without destroying their fragile quantum states.
Each method, however, will likely require a very different physical process – the quantum equivalent of composing a message in English on a typewriter but then sending it by radio in Spanish, without human intervention.
For example, qubits can be created and controlled dependably using extremely cold superconducting circuits with electrical outputs at microwave frequencies. But the best way to send those qubits with high fidelity over significant distances is to encode them in light at hundreds of terahertz** and move them over fiber-optic lines at ambient temperatures.
"So we are faced with a major challenge," says Konrad Lehnert of the National Institute of Standards and Technology's Quantum Physics Division 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 challenge using micromechanics."
Lehnert's research group, in collaboration with a team led by JILA colleague Cindy Regal, has devised an experimental system in which a tiny "drum head" mechanical oscillator can be used as the intermediary medium between electrical signals and light – and vice versa. The scientists will soon begin the process of shrinking it drastically and lowering 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 carrier wave, which contains quantum information in the form of small variations in the carrier waveform, is routed 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 transferring electromagnetic information into mechanical motion.
In the second stage, the membrane's vibrations are coupled to light by placing the membrane between two facing mirrors in an optical cavity through which a light beam passes. The membrane's motion changes the frequency of light that resonates in the cavity, thus transferring mechanical changes into frequency changes in the beam of photons.
"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 encode the information."
Over the past four or five years, Lehnert's group has developed considerable expertise in working with the first stage*** that links the superconducting circuit to the vibrating membrane. "We can take information 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 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 microwaves. That was kind of a breakthrough 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 helium). That's cold enough to make the metal in the resonant circuit superconducting. But it is not cold enough for the circuit to be in its lowest-energy condition, or "ground state," where thermal noise – electrical fluctuation resulting from heat – effectively stops.
In addition, further miniaturization will be needed. "The frequency "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 membrane," Lehnert says. "It can't be too low. But the only way of increasing the frequency is to make the membrane smaller so that it will vibrate faster. That means miniaturizing all the other structures as well. But at some point, sliding a membrane between 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 superconducting circuit, possibly raising the circuit above the critical temperature at which superconductivity disappears. "At the current scale, we haven't observed any deleterious effects of having the light near the circuit," Lehnert says. "But with the next reduction in temperature factor of 20 or so, it has to work much better, and be orders of magnitude more sensitive. That's our biggest technical 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 – superconducting circuits – to be combined with the only method of transporting quantum information over long distances."
* Conventional computers store and process information in the form of binary digits, or "bits," which can have only one of two values. Quantum bits, by contrast, can exist in a "superposition" of two values at the same time until they are measured.
** Terahertz radiation has a frequency in the range of 1012 cycles per second, approximately a thousand times higher than microwave radiation, with typical frequencies in gigahertz (109 cycles per second). The optical output of the system is 280 THz.
*** The group reports recent research in "Quantum-enabled temporal and spectral mode conversion of microwave signals," R.W. Andrews et al, Nature Communications, 30 Nov. 2015, DOI: 10.1038/ncomms10021