Quantum physics proposes a new way to study biology – and the results could revolutionize our understanding of how life works

By guest blogger Clarice D. Aiello, faculty at UCLA

Imagine using your cellphone to control the activity of your own cells to treat injuries and disease. It sounds like something from the imagination of an overly optimistic science fiction writer. But this may one day be a possibility through the emerging field of quantum biology.

Over the past few decades, scientists have made incredible progress in understanding and manipulating biological systems at increasingly small scales, from protein folding to genetic engineering. And yet, the extent to which quantum effects influence living systems remains barely understood.

Quantum effects are phenomena that occur between atoms and molecules that can’t be explained by classical physics. It has been known for more than a century that the rules of classical mechanics, like Newton’s laws of motion, break down at atomic scales. Instead, tiny objects behave according to a different set of laws known as quantum mechanics.

For humans, who can only perceive the macroscopic world, or what’s visible to the naked eye, quantum mechanics can seem counterintuitive and somewhat magical. Things you might not expect happen in the quantum world, like electrons “tunneling” through tiny energy barriers and appearing on the other side unscathed, or being in two different places at the same time in a phenomenon called superposition.

I am trained as a quantum engineer. Research in quantum mechanics is usually geared toward technology. However, and somewhat surprisingly, there is increasing evidence that nature – an engineer with billions of years of practice – has learned how to use quantum mechanics to function optimally. If this is indeed true, it means that our understanding of biology is radically incomplete. It also means that we could possibly control physiological processes by using the quantum properties of biological matter.

Quantumness in biology is probably real

Researchers can manipulate quantum phenomena to build better technology. In fact, you already live in a quantum-powered world: from laser pointers to GPS, magnetic resonance imaging and the transistors in your computer – all these technologies rely on quantum effects.

In general, quantum effects only manifest at very small length and mass scales, or when temperatures approach absolute zero. This is because quantum objects like atoms and molecules lose their “quantumness” when they uncontrollably interact with each other and their environment. In other words, a macroscopic collection of quantum objects is better described by the laws of classical mechanics. Everything that starts quantum dies classical. For example, an electron can be manipulated to be in two places at the same time, but it will end up in only one place after a short while – exactly what would be expected classically.

In a complicated, noisy biological system, it is thus expected that most quantum effects will rapidly disappear, washed out in what the physicist Erwin Schrödinger called the “warm, wet environment of the cell.” To most physicists, the fact that the living world operates at elevated temperatures and in complex environments implies that biology can be adequately and fully described by classical physics: no funky barrier crossing, no being in multiple locations simultaneously.

Chemists, however, have for a long time begged to differ. Research on basic chemical reactions at room temperature unambiguously shows that processes occurring within biomolecules like proteins and genetic material are the result of quantum effects. Importantly, such nanoscopic, short-lived quantum effects are consistent with driving some macroscopic physiological processes that biologists have measured in living cells and organisms. Research suggests that quantum effects influence biological functions, including regulating enzyme activitysensing magnetic fieldscell metabolism and electron transport in biomolecules.

How to study quantum biology

The tantalizing possibility that subtle quantum effects can tweak biological processes presents both an exciting frontier and a challenge to scientists. Studying quantum mechanical effects in biology requires tools that can measure the short time scales, small length scales and subtle differences in quantum states that give rise to physiological changes – all integrated within a traditional wet lab environment.

In my work, I build instruments to study and control the quantum properties of small things like electrons. In the same way that electrons have mass and charge, they also have a quantum property called spin. Spin defines how the electrons interact with a magnetic field, in the same way that charge defines how electrons interact with an electric field. The quantum experiments I have been building since graduate school, and now in my own lab, aim to apply tailored magnetic fields to change the spins of particular electrons.

Research has demonstrated that many physiological processes are influenced by weak magnetic fields. These processes include stem cell development and maturationcell proliferation ratesgenetic material repair and countless others. These physiological responses to magnetic fields are consistent with chemical reactions that depend on the spin of particular electrons within molecules. Applying a weak magnetic field to change electron spins can thus effectively control a chemical reaction’s final products, with important physiological consequences.

Currently, a lack of understanding of how such processes work at the nanoscale level prevents researchers from determining exactly what strength and frequency of magnetic fields cause specific chemical reactions in cells. Current cellphone, wearable and miniaturization technologies are already sufficient to produce tailored, weak magnetic fields that change physiology, both for good and for bad. The missing piece of the puzzle is, hence, a “deterministic codebook” of how to map quantum causes to physiological outcomes.

In the future, fine-tuning nature’s quantum properties could enable researchers to develop therapeutic devices that are noninvasive, remotely controlled and accessible with a mobile phone. Electromagnetic treatments could potentially be used to prevent and treat disease, such as brain tumors, as well as in biomanufacturing, such as increasing lab-grown meat production.

A whole new way of doing science

Quantum biology is one of the most interdisciplinary fields to ever emerge. How do you build community and train scientists to work in this area?

Since the pandemic, my lab at the University of California, Los Angeles and the University of Surrey’s Quantum Biology Doctoral Training Centre have organized Big Quantum Biology meetings to provide an informal weekly forum for researchers to meet and share their expertise in fields like mainstream quantum physics, biophysics, medicine, chemistry and biology.

Research with potentially transformative implications for biology, medicine and the physical sciences will require working within an equally transformative model of collaboration. Working in one unified lab would allow scientists from disciplines that take very different approaches to research to conduct experiments that meet the breadth of quantum biology from the quantum to the molecular, the cellular and the organismal.

The existence of quantum biology as a discipline implies that traditional understanding of life processes is incomplete. Further research will lead to new insights into the age-old question of what life is, how it can be controlled and how to learn with nature to build better quantum technologies.


This article is republished from The Conversation under a Creative Commons license. Read the original article.


Clarice D. Aiello is a quantum engineer interested in how quantum physics informs biology at the nanoscale. She is an expert on nanosensors that harness room-temperature quantum effects in noisy environments. Aiello received a bachelor’s in physics from the Ecole Polytechnique, France; a master’s degree in physics from the University of Cambridge, Trinity College, UK; and a PhD in electrical engineering from the Massachusetts Institute of Technology. She held postdoctoral appointments in bioengineering at Stanford University and in chemistry at the University of California, Berkeley. Two months before the pandemic, she joined the University of California, Los Angeles, where she leads the Quantum Biology Tech (QuBiT) Lab.


The author thanks Nicole Yunger Halpern and Spyridon Michalakis for the opportunity to talk about quantum biology to the physics audience of this wonderful blog!

Identical twins and quantum entanglement

“If I had a nickel for every unsolicited and very personal health question I’ve gotten at parties, I’d have paid off my medical school loans by now,” my doctor friend complained. As a physicist, I can somewhat relate. I occasionally find myself nodding along politely to people’s eccentric theories about the universe. A gentleman once explained to me how twin telepathy (the phenomenon where, for example, one twin feels the other’s pain despite being in separate countries) comes from twins’ brains being entangled in the womb. Entanglement is a nonclassical correlation that can exist between spatially separated systems. If two objects are entangled, it’s possible to know everything about both of them together but nothing about either one. Entangling two particles (let alone full brains) over tens of kilometres (let alone full countries) is incredibly challenging. “Using twins to study entanglement, that’ll be the day,” I thought. Well, my last paper did something like that. 

In theory, a twin study consists of two people that are as identical as possible in every way except for one. What that allows you to do is isolate the effect of that one thing on something else. Aleksander Lasek (postdoc at QuICS), David Huse (professor of physics at Princeton), Nicole Yunger Halpern (NIST physicist and Quantum Frontiers blogger), and I were interested in isolating the effects of quantities’ noncommutation (explained below) on entanglement. To do so, we first built a pair of twins and then compared them

Consider a well-insulated thermos filled with soup. The heat and the number of “soup particles” inside the thermos are conserved. So the energy and the number of “soup particles” are conserved quantities. In classical physics, conserved quantities commute. This means that we can simultaneously measure the amount of each conserved quantity in our system, like the energy and number of soup particles. However, in quantum mechanics, this needn’t be true. Measuring one property of a quantum system can change another measurement’s outcome.

Conserved quantities’ noncommutation in thermodynamics has led to some interesting results. For example, it’s been shown that conserved quantities’ noncommutation can decrease the rate of entropy production. For the purposes of this post, entropy production is something that limits engine efficiency—how well engines can convert fuel to useful work. For example, if your car engine had zero entropy production (which is impossible), it would convert 100% of the energy in your car’s fuel into work that moved your car along the road. Current car engines can convert about 30% of this energy, so it’s no wonder that people are excited about the prospective application of decreasing entropy production. Other results (like this one and that one) have connected noncommutation to potentially hindering thermalization—the phenomenon where systems interact until they have similar properties, like when a cup of coffee cools. Thermalization limits memory storage and battery lifetimes. Thus, learning how to resist thermalization could also potentially lead to better technologies, such as longer-lasting batteries. 

One can measure the amount of entanglement within a system, and as quantum particles thermalize, they entangle. Given the above results about thermalization, we might expect that noncommutation would decrease entanglement. Testing this expectation is where the twins come in.

Say we built a pair of twins that were identical in every way except for one. Nancy, the noncommuting twin, has some features that don’t commute, say, her hair colour and height. This means that if we measure her height, we’ll have no idea what her hair colour is. For Connor, the commuting twin, his hair colour and height commute, so we can determine them both simultaneously. Which twin has more entanglement? It turns out it’s Nancy.

Disclaimer: This paragraph is written for an expert audience. Our actual models consist of 1D chains of pairs of qubits. Each model has three conserved quantities (“charges”), which are sums over local charges on the sites. In the noncommuting model, the three local charges are tensor products of Pauli matrices with the identity (XI, YI, ZI). In the commuting model, the three local charges are tensor products of the Pauli matrices with themselves (XX, YY, ZZ). The paper explains in what sense these models are similar. We compared these models numerically and analytically in different settings suggested by conventional and quantum thermodynamics. In every comparison, the noncommuting model had more entanglement on average.

Our result thus suggests that noncommutation increases entanglement. So does charges’ noncommutation promote or hinder thermalization? Frankly, I’m not sure. But I’d bet the answer won’t be in the next eccentric theory I hear at a party.

Mo’ heights mo’ challenges – Climbing mount grad school

My wife’s love of mountain hiking and my interest in quantum thermodynamics collided in Telluride, Colorado.

We spent ten days in Telluride, where I spoke at the Information Engines at the Frontiers of Nanoscale Thermodynamics workshop. Telluride is a gorgeous city surrounded by mountains and scenic trails. My wife (Hasti) and I were looking for a leisurely activity one morning. We chose hiking Bear Creek Trail, a 5.1-mile hike with a 1092-foot elevation. This would have been a reasonable choice… back home.

Telluride’s elevation is 8,750 feet (ten times that of our hometown’s). This meant there was nothing leisurely about the hike. Ill-prepared, I dragged myself up the mountain in worn runners and tight jeans. My gasps for breath reminded me how new heights (a literal one in this case) could bring unexpected challenges – A lesson I’ve encountered many times as a graduate student. 

My wife and I atop bear creek trail

I completely squandered my undergrad. One story sums it up best. I was studying for my third-year statistical mechanics final when I realized I could pass the course without writing the final. So, I didn’t write the final. After four years of similar negligence, I somehow graduated, certain I’d left academics forever. Two years later, I rediscovered my love for physics and grieved about wasting my undergraduate. I decided to try again and apply for grad school. I learned Canada had 17 Universities I could apply to with a 70 average; each one rejected me.

I was ecstatic to eventually be accepted into a master’s of math. But, the high didn’t last. Learning math and physics from popular science books and PBS videos is very different from studying for University exams. If I wanted to keep this opportunity, I had to learn how to study.

16 months later, I graduated with a 97 average and an offer to do a master’s of physics at the University of Waterloo. I would be working at the Institute for Quantum Computing (IQC), supervised by Raymond Laflamme (one of the co-founders of the field of quantum computing). My first day at IQC felt surreal. I had become an efficient student and felt ready for the IQC. But, like the bear creek trail, another height would bring another challenge. Ultimately, grad school isn’t about getting good grades; it’s about researching. Raymond (Ray) gave me my first research project, and I was dumbfounded about where to start and too insecure about asking for help.

With Ray and Jonathan Halliwell’s (professor at Imperial College London and guitarist-extraordinaire) guidance, I published my first paper and accepted a Ph.D. offer from Ray. After publishing my second paper, I thought it would be smooth sailing through my Ph.D..  Alas, I was again mistaken. It’s also not enough to solve problems others give you; you need to come up with some problems on your own. So, I tried. I spent the first 8-months of my Ph.D. pursuing a problem I came up with, and It was a complete dud. It turns out the problems also need to be worth solving. For those keeping track, this is challenge number four.

I have now finished the third year of my Ph.D. (two, if you don’t count the year I “took off” to write a textbook with Ray and superconducting-qubit experimentalist, Prof. Chris Wilson). During that time, Nicole Yunger Halpern (NIST physicist, my new co-advisor, and Quantum Frontiers blogger) introduced me to the field of quantum thermodynamics. We’ve published a paper together (related blog post and Vanier interview) and have a second on the way. Despite that, I’m still grappling with that last challenge. I have no problem finding research questions that would be fun to solve. However, I’m still not sure which ones are worth solving. But, like the other challenges, I’m hopeful I’ll figure it out.

While this lesson was inspiring, the city of Telluride inspired me the most. Telluride is at a local minimum elevation, surrounded by mountains. Meaning there is virtually nowhere to go but up. I’m hoping the same is true for me.

Distilling Quantum Particles

This is a story about distillation—a process that has kept my family busy for generations.

My great, great, great, great grandfather was known as Brännvinskungen, loosely translated as the Vodka King. This “royal” ancestor of mine lived in the deepest forests of Småland, Sweden; the forests that during his time would populate the US state of Minnesota with emigrants fleeing the harshest lands of Europe. The demand for alcoholic beverages among their inhabitants was great. And the Vodka King had refined both his recipe and the technology to meet the demand. He didn’t claim to compete with big Stockholm-based companies in terms of quality or ambition. Nevertheless, his ability to, using simple means and low cost, turn water into (fortified) wine earned him his majestic title.

I’m not about to launch the concept of quantum vodka. Instead, I’m about to tell you about my and my stellar colleagues’ results on the distillation of quantum particles. In the spirit of the Vodka King, I don’t intend to compete with the big players of quantum computing. Instead, I will describe how a simple and low-cost method can distil information in quantum particles and improve technologies for measurements of physical things. Before I tell you about how quantum distillation can improve measurements, I need to explain why anyone would use quantum physics to do measurements in the first place, something known as quantum metrology.

According to Wikipedia, “metrology is the scientific study of measurement”. And just about any physical experiment or technology relies on measurements. Quantum metrology is the field of using quantum phenomena, such as entanglement, to improve measurements [1]. The ability to quantum-boost technologies for measurements has fostered a huge interest in quantum metrology. My hope is that speedometers, voltmeters, GPS devices and clocks will be improved by quantum metrology in the near future. 

There are some problems to overcome before quantum metrology will make it to the mainstream. Just like our eyes on a bright day, quantum-measurement devices saturate (are blinded) if they are subjected to overly intense beams of quantum particles. Very often the particle detectors are the limiting factor in quantum metrology: one can prepare incredibly strong beams of quantum particles, but one cannot detect and access all the information they contain. To remedy this, one could use lower-intensity beams, or insert filters just before the detectors. But ideally, one would distil the information from a large number of particles into a few, going from high to low intensity without losing any information. 

Figure 1: Rough workings of non-polarising sunglasses (left), polarising sunglasses (middle) and the new quantum filter (right). Light-particles are represented by bottles, and information by the bottles’ content.

Collaborators and I have developed a quantum filter that solves this precise problem [2, 3]. (See this blog post for more details on our work.) Our filter provides sunglasses for quantum-metrology technologies. However, unlike normal sunglasses, our quantum filters increase the information content of the individual particles that pass through them. Figure 1 compares sunglasses (polarising and non-polarising) with our quantum filter; miniature bottles represent light-particles, and their content represents information.

  • The left-most boxes show the effect of non-polarising sunglasses, which can be used when there is a strong beam of different types of light particles that carry different amounts of information. The sunglasses block a fraction of the light particles. This reduces glare and avoids eyes’ being blinded. However, information is lost with the blocked light particles. 
  • When driving a car, you see light particles from the surroundings, which vibrate both horizontally and vertically. The annoying glare from the road, however, is made of light particles which vibrate predominantly horizontally. In this scenario, vertical light carries more information than horizontal light. Polarising sunglasses (middle boxes) can help. Irritating horizontal light particles are blocked, but informative vertical ones aren’t. On the level of the individual particles, however, no distillation takes place; the information in a vertical light particle is the same before and after the filter.
  • The right-most boxes show the workings of our quantum filter. In quantum metrology, often all particles are the same, and all carry a small amount of information. Our filter blocks some particles, but compresses their information into the particles that survive the filter. The number of particles is reduced, but the information isn’t.

Our filter is not only different to sunglasses, but also to standard distillation processes. Distillation of alcohol has a limit: 100%. Given 10 litres of 10% wine, one could get at most 1 litre of 100% alcohol, not ½ litres of 200% alcohol. Our quantum filters are different. There is no cap on how much information can be distilled into a few particles; the information of a million particles can all be compressed into a single quantum particle. This exotic feature relies on negativity [4]. Quantum things cannot generally be described by probabilities between 0% and 100%, sometimes they require the exotic occurrence of negative probabilities. Experiments whose explanations require negative probabilities are said to possess negativity. 

Figure 2: Quantum metrology with laser-light particles. (a) Without quantum filter. (b) With quantum filter.

In a recent theory-experiment collaboration, spearheaded by Aephraim Steinberg’s quantum-optics group, our multi-institutional team designed a measurement device that can harness negativity [5]. Figure 2 shows an artistic model of our technology. We used single light particles to measure the optical rotation induced by a piece of crystal. Light particles were created by a laser, and then sent through the crystal. The light particles were rotated by the crystal: information about the degree of rotation was encoded in the particles. By measuring these particles, we could access this information and learn what the rotation was. In Figure 2(a) the beam of particles is too strong, and the detectors do not work properly. Thus, we insert our quantum filter [Figure 2(b)]. Every light particle that passed our quantum filter carried the information of over 200 blocked particles. In other words, the number of particles that reached our detector was 200 times less, but the information the detector received stayed constant. This allowed us to measure the optical rotation to a level impossible without our filter. 

Our ambition is that our proof-of-principle experiment will lead to the development of filters for other measurements, beyond optical rotations. Quantum metrology with light particles is involved in technologies ranging from quantum-computer calibration to gravitational-wave detection, so the possibilities for our metaphorical quantum vodka are many.

David Arvidsson-Shukur, Cambridge (UK), 14 April 2022

David is a quantum researcher at the Hitachi Cambridge Laboratory. His research focuses on both fundamental aspects of quantum phenomena, and on practical aspects of bringing such phenomena into technologies.

[1] ‘Advances in quantum metrology’, V. Giovannetti, S. Lloyd, L. Maccone, Nature photonics, 5, 4, (2011), https://www.nature.com/articles/nphoton.2011.35

[2] ‘Quantum Advantage in Postselected Metrology’, D. R. M. Arvidsson-Shukur, N. Yunger Halpern, H. V. Lepage, A. A. Lasek, C. H. W. Barnes, and S. Lloyd, Nature Communications, 11, 3775 (2020), https://doi.org/10.1038/s41467-020-17559-w

[3] ‘Quantum Learnability is Arbitrarily Distillable’, J. Jenne, D. R. M. Arvidsson-Shukur, arXiv, (2020), https://arxiv.org/abs/2104.09520

[4] ‘Conditions tighter than noncommutation needed for nonclassicality’, D. R. M. Arvidsson-Shukur, J. Chevalier Drori, N. Yunger Halpern, J. Phys. A: Math. Theor., 54, 284001, (2021), https://iopscience.iop.org/article/10.1088/1751-8121/ac0289

[5] ‘Negative quasiprobabilities enhance phase-estimation in quantum-optics experiment’, N. Lupu-Gladstein, Y. B. Yilmaz, D. R. M. Arvidsson-Shukur, A. Broducht, A. O. T. Pang, Æ. Steinberg, N. Yunger Halpern, P.R.L (in production), (2022), https://arxiv.org/abs/2111.01194

Quantum Encryption in a Box

Over the last few decades, transistor density has become so high that classical computers have run into problems with some of the quirks of quantum mechanics. Quantum computers, on the other hand, exploit these quirks to revolutionize the way computers work. They promise secure communications, simulation of complex molecules, ultrafast computations, and much more. The fear of being left behind as this new technology develops is now becoming pervasive around the world. As a result, there are large, near-term investments in developing quantum technologies, with parallel efforts aimed at attracting young people into the field of quantum information science and engineering in the long-term.

I was not surprised then that, after completing my master’s thesis in quantum optics at TU Berlin in Germany, I was invited to participate in a program called Quanten 1×1 and hosted by the Junge Tueftler (Young Tinkerers) non-profit, to get young people excited about quantum technologies. As part of a small team, we decided to develop tabletop games to explain the concepts of superposition, entanglement, quantum gates, and quantum encryption. In the sections that follow, I will introduce the thought process that led to the design of one of the final products on quantum encryption. If you want to learn more about the other games, you can find the relevant links at the end of this post.

The price of admission into the quantum realm

How much quantum mechanics is too much? Is it enough for people to know about the health of Schrödinger’s cat, or should we use a squishy ball with a smiley face and an arrow on it to get people excited about qubits and the Bloch sphere? In other words, what is the best way to go beyond metaphors and start delving into the real stuff? After all, we are talking about cutting-edge quantum technology here, which requires years of study to understand. Even the quantum experts I met with during the project had a hard time explaining their work to lay people.

Since there is no standardized way to explain these topics outside a university, the goal of our project was to try different models to teach quantum phenomena and make the learning as entertaining as possible. Compared to methods where people passively absorb the information, our tabletop-games approach leverages people’s curiosity and leads to active learning through trial and error.

A wooden quantum key generator (BB84)

Everybody has secrets

Most of the (sensitive) information that is transmitted over the Internet is encrypted. This means that only those with the right “secret key” can unlock the digital box and read the private message within. Without the secret key used to decrypt, the message looks like gibberish – a series of random characters. To encrypt the billions of messages being exchanged every day (over 300 billion emails alone), the Internet relies heavily on public-key cryptography and so-called one-way functions. These mathematical functions allow one to generate a public key to be shared with everyone, from a private key kept to themselves. The public key plays the role of a digital padlock that only the private key can unlock. Anyone (human or computer) who wants to communicate with you privately can get a digital copy of your padlock (by copying it from a pinned tweet on your Twitter account, for example), put their private message inside a digital box provided by their favorite app or Internet communication protocol running behind the scenes, lock the digital box using your digital padlock (public-key), and then send it over to you (or, accidentally, to anyone else who may be trying to eavesdrop). Ingeniously, only the person with the private key (you) can open the box and read the message, even if everyone in the world has access to that digital box and padlock.

But there is a problem. Current one-way functions hide the private key within the public key in a way that powerful enough quantum computers can reveal. The implications of this are pretty staggering. Your information (bank account, email, bitcoin wallet, etc) as currently encrypted will be available to anyone with such a computer. This is a very serious issue of global importance. So serious indeed, that the President of the United States recently released a memo aimed at addressing this very issue. Fortunately, there are ways to fight quantum with quantum. That is, there are quantum encryption protocols that not even quantum computers can break. In fact, they are as secure as the laws of physics.

Quantum Keys

A popular way of illustrating how quantum encryption works is through single photon sources and polarization filters. In classroom settings, this often boils down to lasers and small polarizing filters a few meters apart. Although lasers are pretty cool, they emit streams of photons (particles of light), not single photons needed for quantum encryption. Moreover, measuring polarization of individual photons (another essential part of this process) is often very tricky, especially without the right equipment. In my opinion the concept of quantum mechanical measurement and the collapse of wave functions is not easily communicated in this way.

Inspired by wooden toys and puzzles my mom bought for me as a kid after visits to the dentist, I tried to look for a more physical way to visualize the experiment behind the famous BB84 quantum key distribution protocol. After a lot of back and forth between the drawing board and laser cutter, the first quantum key generator (QeyGen) was built. 

How does the box work?

Note: This short description leaves out some details. For a deeper dive, I recommend watching the tutorial video on our Youtube channel.

The quantum key generator (QeyGen) consists of an outer and an inner box. The outer box is used by the person generating the secret key, while the inner box is used by the person with whom they wish to share that key. The sender prepares a coin in one of two states (heads = 0, tails = 1) and inserts it either into slot 1 (horizontal basis), or slot 2 (vertical basis) of the outer box. The receiver then measures the state of the coin in one of the same two bases by sliding the inner box to the left (horizontal basis = 1) or right (vertical basis = 2). Crucially, if the bases to prepare and measure the coin match, then both sender and receiver get the same value for the coin. But if the basis used to prepare the coin doesn’t match the measurement basis, the value of the coin collapses into one of the two allowed states in the measurement basis with 50/50 chance. Because of this design, the box can be used to illustrate the BB84 protocol that allows two distant parties to create and share a secure encryption key.

Simulating the BB84 protocol

The following is a step by step tutorial on how to play out the BB84 protocol with the QeyGen. You can play it with two (Alice, Bob) or three (Alice, Bob, Eve) people. It is useful to know right from the start that this protocol is not used to send private messages, but is instead used to generate a shared private key that can then be used with various encryption methods, like the one-time pad, to send secret messages.

BB84 Protocol:

  1. Alice secretly “prepares” a coin by inserting it facing-towards (0) or facing-away (1) from her into one of the two slots (bases) on the outer box. She writes down the value (0 or 1) and basis (horizontal or vertical) of the coin she just inserted.
  2. (optional) Eve, the eavesdropper, tries to “measure” the coin by sliding the inner box left (horizontal basis) or right (vertical basis), before putting the coin back through the outer box without anyone noticing.
  3. Bob then secretly measures the coin in a basis of his choice and writes down the value (0 or 1) and basis (horizontal and vertical) as well.
  4. Steps 1 and 3 are then repeated several times. The more times Alice and Bob go through this process, the more secure their secret key will be.

Sharing the key while checking for eavesdroppers:

  1. Alice and Bob publicly discuss which bases they used at each “prepare” and “measure” step, and cross out the values of the coin corresponding to the bases that didn’t match (about half of them on average; here, it would be rounds 1,3,5,6,7, and 11).
  2. Then, they publicly announce the first few (or a random subset of the) values that survive the previous step (i.e. have matching bases; here, it is rounds 2 and 4). If the values match for each round, then it is safe to assume that there was no eavesdrop attack. The remaining values are kept secret and can be used as a secure key for further communication.
  3. If the values of Alice and Bob don’t match, Eve must have measured the coin (before Bob) in the wrong basis (hence, randomizing its value) and put it back in the wrong orientation from the one Alice had originally chosen. Having detected Eve’s presence, Alice and Bob switch to a different channel of communication and try again.

Note that the more rounds Alice and Bob choose for the eavesdropper detection, the higher the chance that the channel of communication is secure, since N rounds that all return the same value for the coin mean a 2^{-N} chance that Eve got lucky and guessed Alice’s inputs correctly. To put this in perspective, a 20-round check for Eve provides a 99.9999% guarantee of security. Of course, the more rounds used to check for Eve, the fewer secure bits are left for Alice and Bob to share at the end. On average, after a total of 2(N+M) rounds, with N rounds dedicated to Eve, we get an M-bit secret key.

What do people learn?

When we play with the box, we usually encounter three main topics that we discuss with the participants.

  1. qm states and quantum particles: We talk about superposition of quantum particles and draw an analogy from the coin to polarized photons.
  2. qm measurement and basis: We ask about the state of the coin and discuss how we actually define a state and a basis for a coin. By using the box, we emphasize that the measurement itself (in which basis the coin is observed) can directly affect the state of the coin and collapse its “wavefunction”.
  3. BB84 protocol: After a little playtime of preparing and measuring the coin with the box, we introduce the steps to perform the BB84 protocol as described above. The penny-dropping moment (pun intended) often happens when the participants realize that a spy intervening between preparation and measurement can change the state of the coin, leading to contradictions in the subsequent eavesdrop test of the protocol and exposing the spy.

I hope that this small outline has provided a rough idea of how the box works and why we developed it. If you have access to a laser cutter, I highly recommend making a QeyGen for yourself (link to files below). For any further questions, feel free to contact me at t.schubert@fu-berlin.de.

Resources and acknowledgments

Project page Junge Tueftler: tueftelakademie.de/quantum1x1
Video series for the QeyGen: youtube.com/watch?v=YmdoAP1TJRo
Laser cut files: thingiverse.com/thing:5376516

The program was funded by the Federal Ministry of Education and Research (Germany) and was a collaboration between the Jungen Tueftlern and the Technical University of Berlin.
A special thanks to Robert from Project Sci.Com who helped me with the development.

Life among the experimentalists

I used to catch lizards—brown anoles, as I learned to call them later—as a child. They were colored as their name suggests, were about as long as one of my hands, and resented my attention. But they frequented our back porch, and I had a butterfly net. So I’d catch lizards, with my brother or a friend, and watch them. They had throats that occasionally puffed out, exposing red skin, and tails that detached and wriggled of their own accord, to distract predators.

Some theorists might appreciate butterfly nets, I imagine, for catching experimentalists. Some of us theorists will end a paper or a talk with “…and these predictions are experimentally accessible.” A pause will follow the paper’s release or the talk, in hopes that a reader or an audience member will take up the challenge. Usually, none does, and the writer or speaker retires to the Great Deck Chair of Theory on the Back Patio of Science.

So I was startled when an anole, metaphorically speaking, volunteered a superconducting qubit for an experiment I’d proposed.

The experimentalist is one of the few people I can compare to a reptile without fear that he’ll take umbrage: Kater Murch, an associate professor of physics at Washington University in St. Louis. The most evocative description of Kater that I can offer appeared in an earlier blog post: “Kater exudes the soberness of a tenured professor but the irreverence of a Californian who wears his hair slightly long and who tattooed his wedding band on.”

Kater expressed interest in an uncertainty relation I’d proved with theory collaborators. According to some of the most famous uncertainty relations, a quantum particle can’t have a well-defined position and a well-defined momentum simultaneously. Measuring the position disturbs the momentum; any later momentum measurement outputs a completely random, or uncertain, number. We measure uncertainties with entropies: The greater an entropy, the greater our uncertainty. We can cast uncertainty relations in terms of entropies.

I’d proved, with collaborators, an entropic uncertainty relation that describes chaos in many-particle quantum systems. Other collaborators and I had shown that weak measurements, which don’t disturb a quantum system much, characterize chaos. So you can check our uncertainty relation using weak measurements—as well as strong measurements, which do disturb quantum systems much. One can simplify our uncertainty relation—eliminate the chaos from the problem and even eliminate most of the particles. An entropic uncertainty relation for weak and strong measurements results.

Kater specializes in weak measurements, so he resolved to test our uncertainty relation. Physical Review Letters published the paper about our collaboration this month. Quantum measurements can not only create uncertainty, the paper shows, but also reduce it: Kater and his PhD student Jonathan Monroe used light to measure a superconducting qubit, a tiny circuit in which current can flow forever. The qubit had properties analogous to position and momentum (the spin’s z– and x-components). If the atom started with a well-defined “position” (the z-component) and the “momentum” (the x-component) was measured, the outcome was highly random; the total uncertainty about the two measurements was large. But if the atom started with a well-defined “position” (z-component) and another property (the spin’s y-component) was measured before the “momentum” (the x-component) was measured strongly, the total uncertainty was lower. The extra measurement was designed not to disturb the atom much. But the nudge prodded the atom enough, rendering the later “momentum” measurement (the x measurement) more predictable. So not only can quantum measurements create uncertainty, but gentle quantum measurements can also reduce it.

I didn’t learn only physics from our experiment. When I’d catch a lizard, I’d tip it into a tank whose lid contained a magnifying lens, and I’d watch the lizard. I didn’t trap Kater and Jonathan under a magnifying glass, but I did observe their ways. Here’s what I learned about the species experimentalus quanticus.

1) They can run experiments remotely when a pandemic shuts down campus: A year ago, when universities closed and cities locked down, I feared that our project would grind to a halt. But Jonathan twiddled knobs and read dials via his computer, and Kater popped into the lab for the occasional fixer-upper. Jonathan even continued his experiment from another state, upon moving to Texas to join his parents. And here we theorists boast of being able to do our science almost anywhere.

2) They speak with one less layer of abstraction than I: We often discussed, for instance, the thing used to measure the qubit. I’d call the thing “the detector.” Jonathan would call it “the cavity mode,” referring to the light that interacts with the qubit, which sits in a box, or cavity. I’d say “poh-tay-toe”; they’d say “poh-tah-toe”; but I’m glad we didn’t call the whole thing off.

Fred Astaire: “Detector.”
Ginger Rogers: “Cavity mode.”

3) Experiments take longer than expected—even if you expect them to take longer than estimated: Kater and I hatched the plan for this project during June 2018. The experiment would take a few months, Kater estimated. It terminated last summer.

4) How they explain their data: Usually in terms of decoherence, the qubit’s leaking of quantum information into its environment. For instance, to check that the setup worked properly, Jonathan ran a simple test that ended with a measurement. (Experts: He prepared a \sigma_z eigenstate, performed a Hadamard gate, and measured \sigma_z.) The measurement should have had a 50% chance of yielding +1 and a 50% chance of yield -1. But the -1 outcome dominated the trials. Why? Decoherence pushed the qubit toward toward -1. (Amplitude damping dominated the noise.)

5) Seeing one’s theoretical proposal turn into an experiment feels satisfying: Due to point (3), among other considerations, experiments aren’t cheap. The lab’s willingness to invest in the idea I’d developed with other theorists was heartening. Furthermore, the experiment pushed us to uncover more theory—for example, how tight the uncertainty bound could grow.

After getting to know an anole, I’d release it into our backyard and bid it adieu.1 So has Kater moved on to experimenting with topology, and Jonathan has progressed toward graduation. But more visitors are wriggling in the Butterfly Net of Theory-Experiment Collaboration. Stay tuned.

1Except for the anole I accidentally killed, by keeping it in the tank for too long. But let’s not talk about that.

May you go from weakness to weakness

I used to eat lunch at the foundations-of-quantum-theory table. 

I was a Masters student at the Perimeter Institute for Theoretical Physics, where I undertook a research project during the spring term. The project squatted on the border between quantum information theory and quantum foundations, where my two mentors worked. Quantum foundations concerns how quantum physics differs from classical physics; which alternatives to quantum physics could govern our world but don’t; and those questions, such as about Schrödinger’s cat, that fascinate us when we first encounter quantum theory, that many advisors warn probably won’t land us jobs if we study them, and that most physicists argue about only over a beer in the evening.

I don’t drink beer, so I had to talk foundations over sandwiches around noon.

One of us would dream up what appeared to be a perpetual-motion machine; then the rest of us would figure out why it couldn’t exist. Satisfied that the second law of thermodynamics still reigned, we’d decamp for coffee. (Perpetual-motion machines belong to the foundations of thermodynamics, rather than the foundations of quantum theory, but we didn’t discriminate.) I felt, at that lunch table, an emotion blessed to a student finding her footing in research, outside her country of origin: belonging.

The quantum-foundations lunch table came to mind last month, when I learned that Britain’s Institute of Physics had selected me to receive its International Quantum Technology Emerging Researcher Award. I was very grateful for the designation, but I was incredulous: Me? Technology? But I began grad school at the quantum-foundations lunch table. Foundations is to technology as the philosophy of economics is to dragging a plow across a wheat field, at least stereotypically.

Worse, I drag plows from wheat field to barley field to oat field. I’m an interdisciplinarian who never belongs in the room I’ve joined. Among quantum information theorists, I’m the thermodynamicist, or that theorist who works with experimentalists; among experimentalists, I’m the theorist; among condensed-matter physicists, I’m the quantum information theorist; among high-energy physicists, I’m the quantum information theorist or the atomic-molecular-and-optical (AMO) physicist; and, among quantum thermodynamicists, I do condensed matter, AMO, high energy, and biophysics. I usually know less than everyone else in the room about the topic under discussion. An interdisciplinarian can leverage other fields’ tools to answer a given field’s questions and can discover questions. But she may sound, to those in any one room, as though she were born yesterday. As Kermit the Frog said, 

Grateful as I am, I’d rather not dwell on why the Institute of Physics chose my file; anyone interested can read the citation or watch the thank-you speech. But the decision turned out to involve foundations and interdisciplinarity. So I’m dedicating this article to two sources of inspiration: an organization that’s blossomed by crossing fields and an individual who’s driven technology by studying fundamentals.

Britain’s Institute for Physics has a counterpart in the American Physical Society. The latter has divisions, each dedicated to some subfield of physics. If you belong to the society and share an interest in one of those subfields, you can join that division, attend its conferences, and receive its newsletters. I learned about Division of Soft Matter from this article, which I wish I could quote almost in full. This division’s members study “a staggering variety of materials from the everyday to the exotic, including polymers such as plastics, rubbers, textiles, and biological materials like nucleic acids and proteins; colloids, a suspension of solid particles such as fogs, smokes, foams, gels, and emulsions; liquid crystals like those found in electronic displays; [ . . . ] and granular materials.” Members belong to physics, chemistry, biology, engineering, and geochemistry. 

Despite, or perhaps because of, its interdisciplinarity, the division has thrived. The group grew from a protodivision (a “topical group,” in the society’s terminology) to a division in five years—at “an unprecedented pace.” Intellectual diversity has complemented sociological diversity: The division “ranks among the top [American Physical Society] units in terms of female membership.” The division’s chair observes a close partnership between theory and experiment in what he calls “a vibrant young field.”

And some division members study oobleck. Wouldn’t you like to have an excuse to say “oobleck” every day?

The second source of inspiration lives, like the Institute of Physics, in Britain. David Deutsch belongs at the quantum-foundations table more than I. A theoretical physicist at Oxford, David cofounded the field of quantum computing. He explained why to me in a fusion of poetry and the pedestrian: He was “fixing the roof” of quantum theory. As a graduate student, David wanted to understand quantum foundations—what happens during a measurement—but concluded that quantum theory has too many holes. The roof was leaking through those holes, so he determined to fix them. He studied how information transformed during quantum processes, married quantum theory with computer science, and formalized what quantum computers could and couldn’t accomplish. Which—years down the road, fused with others’ contributions—galvanized experimentalists to harness ions and atoms, improve lasers and refrigerators, and build quantum computers and quantum cryptography networks. 

David is a theorist and arguably a philosopher. But he’d have swept the Institute of Physics’s playing field, could he have qualified as an “emerging researcher” this autumn (David began designing quantum algorithms during the 1980s).

I returned to the Perimeter Institute during the spring term of 2019. I ate lunch at the quantum-foundations table, and I felt that I still belonged. I feel so still. But I’ve eaten lunch at other tables by now, and I feel that I belong at them, too. I’m grateful if the habit has been useful.

Congratulations to Hannes Bernien, who won the institute’s International Quantum Technology Young Scientist Award, and to the “highly commended” candidates, whom you can find here!

Quantum steampunk invades Scientific American

London, at an hour that made Rosalind glad she’d nicked her brother’s black cloak instead of wearing her scarlet one. The factory alongside her had quit belching smoke for the night, but it would start again soon. A noise caused her to draw back against the brick wall. Glancing up, she gasped. An oblong hulk was drifting across the sky. The darkness obscured the details, but she didn’t need to see; a brass-colored lock would be painted across the side. Mellator had launched his dirigible.

A variation on the paragraph above began the article that I sent to Scientific American last year. Clara Moskowitz, an editor, asked which novel I’d quoted the paragraph from. I’d made the text up, I confessed. 


Most of my publications, which wind up in physics journals, don’t read like novels. But I couldn’t resist when Clara invited me to write a feature about quantum steampunk, the confluence of quantum information and thermodynamics. Quantum Frontiers regulars will anticipate paragraphs two and three of the article:

Welcome to steampunk. This genre has expanded across literature, art and film over the past several decades. Its stories tend to take place near nascent factories and in grimy cities, in Industrial Age England and the Wild West—in real-life settings where technologies were burgeoning. Yet steampunk characters extend these inventions into futuristic technologies, including automata and time machines. The juxtaposition of old and new creates an atmosphere of romanticism and adventure. Little wonder that steampunk fans buy top hats and petticoats, adorn themselves in brass and glass, and flock to steampunk conventions. 

These fans dream the adventure. But physicists today who work at the intersection of three fields—quantum physics, information theory and thermodynamics—live it. Just as steampunk blends science-fiction technology with Victorian style, a modern field of physics that I call “quantum steampunk” unites 21st-century technology with 19th-century scientific principles. 

The Scientific American graphics team dazzled me. For years, I’ve been hankering to work with artists on visualizing quantum steampunk. I had an opportunity after describing an example of quantum steampunk in the article. The example consists of a quantum many-body engine that I designed with members Christopher White, Sarang Gopalakrishnan, and Gil Refael of Caltech’s Institute for Quantum Information and Matter. Our engine is a many-particle system ratcheted between two phases accessible to quantum matter, analogous to liquid and solid. The engine can be realized with, e.g., ultracold atoms or trapped ions. Lasers would trap and control the particles. Clara, the artists, and I drew the engine, traded comments, and revised the figure tens of times. In early drafts, the lasers resembled the sketches in atomic physicists’ Powerpoints. Before the final draft, the lasers transformed into brass-and-glass beauties. They evoke the scientific instruments crafted through the early 1900s, before chunky gray aesthetics dulled labs.


Scientific American published the feature this month; you can read it in print or, here, online. Many thanks to Clara for the invitation, for shepherding the article into print, and for her enthusiasm. To repurpose the end of the article, “You’re reading about this confluence of old and new on Quantum Frontiers. But you might as well be holding a novel by H. G. Wells or Jules Verne.”


Figures courtesy of the Scientific American graphics team.

Achieving superlubricity with graphene

Sometimes, experimental results spark enormous curiosity inspiring a myriad of questions and ideas for further experimentation. In 2004, Geim and Novoselov, from The University of Manchester, isolated a single layer of graphene from bulk graphite with the “Scotch Tape Method” for which they were awarded the 2010 Nobel Prize in Physics.  This one experimental result has branched out countless times serving as a source of inspiration in as many different fields.  We are now in the midst of an array of branching-out in graphene research, and one of those branches gaining attention is ultra low friction observed between graphene and other surface materials.  

Much has been learned about graphene in the past 15 years through an immense amount of research, most of which, in non-mechanical realms (e.g., electron transport measurements, thermal conductivity, pseudo magnetic fields in strain engineering).  However, superlubricity, a mechanical phenomenon, has become the focus among many research groups. Mechanical measurements have famously shown graphene’s tensile strength to be hundreds of times that of the strongest steel, indisputably placing it atop the list of construction materials best for a superhero suit.  Superlubricity is a tribological property of graphene and is, arguably, as equally impressive as graphene’s tensile strength.

Tribology is the study of interacting surfaces during relative motion including sources of friction and methods for its reduction.  It’s not a recent discovery that coating a surface with graphite (many layers of graphene) can lower friction between two sliding surfaces.  Current research studies the precise mechanisms and surfaces for which to minimize friction with single or several layers of graphene. 

Research published in Nature Materials in 2018 measures friction between surfaces under constant load and velocity. The experiment includes two groups; one consisting of two graphene surfaces (homogeneous junction), and another consisting of graphene and hexagonal boron nitride (heterogeneous junction).   The research group measures friction using Atomic Force Microscopy (AFM).  The hexagonal boron nitride (or graphene for a homogeneous junction) is fixed to the stage of the AFM while the graphene slides atop.  Loads are held constant at 20 𝜇N and sliding velocity constant at 200 nm/s. Ultra low friction is observed for homogeneous junctions when the underlying crystalline lattice structures of the surfaces are at a relative angle of 30 degrees.  However, this ultra low friction state is very unstable and upon sliding, the surfaces rotate towards a locked-in lattice alignment. Friction varies with respect to the relative angle between the two surface’s crystalline lattice structures. Minimum (ultra low) friction occurs at a relative angle of 30 degrees reaching a maximum when locked-in lattice alignment is realized upon sliding. While in a state of lattice alignment, shearing is rendered impossible with the experimental setup due to the relatively large amount of friction.

Friction varies with respect to the relative angle of the crystalline lattice structures and is, therefore, anisotropic.  For example, the fact it takes less force to split wood when an axe blade is applied parallel to its grains than when applied perpendicularly illustrates the anisotropic nature of wood, as the force to split wood is dependent upon the direction along which the force is applied.  Frictional anisotropy is greater in homogeneous junctions because the tendency to orient into a stuck, maximum friction alignment, is greater than with heterojunctions.  In fact, heterogeneous junctions experience frictional anisotropy three orders of magnitude less than homogeneous junctions. Heterogenous junctions display much less frictional anisotropy due to a lattice misalignment when the angle between the lattice vectors is at a minimum.  In other words, the graphene and hBN crystalline lattice structures are never parallel because the materials differ, therefore, never experience the impact of lattice alignment as do homogenous junctions. Hence, heterogeneous junctions do not become stuck in a high friction state that characterizes homogeneous ones, and experience ultra low friction during sliding at all relative crystalline lattice structure angles.

Presumably, to increase applicability, upscaling to much larger loads will be necessary. A large scale cost effective method to dramatically reduce friction would undoubtedly have an enormous impact on a great number of industries.  Cost efficiency is a key component to the realization of graphene’s potential impact, not only as it applies to superlubricity, but in all areas of application.  As access to large amounts of affordable graphene increases, so will experiments in fabricating devices exploiting the extraordinary characteristics which have placed graphene and graphene based materials on the front lines of material research the past couple decades.

A Roman in a Modern Court

Yesterday I spent some time wondering how to explain the modern economy to an ancient Roman brought forward from the first millennium BCE. For now I’ll assume language isn’t a barrier, but not much more. Here’s my rough take:

“There have been five really important things that were discovered since when you left and now.

First, every living thing has a tiny blueprint inside it. We learned how to rewrite those, and now we can make crops that resist pests, grow healthy, and take minimal effort to cultivate. The same tool also let us make creatures that manufacture medicine, as well as animals different from anything that existed before. Food became cheap because of this.

Second, we learned that hot air and steam expand. This means you can burn oil or coal and use that to push air around, which in turn can push against solid objects. With this we’ve made vehicles that can go the span of the Empire from Rome to Londinium and back in hours rather than weeks. Similar mechanisms can be used to work farms, forge metal, and so on. Manufactured goods became cheap as a result.

Third, we discovered an invisible fluid that lives in metals. It flows unimaginably quickly and with minimal force through even very narrow channels, so by pushing on it in one city it may be made to move almost instantly in another. That lets you work with energy as a kind of commodity, rather than something that hooks up and is generated specifically for each device.

Fourth, we found that this fluid can be pushed around by light, including a kind human eyes can’t see. This lets a device make light in one place and push on the fluid in a different device with no metal in between. Communication became fast, cheap, and easy.

Finally, and this one takes some explaining, our machines can make decisions. Imagine you had a channel for water with a fork. You can insert a blade to control which route the water takes. If you attach that blade to a lever you can change the direction of the flow. If you dip that lever in another channel of water, then what flows in one channel can set which way another channel goes. It turns out that that’s all you need to make simple decisions like “If water is in this channel, flow down that other one.”, which can then be turned into useful statements like “Put water in this channel if you’re attacked. It’ll redirect the other channel and release boiling oil.” With enough of these water switches you can do really complicated things like tracking money, searching for patterns, predicting the weather, and so on. While water is hard to work with, you can make these channels and switches almost perfect for the invisible fluid, and you can make them tiny, vastly smaller than the width of a hair. A device that fits in your hand might have more switches than there are grains in a cubic meter of sand. The number of switches we’ve made so far outnumbers all the grains of sand on Earth, and we’re just getting started.”