Decoherence Is a Problem for Quantum Computing, But…

It seems that quantum computing is becoming more and more advanced by the day. The qubits are getting cleaner, the gates are getting better, and the algorithms are getting more complex. It is apparently only a matter of time before quantum computing becomes a staple technology. However, a major hurtle remains that will require immense efforts to overcome: decoherence.

Quantum computers promise exponential speedup in solving certain types of problems by using quantum principles like superposition and entanglement, but the use of quantum states also leaves the quantum computer much more vulnerable to errors than a classical computer would be. These errors arise from decoherence, a process in which the environment interacts with the qubits, uncontrollably changing their quantum states and causing information stored by the quantum computer to be lost.

Decoherence could come from many aspects of the environment: changing magnetic and electric fields, radiation from warm objects nearby, or cross talk between qubits. Quantum scientists have their work cut out for them in wrangling all of these potential sources of decoherence.

But the point of this essay is not to belittle quantum computing. It is to highlight another application of quantum mechanics—one that exploits rather than laments a quantum state’s extreme sensitivity to its environment. This is the field of quantum measurement.

The purpose of quantum measurement is to use superposition and entangled states that are much more sensitive to the environment than regular, classically behaving states. The higher sensitivity of these states allows us to measure the environment more precisely than we would otherwise be able to.

Here’s how this works. The Heisenberg uncertainty principle says that if we measure one aspect of a system very precisely, for example, an object’s position, then we lose information about a different aspect of the system, e.g., how fast the object is moving. In a regular, classical state that doesn’t behave quantum mechanically, the uncertainty in the position and momentum are fixed values. But if we have quantum control of a state, we can bend the rules in our favor.

Imagine this uncertainty as a round balloon. If you do nothing to it, then the dimensions are all the same. Now imagine holding this balloon between your hands and squeezing it. It will become skinnier in the dimension in which you’re squeezing, but consequently will stretch out in the other dimensions, leaving the overall volume the same (provided you don’t squeeze so hard you pop the balloon). This is essentially what we do when we squeeze a quantum state. We make the uncertainty in the dimension we care about skinnier, but as a result we suffer a larger uncertainty in the other dimension, in keeping with the uncertainty principle.

Just as in the analogy of the balloon being squeezed, the quantum process of reducing the uncertainty in one direction is also aptly referred to as squeezing. And recently squeezing has been getting a ton of attention in the physics community, thanks to its adoption by the famous gravitational wave search of the LIGO collaboration.

LIGO operates an interferometer that uses the interference of light traveling in two different paths to detect small changes in the relative distance of those two paths—a signature of a gravitational wave. It’s an incredible feat of engineering. With two four-kilometer-long arms and 200 kilowatts of laser power bouncing off of huge mirrors suspended like pendula to isolate the experiment from seismic events, the sensitivity to gravitational waves that they’ve achieved is impressive. And, in order to establish more confidence in each detected event, they didn’t just make one interferometer. They made three (including the other gravitational wave interferometer collaboration, VIRGO).

With all of these impressive features designed into the interferometer, researchers at LIGO have detected many events that caused gravitational waves, but they wanted to increase their sensitivity even further to be capable of detecting smaller or more distant events that cause gravitational waves. Recently, they accomplished this by adding that special quantum state of light, squeezed light, into one of the paths of their detector. This squeezing made it much easier to detect a small difference in the lengths of the two arms of the interferometer resulting from gravitational waves, allowing them to detect 50 percent more events than before.

Let’s be clear: decoherence is still a problem for quantum sensing. It causes the signal to wash out. Trying to make measurements in the presence of decoherence is like taking a long-exposure photograph of someone who’s moving; the photo becomes blurry, and it becomes difficult to discern exactly how the person is moving.

However, the underlying physics of why decoherence happens, i.e., interactions with the environment, is exactly why quantum measurement works in the first place; we just have to be clever about how to design the experiment so that the measurement is sensitive to the thing we are trying to measure, but not to the rest of the environment. LIGO, for example, accomplished this by working really hard to isolate the interferometer from anything that would produce a false signal, like vibrations from seismic activity, air currents or even heavy-footed scientists stomping around the lab.

LIGO is just one of many examples of experiments that are using quantum mechanics to boost their sensitivity. Researchers are putting atoms in quantum states of motion to detect electric fields more precisely, creating squeezed states of atomic clocks to boost their precision in measuring time, and working towards using entangled states of atoms to measure gravity more precisely.

Quantum computing experts are finding ways suppress decoherence, and they’re making big improvements every year. With continued efforts, quantum computers will one day fill a niche in computing, solving certain types of problems that are classically intractable. But let’s not overlook the fact that the very thing quantum computers are battling is allowing the field of quantum measurements to blossom.



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