This years’s Nobel prize in physics was awarded to Alain Aspect, John Clauser and Anton Zeilinger for *“for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. *This year’s prize has a few special connections with UW. Alain Aspect received an honorary doctorate from the faculty of Science in 2014. In addition a number of our own faculty within IQC have worked in the Zeilinger group. One of those faculty is Waterloo’s own Kevin Resch. Kevin is a Canada Research chair in Optical Quantum technologies and a faculty member in the Physics and Astronomy department and part of UW’s institute for Quantum Computing and served as the acting director from 2017 to 2020. We caught up with Kevin to ask him about his perspective on this year’s prize.

**First can you tell us a little bit about yourself?**

* KR:* I did my Ph.D. Toronto with Aephraim Steinberg, and did postdoctoral work in Vienna with Anton Zeilinger and Brisbane with Andrew White. Started as a faculty member in waterloo in 2006 in physics/iqc. I lead an Experimental quantum optics group focusing on quantum states of light like entangled states and single photons, quantum nonlinear optics, and foundations of quantum mechanics.

**What is this entanglement thing? I don’t remember learning about this in undergraduate quantum mechanics years ago.**

* KR:* Times have changed since we were undergraduates. Students in our program learn about entanglement and Bell’s inequalities as early as second year in PHYS234 now. They see a little more in PHYS334 when they study angular momentum. Then we offer a quantum information course in 4

^{th}year (PHYS467) that focuses on quantum technologies.

Entanglement is a consequence of the superposition principle from quantum mechanics applied to multiple particles. In an entangled state the properties of one of the particles can’t be described on its own. However, their properties can be very strongly correlated to those of the other particles in the state.

**So why is this spooky action at a distance thing important from a fundamental understanding point of view?**

* KR:* In quantum mechanics the state of a system is described by a wavefunction that encodes all the possible measurement outcomes and their probabilities. When we perform a measurement, the quantum system abruptly changes to a new state corresponding to the measurement outcome. This is called the “collapse of the wavefunction”.

“Spooky action at a distance” (spukhafte Fernwirkung) is the term Einstein used to describe the collapse of the wavefunction applied to entangled states. Before the measurement, the particles are described by an entangled wavefunction. When a measurement is made on one particle, the collapse applies to all the other particles, regardless of the distance between them, instantaneously.

Collapse and instantaneous change across large distances cause serious issues, although exactly which problem they create depends on how one interprets the rules of quantum mechanics. As just one example, if collapse is a consequence of obtaining information about some preexisting properties of the system—a view that Einstein suggested in the famous EPR paper—then the combination of Bell’s theorem and experiments like the ones conducted by Clauser, Aspect, and Zeilinger (and others) show that nature cannot be local, in stark contrast with classical physics and our intuition of how the universe works.

I would be remiss if I didn’t point out that even with this tension between quantum mechanics and special relativity/local realism, entangled quantum states cannot be used to send messages faster than the speed of light.

**The press release talked about new technologies because of entanglement. Are there REALLY new technologies that are possible with this, and are there any that we are starting to see now? What is the biggest area of impact we might expect to see this used in?**

* KR:* There are some entanglement-based technologies that exist today. Many precision measurement techniques rely on measuring the phase of a signal. Quantum effects can increase the size of that phase shift or reduce the noise in the measurement, both of which increase precision. Atomic clocks that use entangled atoms or optical interferometers (such as the LIGO experiment used to detect gravity waves) using entanglement and a related property called squeezing can get a quantum boost in performance. There is a lot of related work in this area called quantum metrology or sensing, applying quantum systems in applications for in measuring physical properties (like electric or magnetic fields, optical properties, rotations, accelerations/gravity) with the best precision nature allows.

Other technologies have been demonstrated such as using entanglement for quantum cryptography, a communication method where security is guaranteed by the laws of nature. Entanglement or other types of quantum correlations plays a crucial role in the speedup of quantum computing algorithms. Scaling up the size and complexity of quantum systems for computing is a very active field of research today where the progress has been accelerating in recent years.

**How big a part is entangled quantum states in your own research?**

* KR:* My group works in experimental quantum optics and quantum information. Almost all of the work that we do requires quantum states of light, like entangled photons, and then uses them for an application. Examples of applications include new types of interferometry and imaging and fundamental tests of physics.

**If you had to guess the next Nobel prize in quantum stuff, what would it be?**

* KR:* The Breakthrough Prize ($3M) this year went to people who invented quantum cryptography and some of the first algorithms that could run on a quantum computer. Those advances would be high up on my list as potential future Nobel winners. I could see some of the very recent results in ramping up the size of controllable quantum systems in various systems for quantum computing also winning a Nobel at some point.

**Do you have any advice for undergrads who think this stuff is way too hard?**

* KR:* Most things worth doing are difficult. Waterloo has all the ingredients required to learn about quantum mechanics and quantum technologies. We have excellent students, dedicated professors, expert researchers in quantum and a critical mass of activities in this field in the Physics Department and at the Institute for Quantum Computing. We have the world’s most comprehensive graduate program in quantum information science and a new course-based MSc in Quantum Technology (complete with a hands-on entangled photon/Bell inequality experiment). So if people are interested in learning about this field Waterloo provides the opportunity.