Researchers demonstrate new type of laser

March 3, 2017, Delft University of Technology
Researchers demonstrate new type of laser
Researchers at QuTech have built an on-chip microwave laser based on a fundamental aspect of superconductivity, the AC Josephson effect. The device is made from a single nanoscale Josephson junction strongly coupled to a superconducting cavity. When a small DC voltage is applied across the junction by a battery, the difference in energy causes microwaves to be released when a Cooper pair tunnels across the junction. The cavity then provides amplification, resulting in a beam of coherent microwave light to be emitted from the cavity. The device may have applications in building a scalable quantum computer.  Credit: Delft University of Technology

Lasers are everywhere nowadays: Doctors use them to correct eyesight, cashiers to scan your groceries, and quantum scientist to control qubits in the future quantum computer. For most applications, the current bulky, energy-inefficient lasers are fine, but quantum scientist work at extremely low temperatures and on very small scales. For over 40 years, they have been searching for efficient and precise microwave lasers that will not disturb the very cold environment in which quantum technology works.

A team of researchers led by Leo Kouwenhoven at TU Delft has demonstrated an on-chip laser based on a fundamental property of superconductivity, the ac Josephson effect. They embedded a small section of an interrupted superconductor, a Josephson junction, in a carefully engineered on-chip cavity. Such a device opens the door to many applications in which microwave radiation with minimal dissipation is key, for example in controlling qubits in a scalable computer.

The scientists have published their work in Science on the 3rd of March.

Lasers have the unique ability to emit perfectly synchronized, coherent light. This means that the linewidth (corresponding to the color) is very narrow. Typically lasers are made from a large number of emitters (atoms, molecules, or semiconducting carriers) inside a cavity. These conventional lasers are often inefficient, and dissipate a lot of heat while lasing. This makes them difficult to operate in cryogenic environments, such as what is required for operating a quantum computer.

Superconducting Josephson junction

In 1911, the Dutch physicist Heike Kamerlingh Onnes discovered that some materials transition to a superconducting state at very low temperatures, allowing electrical current to flow without any loss of energy. One of the most important applications of superconductivity is the Josephson effect: if a very short barrier interrupts a piece of superconductor, the electrical carriers tunnel through this non-superconducting material by the laws of quantum mechanics. Moreover, they do so at a very characteristic frequency, which can be varied by an externally applied DC voltage. The Josephson junction is therefore a perfect voltage to light (frequency) converter.

Josephson junction laser

The scientists at QuTech coupled such a single Josephson junction to a high-quality factor superconducting micro-cavity, no bigger than an ant. The Josephson junction acts like a single atom, while the cavity can be seen as two mirrors for microwave light. When a small DC voltage is applied to this Josephson junction, it emits that are on resonance with the cavity frequency. The photons bounce back and forth between two superconducting mirrors, and force the Josephson junction to emit more photons synchronized with the photons in the cavity. By cooling the device down to ultra- (< 1 Kelvin) and applying a small DC voltage to the Josephson junction, the researchers observe a coherent beam of microwave photons emitted at the output of the cavity. Because the on-chip laser is made entirely from superconductors, it is very energy efficient and more stable than previously demonstrated semiconductor-based lasers. It uses less than a picoWatt of power to run, more than 100 billion times less than a light globe.

Low-loss quantum control

Efficient sources of high quality coherent microwave light are essential in all current designs of the future quantum computer. Microwave bursts are used to read out and transfer information, correct errors and access and control the individual quantum components. While current microwave sources are expensive and inefficient, the Josephson junction laser created at QuTech is energy efficient and offers an on-chip solution that is easy to control and modify. The group is extending their design to use tunable Josephson junctions made from nanowires to allow for microwave burst for fast control of multiple quantum components. In the future, such a device may be able to generate so-called "amplitude-squeezed" light with has smaller intensity fluctuations compared to conventional lasers, this is essential in most quantum communication protocols. This work marks an important step towards the control of large quantum systems for .

Explore further: Superconductors that work by themselves: Scientists discover new possibilities in chryoelectronics

More information: M. C. Cassidy et al. Demonstration of an ac Josephson junction laser, Science (2017). DOI: 10.1126/science.aah6640

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eachus
5 / 5 (4) Mar 03, 2017
Shouldn't this be called a maser? Masers came first, and when scientists figured out how to boost the frequencies substantially, they started calling the result, lasers. So a laser may be a light frequency maser, but it doesn't work the other way around.
Da Schneib
4.8 / 5 (4) Mar 04, 2017
This is pretty cool. Heh, sorry, just had to.

It's a fairly ingenious way of coupling a cavity resonator with a Josephson junction. Its one problem is the need for supercooling. This is probably going to be the biggest problem with quantum computing, unless we can get some (near) room temperature superconductors.
eachus
5 / 5 (2) Mar 04, 2017
The ceramic superconductors are starting to move out of the laboratory and into products, which is at least an order of magnitude reduction in cooling costs and complexity. But any idea that pocket sized quantum computers will be available in the near or meduim term is unlikely. Quantum computers will be room-sized boxes with serious power and cooling demands initially, and may not shrink all that much over time.

Of course, in one sense, we won't need widespread deployment of quantum computers. Ordinary computer networking can carry data to "cloud" quantum computers, and carry results back. (Putting them in space, or on the moon, if superconductors are necessary would certainly simplify the cooling issues, and quantum cryptography can protect the communications.)

Seriously, I'd love to have access to a kilo-qubit or larger QC, but the programs I would run would take maybe five minutes, And I suspect that most scientists needing access would also need only minutes per yesr.
Da Schneib
5 / 5 (1) Mar 04, 2017
The ceramic superconductors are starting to move out of the laboratory and into products, which is at least an order of magnitude reduction in cooling costs and complexity. But any idea that pocket sized quantum computers will be available in the near or meduim term is unlikely. Quantum computers will be room-sized boxes with serious power and cooling demands initially, and may not shrink all that much over time.
Agreed. At this time there is no reason to expect room temperature superconductors, but the LHC makes it clear that metastable superconductors at reasonable temperatures are definitely technically feasible. Now that quantum optics seems to be coming into its own, the cooling requirements appear to be less challenging than that by quite a bit; one has isolated chips in a QC that require extreme cooling, but they can be connected together with equipment that doesn't require this.

[contd]

Da Schneib
5 / 5 (1) Mar 04, 2017
[contd]
Of course, in one sense, we won't need widespread deployment of quantum computers. Ordinary computer networking can carry data to "cloud" quantum computers, and carry results back. (Putting them in space, or on the moon, if superconductors are necessary would certainly simplify the cooling issues, and quantum cryptography can protect the communications.)
Correct. The big QCs will be built in space, where they are much more feasible. I also suspect that we will build supercolliders on the Moon that will be based on the recent advances in laser-plasma technology mated with more traditional methods, though I don't look for this in my lifetime. Using QCs to simulate and process the results we will get from these devices, as well as from cosmic ray studies where particle energies are orders of magnitude greater than anything we are every likely to do, will also make a big difference in particle physics.
[contd]
Da Schneib
5 / 5 (1) Mar 04, 2017
[contd]
Seriously, I'd love to have access to a kilo-qubit or larger QC, but the programs I would run would take maybe five minutes, And I suspect that most scientists needing access would also need only minutes per yesr.
I actually think this may turn out to be the biggest barrier to QC. It's not a matter of what the hardware can do, it's a matter of what algorithms there are that need a QC. Based upon how long it took for microprocessors, software engineering will be the longer barrier and require the most man-hours to implement. Only a handful of algorithms currently exist that require QCs. As always, the question after "how do we make these" is "how do we use them."
Whydening Gyre
5 / 5 (1) Mar 05, 2017
Interesting connection to Asimov, with the concept of QC's in space, Along with DS's last comment about "the question will be ...":-)
Is it the Last question...:-)?
eachus
5 / 5 (2) Mar 05, 2017
I actually think this may turn out to be the biggest barrier to QC. It's not a matter of what the hardware can do, it's a matter of what algorithms there are that need a QC. Based upon how long it took for microprocessors, software engineering will be the longer barrier and require the most man-hours to implement. Only a handful of algorithms currently exist that require QCs. As always, the question after "how do we make these" is "how do we use them."


Um, have you had a course in Analysis of Algorithms or the like? There are lots of real problems which currently fall into the NP category. (Answers can be proved true in polynomial time, but non-polynomial time is (currently) required to find a solution.) Since any NP problem--including NP-complete problems--can be mapped to any NP-complete problem in polynomial time (and space), all that is needed is a QC algorithm for one NP-complete problem to make QCs useful for thousands of problems, like protein folding.
Da Schneib
5 / 5 (1) Mar 05, 2017
@eachus, having been a programmer myself and seen the lackluster mathematical skills of most commercial software programmers working at the highest levels in the industry, while I understand the nature of NP and NP complete problems, I assure you that they by and large do not.

I will also point out that such algorithms do not currently exist, and will require some pretty serious work by some pretty serious people to be brought into existence. And it is insufficient to have such an algorithm; to use it effectively one must understand it. And it's not currently taught in CS curriculum that I've seen, nor is the mathematics required to understand it.

@Whyde, you should read some Frederic Brown. Heinlein dedicated Stranger In A Strange Land to him, you know.
Da Schneib
5 / 5 (1) Mar 05, 2017
@eachus, just so you know, I have worked at several companies whose names you would instantly recognize, and I am talking about some of the top programmers in business. These are people who make software for Fortune 10 companies. And they are not aware of the implications of dates and times, never mind Boolean algebra much less NP-complete problems. If you pretend that teaching them higher mathematics is possible you are not aware of the limitations of commercial programmers.
eachus
5 / 5 (2) Mar 05, 2017
If you pretend that teaching them higher mathematics is possible you are not aware of the limitations of commercial programmers.


I have also worked at some of those companies, and more important tried to teach software engineering to some of those programmers. (I taught evenings at what is now UMass Lowell, in addition to some in-house courses for programming staff.) Believe me, I know the huge difference between average programmers and good software engineers. Robert Dewar, who taught at NYU used to explain the problem by asking some of his Computer Science classes if they enjoyed debugging. A majority of the students would raise their hands. I remember the last time I used a debugger. It was a bug in Solaris. That was about 20 years ago. I'm retired now. I still code, but not as much. Yes, I'll have typing errors found by the compiler. But I expect once the code compiles, that any changes I make will be to make the output or the code prettier.
eachus
5 / 5 (2) Mar 05, 2017
I will also point out that such algorithms do not currently exist, and will require some pretty serious work by some pretty serious people to be brought into existence.


I won't disagree, but I suspect that the NSA may have written QC code to solve knapsack problems, or SAT by now. The thing that will take time is finding algorithms which use as few qubits as possible for a given problem size. Early QCs will have limits, so just having an algorithm ready to go is not realistic. I am sure we could spend days arguing whether the D-Wave machines are 'real' QCs, but they do solve one set of NP-hard problems. And now open source software is available which allows ordinary programmers to access the machines: https://www.wired...mputing/

As for the problems that interest me? I should be able to cast them as optimization problems. Again the trick will be sizing them for existing D-Wave machines.

Da Schneib
not rated yet Mar 05, 2017
My goodness, I haven't been paying attention. It looks like you're a long way ahead of me. This is a fascinating conversation.
Da Schneib
not rated yet Mar 05, 2017
Did a little research, seems that a D-wave machine can't execute Shor's algorithm. But I think your suspicions may be right about the NSA. The question is whether they have a real QC to test on. Or more properly when they will.

This is coming on faster than I expected. I've been too busy to keep track of it.
Hyperfuzzy
not rated yet Mar 08, 2017
The current is not a resistive current but is f(E,z,t), i.e. changing field at the barrier. How thin can the barrier be and how low may v(t) be, and what's the function due to direction? Looks like current, behaves like current, let's call it a current that begets current. Anyway, the field wrinkles!
Hyperfuzzy
not rated yet Mar 08, 2017
...
Seriously, I'd love to have access to a kilo-qubit or larger QC, but the programs I would run would take maybe five minutes, And I suspect that most scientists needing access would also need only minutes per yesr.[/q
You only need an optical computer. However this has some interesting qualities. Think, an optical junction may perform a "set" of task at the same time, cascade, get it ...
Hyperfuzzy
not rated yet Mar 08, 2017
The current is not a resistive current but is f(E,z,t), i.e. changing field at the barrier. How thin can the barrier be and how low may v(t) be, and what's the function due to direction? Looks like current, behaves like current, let's call it a current that begets current. Anyway, the field wrinkles!

Suspect with an atom thick insulator, if possible, very high frequency and multiple operations per clock cycle, extreme computing. I have some ideas on building a tool to build tools. juz say'n

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