EU targets holy grail of quantum superconducting

Article By : R. Colin Johnson

One grant for an ultra-sensitive superconducting photon director could clear the way to a universal supercomputing quantum computer.

Quantum superconducting computers are already available from D-Wave, but they work only for special problems (solving parameter optimisation problems with combinatorial linear equations). Now the European Union is aiming for the Holy Grail—a universal superconducting quantum computer that can solve any problem digital computers can plus use qubits to solve problems that require too much time on digital computers.

One recent European Research Council (ERC) grant for an ultra-sensitive superconducting photon detector, by Aalto University (Helsinki, Finland) scientists Mikko Möttönen, Joonas Govenius, Russell E. Lake and Kuan Yen Tan, could clear the way to a universal supercomputing quantum computer.

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__Figure 1:__ *Artistic rendering of a hybrid superconducting single photon microwave detector. Credit: Ella Maru Sudio. (Source: Aalto)*

"This detector was developed with my first ERC grant—a so-called starting grant, which amounts to $1.7 million (Rs. 11.49 crore) for five years," Möttönen, leader of the record-breaking Quantum Computing and Devices research group, told EE Times. Because of the success of its starting-grant phase, their group received "the soon-starting ERC Proof-of-Concept Grant of $166,000 (Rs. 1.12 crore) for 18 months. In the PoC, we will develop the detector towards a commercial product. Furthermore, it will be used in the follow-up ERC Consolidator Grant, $2.2 million (Rs. 14.86 crore) for five years starting in the beginning of 2017, to develop engineered dissipation for quantum electric devices such as the superconducting quantum computer."

The problem with superconducting quantum computers today, is that emitters of photons—encoded with quantum information (qubits)—are relatively simple to construct, but detectors are bulky, expensive and difficult to build. Möttönen's group, on the other hand, claims to be on-track to solve that problem with a new architecture that is ultra small, simple to construct and 14-times more sensitive than today's photodiodes.

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__Figure 2:__ *Scanning-electron microscope image of the single photon microwave detector. The nanowire (yellow) spans and the superconducting aluminium parts over it. Photons arrive from the left, are absorbed by the long nanowire which elevates the temperature thus weakening its superconductivity which consequently works as a sensitive thermometer. Credit: Joonas Govenius. (Source: Aalto)*

Möttönen attributes their success, where others have failed, as the result of "key enabling" technologies developed exclusively at Aalto University. The first was marrying superconducting aluminium with a non-superconducting gold alloy (gold-palladium) nanowire, forming a proximity-induced Josephson-junction to efficiently absorb a single photon and accurately amplify its signal. The resultant efficient absorption of the photon by the gold alloy and the super-sensitive readout by the superconducting aluminium is done with a detector measuring just a few microns in size.

"We call the device partially superconducting since it has a superconducting part [aluminium] that works as a thermometer and a normal-metal part [gold-palladium] that absorbs the in-coming photons. We are planning to have the first commercial devices work at below 100mK temperature. This is not a problem since the first customers to use these systems routinely reach these temperatures," Möttönen told EE Times. "The thermometer, the nanowire, is so close to the superconducting aluminium that it also turns superconducting. This phenomenon is generally referred to as the proximity effect."

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__Figure 3:__ *Artistic rendering of the microwave photo detector in action. Credit: Heikka Valja. (Source: Aalto)*

The energy detected is extremely small—a single zepto-joule—equivalent to the energy needed to lift a red blood cell by just a single nanometer. To transfer the signal from the micron-sized thermometer its qubits must be faithfully amplified, which the researchers achieve with a novel use of temperature change to create positive feedback at microwave frequencies using a resonant LC (inductor-capacitor) "tank" circuit.

"We use a relatively strong probe microwave tone, but it is tuned significantly below the resonance of the LC tank circuit. When the photons get absorbed and the electron temperature of the detector rises, inductance goes up, and hence the resonance shifts down. This results in the probe tone moving closer to resonance and hence more heating from the probe—thus the resultant positive feedback for temperature changes," Möttönen told EE Times.

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__Figure 4:__ *Doctoral candidate Joonas Govenius (left) and professor Mikko Mottonen (right) at Aalto University (Helsinki, Finland). Credit: Vilja Pursiainen/Kaskas Media.
(Source: Aalto)*

After perfecting the current photon detector, Möttönen's group hopes to get the go-ahead from the ERC to start building an actual single photon detector specifically designed for superconducting quantum computers communicating qubits at the traditional microwave frequencies.

"We are strongly pushing for a single-photon detector for microwaves. Currently, I cannot promise when it will be ready—there are many things to do to improve our current device. However, we are not at a dead end," Möttönen told EE Times.

For all the details, read the paper Detection of zeptojoule microwave pulses using electrothermal feedback in proximity-induced Josephson junctions published in Physical Review Letters.

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