Preserving coherence keeps quantum systems clear and comprehensible.
We say a message is incoherent when we can’t make it out, or when it doesn’t make sense. A scribbled note, a drunken argument or a conversation taking place five tables down in a crowded cafe might all be incoherent. In general, “coherent” means the opposite — consistent, connected, clear.
In science, the word coherence takes on more specific, mathematical definitions, but they all get at a similar concept: Something is coherent if it can be understood, if it forms a unified whole and if those first two qualities persist.
Scientists originally developed the concept of coherence to understand and describe the wave-like behavior of light. Since then, the concept has been generalized to other systems involving waves, such as acoustic, electronic and quantum mechanical systems.
“Coherence is a measure of how well certain systems will maintain their relationships with each other and how well we are able to predict the evolution of those systems,” said Martin Holt, a scientist at the U.S. Department of Energy’s (DOE) Argonne National Laboratory and a member of Q-NEXT, a DOE National Quantum Information Science Research Center led by Argonne. “Understanding and controlling coherence in quantum technologies is crucial because the relationships involved need to be very long-lived and well-understood.”
Like researchers around the globe, Q-NEXT scientists are studying and improving coherence in quantum systems for technologies such as quantum sensing and quantum computing. Once realized, these technologies will leverage coherence to perform complex calculations, take high-resolution measurements and transmit unhackable messages, potentially revolutionizing our approach to communication, cybersecurity, simulation, optimization and more.
Coherent Waves
Imagine a wave, rising and falling periodically at a certain speed (frequency) and with a certain height or intensity (amplitude). Now, throw in a second wave. If the two waves are offset from each other — if they are not rising and falling together — they are said to be out of phase. It’s this phase difference that determines whether the waves will interfere to have an amplifying or canceling effect on each other, or something in between.
We see this in everyday life. Constructive interference occurs when two singers amplify each other’s voices, or when you double bounce a friend on the trampoline. Destructive interference of sound waves is the principle behind noise-cancelling headphones.
Two waves are coherent when there is a meaningful relationship between their phases or when their interference creates a well-understood pattern. In essence, coherence is a measure of how in sync the waves are with each other. There are degrees of coherence; waves can be more or less coherent with each other.
Lasers, for example, are designed to emit highly coherent light. They contain atoms that are excited with energy and, upon their decay, emit photons (particles of light) with the same frequency and phase as each other. These photons bounce off mirrors within the laser, which serve to amplify the light traveling only in a certain direction and with a certain frequency. This special interference — or coherence — between photons results in a highly focused and uniform beam of light. Sound waves can be similarly coherent, and scientists have even created sound lasers, or sasers.
Quantum Coherence
In quantum mechanics, objects can be represented as either a combination of waves or particles. In principle, this applies to any object. But this way of looking at things works best when dealing with objects that are very small, like photons, other elementary particles and atoms.
Quantum objects can be described with a special type of identifier called a wavefunction. It’s sort of a wave on steroids, since it can contain an incredible amount of information within its mathematical nooks and crannies.
This is because wavefunctions are composites of waves themselves. Quantum coherence refers to the phase relationships between these waves — the ones that, together, describe the whole object. When these waves interfere in coherent ways, it gives rise to quantum superposition, a central feature of quantum mechanics that allows an object to exist in multiple states simultaneously.
Here’s where it gets uniquely quantum. The waves composing an object’s quantum wavefunction don’t correspond to physical values, like position or energy. Instead, they correspond to the likelihood of different possible ways that the state of the object could evolve — for example, the likelihood that its energy will change over time in a certain way, or the likelihood that it will spin a certain way in a certain location. Quantum coherence is an interference between these different possible future histories of the object.
However, this interference can exist only until the system is observed or disturbed. At that point, the interference between the waves vanishes, and the superposition is lost. The object has apparently experienced only one of the possible histories.
What does it mean for possible future histories to interfere? And for the wavefunction to collapse into just one of those histories? Those are tough questions. Currently, we know more about how to use this feature of quantum mechanics than what it means for the nature of our reality.