On-Demand Driven Dissipation for Cavity Reset and Cooling

We present a superconducting circuit device that provides active, on-demand, tunable dissipation on a target mode of the electromagnetic field. Our device is based on a tunable “dissipator” that can be made lossy when tuned into resonance with a broadband filter mode. When driven parametrically, this dissipator induces loss on any mode coupled to it with energy detuning equal to the drive frequency. We demonstrate the use of this device to reset a superconducting qubit’s readout cavity after a measurement, removing photons with a characteristic rate greater than $50\text{μ}{\text{s}}^{-1}$. We also demonstrate that the dissipation can be driven constantly to simultaneously damp and cool the cavity, effectively eliminating thermal photon fluctuations as a relevant decoherence channel. Our results demonstrate the utility of our device as a modular tool for environmental engineering and entropy removal in circuit quantum electrodynamics.

Popular Summary

Quantum bits made from superconducting circuits (superconducting qubits) are a promising quantum computing technology, offering fast operation speed, extended coherence, and the potential for scalability. However, noisy interactions with the environment cause decoherence, scrambling qubit states and degrading performance. One limiting noise source is residual photons in the resonators used to measure qubits. These photons “measure” the qubit when a measurement was not intended, scrambling the quantum state. Residual photons may be left over from a prior measurement or may be due to the resonator’s nonzero temperature.

In this work, we address these challenges by introducing a “dissipator” device, consisting of a lossy parametric coupler. By pumping the dissipator, we transfer photons from a resonator into the dissipator. The dissipator’s loss causes the photons to quickly decay, thus emptying the resonator of photons. We demonstrate that using this approach, we can reset a resonator after a qubit measurement more than 10 times faster than the natural decay process. We also demonstrate that we can continuously cool the resonator, preserving qubit coherence even at elevated temperature. We show that we can accomplish this without disrupting qubit measurements.

Our dissipator device can function as a general-purpose source of tunable, on-demand loss. This enables reset of resonators and qubits, dissipative state preparation and stabilization, and novel operation schemes. It can even be used for experiments on foundational quantum thermodynamics and the quantum measurement problem.

Figure 1

(a) Energy level diagram depicting the driven dissipation process, showing the lowest two dissipator states ($|g⟩$ and $|e⟩$) and lowest four cavity photon states ($|0⟩$, $|1⟩$, $|2⟩$, and $|3⟩$). A parametric drive on the dissipator at the dissipator-cavity detuning frequency induces coherent oscillations between the $|g,n⟩$ and $|e,n-1⟩$ states (blue arrows). The dissipator is lossy, and so quickly emits a photon (green) and decays back to the $|g,n-1⟩$ state (dashed black arrow). A photon is thus unconditionally removed from the cavity; continuous driving of this process brings the system to its ground state $|g,0⟩$. (b) Circuit schematic of our device. A measurement feedline (black, ports 1 and 2) is coupled with rate ${\kappa }_{\mathrm{c}}$ to a half-wave cavity (green) used to read out an ordinary transmon qubit (purple). The cavity is also coupled to a tunable coupler, the dissipator (light blue), which can be flux driven via a fast flux (FF) line (port 3). The dissipator is coupled via a bandpass filter (implemented as a half-wave cavity) to a terminated transmission line (port 4), with filter bandwidth ${\kappa }_{\mathrm{f}}$, causing the dissipator to become lossy when near the filter band. (c) False-colored scanning electron microscope image of a device with the same design as the one reported here, with color coding as used in (b). The readout cavity, qubit, dissipator, filter, and terminated line are all connected via coupling capacitors (highlighted in yellow).

Figure 2

(a) Spectra of cavity and filter modes as a function of flux through the dissipator SQUID loop. The upper plot shows power transmitted from the filter (loss) port to the measurement output port (port 4 to port 2), relative to an arbitrary reference power in the unit of decibels. The dashed red line is a fit to the dissipator frequency as a function of flux based on the locations of the avoided crossings. The broadband mode at 8.6 GHz is the filter mode (dashed brown line). The fundamental cavity mode is indicated by the dashed orange line at 5.594 GHz. Also visible are the second cavity mode at 11.2 GHz and another mode at 10 GHz that we attribute to nonlinear mixing of the second cavity and filter modes; the second cavity mode is too narrow to see on this scale. (b) Detailed measurement of the region indicated by the box in (a). Here we show transmission through the ordinary measurement feedline (port 1 to port 2), which couples strongly to the readout cavity mode. The red curves, which appear vertical, are actually the (steep) dissipator flux tuning in this range, and the orange curves are the cavity frequency obtained with the dissipator parameters from the previous fit and by our letting ${\omega }_{\text{c}}$ and $g_{\text{c}}$ be free parameters.

Figure 3

(a) Pulse sequence for cavity ringdown measurement. The cavity is populated with an initial drive pulse. We then drive the dissipator with a parametric flux drive for a variable time $\tau$ before digitizing the signal resulting from the remaining cavity population. (b) Ringdown spectroscopy measurement of cavity ringdown rate as a function of parametric drive frequency and power. The broad feature centered at 2.9 GHz is the detuning between the cavity and the dissipator. Narrower features at approximately $3.35$ GHz and approximately $3.2$ GHz are spurious signals due to crosstalk-induced driving of the qubit transitions. (c) Horizontal slices of the data in (b) at three different frequencies. Only when the parametric drive is on resonance with the dissipator-cavity detuning (black curve) do we see a strong increase of the ringdown rate. The upper horizontal axis, which reports the drive in the unit of the induced parametric coupling rate, is based on estimates of $g_{\mathrm{c}}$ and ${ϵ}_{\mathrm{p}}$ and should be viewed as an estimate itself; it roughly predicts the decay rate seen.

Figure 4

(a) Pulse sequence for cavity reset measurements. We first drive a population pulse to displace the cavity state. We then drive a parametric pulse on the dissipator for a variable time to induce cavity loss. We next apply an ordinary Hahn echo sequence on the qubit and then measure its state. (b) Measured echo decoherence rate ${\mathrm{\Gamma }}_2^{\mathrm{E}}$ as a function of the wait time after an initial population pulse with (blue, left) and without (green, right) the dissipator drive. Decoherence initially is fast, then drops to the background value (gray-shaded region, indicating the variance in the background ${\mathrm{\Gamma }}_2^{\mathrm{E}}$). The dashed curves are fits to the simple model given in Eq. (9), with extracted cavity decay rates given in the legend. At short times without dissipator drive, the photon number changes significantly during the echo measurement, making ${\mathrm{\Gamma }}_2^{\mathrm{E}}$ fits unreliable. This can be seen clearly in the inset curves, which show single echo measurements with (blue) and without (green) the dissipation drive: the measurement without dissipation drive gives a strongly nonexponential curve.

Figure 5

(a) Pulse sequence for the cavity refrigeration measurements. The dissipator is parametrically driven to induce cavity loss while an ordinary Hahn echo sequence is applied on the qubit. During the free evolution of the echo, the cavity is driven with a variable-amplitude population drive to add a fluctuating photon population. (b) Measured echo decoherence rate ${\mathrm{\Gamma }}_2^{\mathrm{E}}$ as a function of parametric drive power for three different cavity drive powers. The legend shows the mean cavity photon numbers $\overline{n}$ under the influence of the cavity drives when the parametric (dissipation) drive is off. The dashed gray line shows the mean value of ${\mathrm{\Gamma }}_2^{\mathrm{E}}$ ($0.17\text{μ}{\text{s}}^{-1}$) in the absence of all other drives. The rate fluctuates, and the gray-shaded region shows the 1-standard-deviation interval. When a cavity drive is present with no cooling drive, fluctuating photon populations $\overline{n}$ of 0.14, 0.35, and 1.10 in the cavity lead to dephasing, giving the increased average decoherence rates of $0.274$, $0.425$, and $0.980\text{μ}{\text{s}}^{-1}$ shown by the dashed blue, green, and red lines, respectively. Shaded regions indicate 1-standard-deviation intervals. When the dissipator is coupled to the cavity with a parametric drive, these excitations are rapidly removed from the cavity and the coherence is improved above its background value. With optimal dissipator driving and no added cavity drive, we read an average decoherence rate of $0.124\text{μ}{\text{s}}^{-1}$, indicated by the dashed yellow line and yellow-shaded region.

Figure 6

Experimental schematic. Qubit and cavity drives are generated with dual-channel baseband pulses fed into in-phase and quadrature mixers and mixed with local oscillators (LOs) to create single sideband microwave pulses. The cavity drives pass through a variable attenuator before being combined with qubit drives and fed into the fridge on a heavily attenuated and filtered input line. At base temperature, the lines are filtered with Eccosorb infrared filters from Quantum Microwave and 12-GHz low-pass filters from K&L. After being transmitted through the device, readout signals pass through terminated circulators (i.e., isolators) before being amplified by a traveling-wave parametric amplifier (TWPA); the TWPA pump is generated separately at room temperature and fed in on its own attenuated and filtered line before being coupled into the amplifier. The amplified signals are further isolated before passing through another stage of filtering and traveling back up the fridge, through a HEMT semiconductor amplifier at approximately $4$ K and successive stages of amplifiers at room temperature, with isolation between each stage. The signal is finally demodulated with the same LO used to generate the readout pulses, before being digitized and further demodulated in software. Microwave fast flux pulses are generated by dual-channel baseband pulses mixed with another LO, passed through a variable attenuator, and fed into the fridge through an attenuated and 12 GHz low-pass-filtered line. Static flux bias travels down an attenuated line with a 250-MHz low-pass filter at base temperature, and they are combined with the microwave flux pulses with a bias tee at base temperature before being fed into the device. The lossy “termination” line connects to the device via heavy attenuation and another K&L filter. Not shown in this configuration, sometimes a VNA is connected to the termination line and the ordinary output line to measure transmission through the filter. ADC, analog-to-digital converter; DAC, digital-to-analog converter; FFL, fast flux line.