When my family and friends asked me what I do in the lab during the first year of my PhD, I had the singular pleasure of telling them that I shine lasers and apply microwaves to diamonds to study quantum mechanics at the atomic scale. But what do all those cool and (quite literally) shiny, words mean? You probably know this already, but if you don’t, quantum computers are made of qubits, the quantum version of the classical computer's bits. Bits on your phone, or the computer you may be reading this blog on, consist of a small voltage that either is or isn't transmitted through a transistor to encode one and zero, respectively. The quantum version of this instead uses qubits, which encode these ones and zeros in quantum states. Because of the quantum properties of superposition and entanglement, quantum computers can handle and process this information in richer ways.
There isn’t just one physical quantum system that has been deemed suitable to create qubits, though. IBM studies superconducting qubits, while other groups study atoms, trapped ions, photons and even a very exotic class of particles called anyons. In my PhD at the University of Chicago (and at many research groups across the world), we look at spin qubits comprised of defects in solids. Specifically, we study the nitrogen vacancy center in diamonds.
/arc-anglerfish-arc2-prod-tronc.s3.amazonaws.com/public/AU7BKRK2T5BAFGXFP4XTL2IL6U.jpg)
An optics table with lenses and mirrors directing photoluminescence from a semiconductor defect to a photon detector in Professor David Awschalom's lab at The University of Chicago, where I work.
[Source: Chicago Tribune.]
Diamonds are made from atomic lattices where each carbon atom links up to four other carbon atoms. Diamond is a very unique material, being incredibly hard, transparent to light at infrared through ultraviolet frequencies, thermally conductive, yet nonetheless, a semiconductor. In most modern electronic devices, semiconductor defects are generally considered to be undesirable. However, in diamond, it turns out that these defects are useful as qubits.
Structure of the NV center in the diamond lattice.
[Source: Heremans F.J. et al, IEEE, 2016.]
A nitrogen vacancy center in a diamond occurs when one carbon atom is missing, and next to this empty lattice site sits a nitrogen atom. Much like an atom in a vacuum binds electrons to it, defect centers, like the nitrogen vacancy center, trap electrons. The spin of these trapped electrons form the basis of our qubit. Spin is a quantum property of particles, albeit a rather mysterious one. Imagine spinning a top, and that the top creates a magnetic field from this motion and also has some angular momentum. Subatomic particles or combinations of particles have a property that has a similar effect as this spinning top on the particle’s magnetic environment which defines its intrinsic angular momentum, though our classical analogies and intuition often fail us at the quantum mechanical scale. For our purposes, think of spin as the quantity that encodes the zero or one of our qubit, where a spin of 0 represents the zero qubit state and a spin of +1 (or -1) represents the one qubit state.
Confocal map of a typical diamond sample (the spots are NV centers themselves).
[Source: Robledo L. et al, PRL 2010.]
How do we manipulate the spin in this qubit? That is, how do we set our qubit to zero, to one, or to a superposition of the two? This is where the lasers and microwaves come in. Our team loves working with these defects because we have many control knobs that we can turn to change their quantum properties. We shine a green laser on the NV center to initialize its spin to the zero state. We also use a technique called confocal microscopy to focus the beam on the defect in our piece of diamond. We then use signal generators, switches, and amplifiers to apply microwaves to this 0 spin state to drive it into the +1 state. The diamond chip with the NV centers embedded in it is connected to a circuit board which has channels to deliver the microwaves to the defects, as is the signal generator which generates the microwaves. The photoluminescence emitted from the diamond sample passes through a different set of lenses and mirrors and hits a photon detector, and from the photoluminescence intensity, we can infer the spin state of the defect.
Another special thing about the nitrogen vacancy center is that we can operate it at room temperature. We usually work with qubits at very very low temperatures, in complicated cooling devices called dilution refrigerators. For the NV center, we don’t need this complication. This leads to the potential for using the NV center as a quantum sensor. Quantum information is lost easily to the environment, and the warmer the environment, the more lattice vibrations, and the more easily this information is lost. This phenomenon is called decoherence, and this is why most quantum computers require very cold (millikelvin!) temperatures. However, the rigid diamond lattice protects the spin of the NV center from thermal decoherence processes, allowing room temperature operation.
Since NV centers work at room temperature, people have used them extensively to study the real-world environment, sensing different external perturbations, especially magnetic fields. An external magnetic field causes the energy levels of the NV center’s spin to change. By observing this evolution, the local magnetic field can then be inferred with nanometer-scale resolution. Researchers have studied the magnetic environment of a wide range of systems, from bacteria to magnetic thin films. This is similar to how an MRI machine takes pictures of our brain, but the NV centers form magnetic sensors that are much smaller and more sensitive than the MRI machine. Think of all the things we could study at resolutions higher than ever before!
NV centers are also used in quantum computing and quantum communication. For instance, a few years ago, a group of scientists at Delft University of Technology entangled two NV centers more than a kilometer apart to do a loophole-free test of one of the most fundamental tests of quantum mechanics: Bell’s theorem. This test also paved the way for implementing a secure quantum communication protocol known as quantum key distribution. Sometimes people like to talk about the quantum research community as racing to find the perfect qubit, but as we can tell from our story of NV centers, different qubits can serve different purposes. So, while the superconducting qubits at IBM are being scaled up to be more and more sophisticated efficient computing devices, NV-center sensors can push the frontiers to sensing smaller and smaller magnetic fields. Attempting the same experiments with different physical systems poses different sets of advantages and disadvantages. As graduate students in the field, we will often read a paper about a different qubit and try to puzzle out: “could my qubit do this better? If not, is it somehow useful to the advancement of this technology?” Different quantum systems with different abilities can also be linked together, through quantum transduction, a way of converting quantum signals so that systems at different energy scales can communicate. To have an entire technological future based on quantum information, we will have quantum computers, quantum sensors and quantum communication networks acting together.
My introduction to the world of quantum information was through NV centers, these defects embedded in tiny diamond chips in the lab, and I am excited to see where they end up featuring in the exciting field of quantum information.
P.S. We are excited to share this first glimpse into the world of quantum information outside of superconducting circuits. We plan to continue branching out and discussing other platforms in the future, so to keep track of what we have covered so far, please refer to our "State of Play" Chart, which we will update over time.
