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Making our first qubit

Computers encode information in units called ‘bits’. Bits have two possible states, 1 or 0. Physically, these correspond to the state of the computer’s electronics, like high or low voltage or current. Stringing bits together lets the computer represent systems with a greater number of states. For example, you can label four states with the bit strings ’00’, ‘01’, ‘10’, and ‘11’.

In most quantum computers, information is also represented in binary, and the two-state units of information are called ‘qubits’. We write qubit states in the Dirac notation as |0> and |1>. In an ion trap quantum computer, qubits are typically encoded in two of the ion’s atomic energy levels. The ion’s energy can be manipulated using electromagnetic fields, so we can perform computations on our qubit with RF signals.

Of course, none of this knowledge is of consequence if you don’t apply it to a system in the real world. But recently, our group has done just that: we have made our first qubit!

We chose to encode our qubit in two of the Zeeman sublevels of the metastable (1.17 s lifetime) D5/2 manifold (a collection of states with a certain energy and orbital angular momentum) of a calcium-40 ion (40Ca+); specifically, |0> corresponds to the m = +5/2 sublevel and |1> to the m = +3/2 sublevel. We are interested in this metastable qubit because we believe it affords several advantages over standard schemes which have at least one of the qubit states encoded in the ground S1/2 manifold (see here and here for details). While such qubits have already been created and tested to some extent, the creation of our first qubit is an achievement for our lab (which only began trapping in October) and heralds our deeper investigation of the metastable qubit scheme.

To prepare our qubit, we began by applying a static magnetic field to the ion. This induces Zeeman shifts in the energy of the sublevels, which lifts the degeneracy of the states (i.e., creates a difference in energy between the sublevels). Then we used lasers to pump population out of the S1/2 ground manifold and the D3/2 manifold. When we chose the right laser polarizations and pumped long enough, we got ~99% of the population into the m = +5/2 sublevel of D5/2.

Now we needed a way to control the qubit. Since all adjacent Zeeman sublevels have the same splitting, we have to lift the degeneracy of the transitions to ensure we only drive the qubit. We accomplished this by applying an 854 nm laser beam with a polarization that couples all sublevels besides the qubit states to the P manifolds. This creates a Stark shift in the energy of the affected states which is large enough to isolate them from the qubit. The action of the Stark shift on the sublevels is shown in the figure below.

40Ca+ atomic structure and Stark-shifted D5/2 sublevels. The qubit states are undisturbed by the Stark shift beam because its polarization does not permit them to couple to the P manifolds. Figure from https://doi.org/10.1103/PhysRevLett.111.180501

After all this preparation, our qubit was ready to go. We performed a quantum logic gate by applying an RF signal tuned to the qubit separation frequency. We used this to drive population oscillations (called ‘Rabi flops’) between our two qubit states and measured the population in one of the states as a function of the duration of the RF pulse. In this situation, we expect population to transfer sinusoidally between the two qubit states, i.e., the population in |0> will go as cos(Ωt) and the population in |1> will go as sin(Ωt), where ‘Ω’ is called the Rabi frequency. The process of measuring the qubit state is called ‘readout’, and we achieved it by hitting the ion with a specifically polarized laser beam to selectively pump population out of all Zeeman levels in D5/2 except for m = +5/2. After this, we checked for fluorescence on the S1/2 ↔ P1/2 transition. The fraction of experiments in which we detected fluorescence told us how often the qubit was in the state |1> (m = +3/2).

In the plot below, you can see that we were able to achieve Rabi flopping at a rate (Rabi frequency) of 83.33 kHz. Our measurements and error bars are in blue and our fit to the data is in orange. On the timescale shown, we achieve clean sine-squared behavior. However, if we continued for a long time, we would observe the troughs of the curve asymptotically approaching 1, due to the finite lifetime of the D5/2 manifold.

Rabi flopping in our metastable qubit. No noticeable decay of oscillations on this timescale because the lifetime of D5/2 is comparatively long.

Trapping ions at the University of Oregon

The Oregon ions group is now officially an ion trap group. Earlier this month we successfully trapped 40Ca+ ions!

Five 40Ca+ ions loaded into our linear Paul trap. The ions are spaced about 5 microns apart.

We trap ions by first ionizing neutral Calcium via a two-photon process that strips a single valence electron from the selected isotope of Calcium. We select the isotope by tuning our laser near resonance of isotope we want to load. The neutral atoms come from a thermal oven, which produces a atomic beam that intersects with our photo-ionization lasers at the center of the trap. At its center, the trap is deeper than the initial kinetic energy of the ions.

Spectroscopy of neutral calcium showing the different isotopes present in our sample. We selectively
load an isotope by tuning our lasers near the resonance of a that isotope.

We form a Coulomb crystal by cooling the ions using a laser beam tuned below the resonance of the relatively broad S1/2 – P1/2 transition in Calcium. This process is called Doppler cooling and as we scatter tens of millions of photons a second, we quickly cool the ions down to the millikelvin level where a crystal forms.

The ions are trapped using a combination of static and radio frequency electric fields, called a linear Paul trap. In two dimensions the ions are confined by an oscillating quadrupole field that generates an effective 2-dimensional harmonic potential. In the third dimension, we use static potential on the end caps of the trap.

The vacuum chamber that houses our linear Paul trap sits on an optical table with optics to direct the lasers into the trap. We have built a gold-plated rod trap that uses DC and RF electric fields to trap ions. Electronic control hardware in the rack on the right allows for precise timing of laser pulses.

Now that we can load ions into our trap, we have begun a series of experiments to characterize our trap and ensure everything is working correctly. We have performed spectroscopy of the S1/2 – P1/2 transition. We have also ‘tickled’ our ions by modulating the potential on our compensation field electrodes to determine the frequency our ions oscillate at.

Frequency scans of our laser over the S1/2 – P1/2 in  40Ca+ at different intensities. As the laser power is increased, we broaden the transition and see fluorescence at larger detuning.

We are continuing to develop our experimental code to run more tests on our setup and are currently setting up new beam lines to perform our first experiments using qubits.

Lab Move-In

In the year since our lab space was renovated, we’ve moved in and set up the hardware we need for our first ion trapping experiments.

Cleanroom
We have assembled our cleanroom, used for building up vacuum systems and optics in a dust-free environment. As of now, we have built up two vacuum systems here and have begun assembling our first trap.

Control Room
Our control stations are set up and wired to their respective labs, allowing us to control hardware across labs from a central location.

Ion Lab 1
In Ion Lab 1, we have set up the equipment we need for our first trap, including  laser beam control hardware, an imaging system, and an ARTIQ crate for running experiments.  The last remaining job is to build and install the trap and associated vacuum system.  At this point, we will be ready to trap an ion.

Electronics Fabrication Room
While this room will eventually be used for cryogenic experiments, for now, we have set up the space for building and testing our electronics.

Laser Lab
In our laser lab, we have installed our breadboard-mounted lasers and their associated control hardware.

Oregon Ions part of new Q-SEnSE Institute

Q-SEnSE is a new NSF Quantum Leap Challenge Institute funded with a $25M 5-year grant.

In the Institute, prominent quantum researchers in experiment and theory, science and engineering, from around the U.S. and internationally, collaborate to explore how advanced quantum sensing can discover new fundamental physics, develop and apply novel quantum technologies, provide tools for a national infrastructure in quantum sensing, and train a quantum savvy workforce.

Around the O news article
Q-SEnSE Website

The center includes PIs from:
• JILA at CU Boulder
• Harvard University
• MIT
• Stanford University
• University of Delaware
• University of New Mexico
• University of Innsbruck in Austria
• National Institute of Standards and Technology (NIST)
• Los Alamos National Laboratory
• MIT Lincoln Laboratory
• Sandia National Laboratory

Lab Renovations Complete

The main construction work on our lab is now complete.  We’re excited to start moving our equipment in once the HVAC is tuned up.  Here’s a tour:

Entrance and Cleanroom Bay
This is where we’ll install our cleanroom.  This will allow us to assemble ion trap vacuum systems and optics without dust contamination.

Control Room
The central desk is where all the ion traps will be controlled from.  The sink is for the coffee machine.

Ion Lab 1
This lab is for room temperature experiments and is where we will install our first ion trap.

Ion Lab 2
This lab is for a cryogenic experiment.  The helium to cool the trap will come through that hole in the far wall.

Ion Lab 3
Another cryogenic lab for future expansion.  For now we will use it for electronic fabrication and test.

Laser Lab
The group’s central laser systems will be locked safely away in this cozy environmentally-isolated space.  Light will be distributed to the ion labs over optical fibers.

Mechanical Space
All the heavy cryogenic machinery will be hidden away in this sound-proof and rf-shielded room behind the two cryogenic ion experiment labs.