The Rigetti QPU

A quantum processor unit (QPU) is a physical device that contains a number of interconnected qubits. This page presents technical details and average performance of Acorn, the 19Q Rigetti QPU device that is made available for quantum computation through the cloud. This device has been designed, fabricated and packaged at Rigetti Computing.

Acorn QPU properties

The quantum processor consists of 20 superconducting transmon qubits with fixed capacitive coupling in the planar lattice design shown in Fig. 1. The resonance frequencies of qubits 0–4 and 10–14 are tunable while qubits 5–9 and 15–19 are fixed. The former have two Josephson junctions in an asymmetric SQUID geometry to provide roughly 1 GHz of frequency tunability, and flux-insensitive “sweet spots” near

\(\omega^{\textrm{max}}_{01}/2\pi\approx 4.5 \, \textrm{GHz}\)

and

\(\omega^{\textrm{min}}_{01}/2\pi\approx 3.0 \, \textrm{GHz}\).

These tunable devices are coupled to bias lines for AC and DC flux delivery. Each qubit is capacitively coupled to a quasi-lumped element resonator for dispersive readout of the qubit state. Single-qubit control is effected by applying microwave drives at the resonator ports. Two-qubit gates are activated via RF drives on the flux bias lines.

Due to a fabrication defect, qubit 3 is not tunable, which prohibits operation of the two-qubit parametric gate described below between qubit 3 and its neighbors. Consequently, we will treat this as a 19-qubit processor. This also means that qubit 3 is not accessible for quantum computation through Forest.

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\(\textbf{Figure 1 $|$ Connectivity of Rigetti 19Q. a,}\) Chip schematic showing tunable transmons (green circles) capacitively coupled to fixed-frequency transmons (blue circles). \(\textbf{b}\), Optical chip image. Note that some couplers have been dropped to produce a lattice with three-fold, rather than four-fold connectivity.

1-Qubit Gate Performance

The device is characterized by several parameters:

  • \(\omega_\textrm{01}/2\pi\) is the qubit transition frequency
  • \(\omega_\textrm{r}/2\pi\) is the resonator frequency
  • \(\eta/2\pi\) is the anharmonicity of the qubit
  • \(g/2\pi\) is the coupling strength between a qubit and a resonator
  • \(\lambda/2\pi\) is the coupling strength between two neighboring qubits

In Rigetti 19Q, each tunable qubit is capacitively coupled to one-to-three fixed-frequency qubits. We use a parametric flux modulation to activate a controlled Z gate between tunable and fixed qubits. The typical time-scale of these entangling gates is in the range 100–250 ns.

Table 1 summarizes the main performance parameters of Rigetti 19Q. The resonator and qubit frequencies are measured with standard spectroscopic techniques. The relaxation time \(T_1\) is extracted from repeated inversion recovery experiments. Similarly, the coherence time \(T^*_2\) is measured with repeated Ramsey fringe experiments. Single-qubit gate fidelities are estimated with randomized benchmarking protocols in which a sequence of \(m\) Clifford gates is applied to the qubit followed by a measurement on the computational basis. The sequence of Clifford gates are such that the first \(m-1\) gates are chosen uniformly at random from the Clifford group, while the last Clifford gate is chosen to bring the state of the system back to the initial state. This protocol is repeated for different values of \(m\in \{2,4,8,16,32,64,128\}\). The reported single-qubit gate fidelity is related to the randomized benchmarking decay constant \(p\) in the following way: \(\mathsf{F}_\textrm{1q} = p +(1-p)/2\). Finally, the readout assignment fidelities are extracted with dispersive readouts combined with a linear classifier trained on \(|0\rangle\) and \(|1\rangle\) state preparation for each qubit. The reported readout assignment fidelity is given by expression \(\mathsf{F}_\textrm{RO} = [p(0|0)+p(1|1)]/2\), where \(p(b|a)\) is the probability of measuring the qubit in state \(b\) when prepared in state \(a\).

\(\textbf{Table 1 | Rigetti 19Q performance}\)
  \(\omega^{\textrm{max}}_{\textrm{r}}/2\pi\) \(\omega^{\textrm{max}}_{01}/2\pi\) \(\eta/2\pi\) \(T_1\) \(T^*_2\) \(\mathsf{F}_{\textrm{1q}}\) \(\mathsf{F}_{\textrm{RO}}\)
  \(\textrm{MHz}\) \(\textrm{MHz}\) \(\textrm{MHz}\) \(\mu\textrm{s}\) \(\mu\textrm{s}\)    
0 5592 4386 -208 15.2 \(\pm\) 2.5 7.2 \(\pm\) 0.7 0.9815 0.938
1 5703 4292 -210 17.6 \(\pm\) 1.7 7.7 \(\pm\) 1.4 0.9907 0.958
2 5599 4221 -142 18.2 \(\pm\) 1.1 10.8 \(\pm\) 0.6 0.9813 0.97
3 5708 3829 -224 31.0 \(\pm\) 2.6 16.8 \(\pm\) 0.8 0.9908 0.886
4 5633 4372 -220 23.0 \(\pm\) 0.5 5.2 \(\pm\) 0.2 0.9887 0.953
5 5178 3690 -224 22.2 \(\pm\) 2.1 11.1 \(\pm\) 1.0 0.9645 0.965
6 5356 3809 -208 26.8 \(\pm\) 2.5 26.8 \(\pm\) 2.5 0.9905 0.84
7 5164 3531 -216 29.4 \(\pm\) 3.8 13.0 \(\pm\) 1.2 0.9916 0.925
8 5367 3707 -208 24.5 \(\pm\) 2.8 13.8 \(\pm\) 0.4 0.9869 0.947
9 5201 3690 -214 20.8 \(\pm\) 6.2 11.1 \(\pm\) 0.7 0.9934 0.927
10 5801 4595 -194 17.1 \(\pm\) 1.2 10.6 \(\pm\) 0.5 0.9916 0.942
11 5511 4275 -204 16.9 \(\pm\) 2.0 4.9 \(\pm\) 1.0 0.9901 0.900
12 5825 4600 -194 8.2 \(\pm\) 0.9 10.9 \(\pm\) 1.4 0.9902 0.942
13 5523 4434 -196 18.7 \(\pm\) 2.0 12.7 \(\pm\) 0.4 0.9933 0.921
14 5848 4552 -204 13.9 \(\pm\) 2.2 9.4 \(\pm\) 0.7 0.9916 0.947
15 5093 3733 -230 20.8 \(\pm\) 3.1 7.3 \(\pm\) 0.4 0.9852 0.970
16 5298 3854 -218 16.7 \(\pm\) 1.2 7.5 \(\pm\) 0.5 0.9906 0.948
17 5097 3574 -226 24.0 \(\pm\) 4.2 8.4 \(\pm\) 0.4 0.9895 0.921
18 5301 3877 -216 16.9 \(\pm\) 2.9 12.9 \(\pm\) 1.3 0.9496 0.930
19 5108 3574 -228 24.7 \(\pm\) 2.8 9.8 \(\pm\) 0.8 0.9942 0.930

Qubit-Qubit Coupling

The coupling strength between two qubits can be extracted from a precise measurement of the shift in qubit frequency after the neighboring qubit is in the excited state. This protocol consists of two steps: a \(\pi\) pulse is applied to the first qubit, followed by a Ramsey fringe experiment on the second qubit which precisely determines its transition frequency (see Fig. 2a). The effective shift is denoted by \(\chi_\textrm{qq}\) and typical values are in the range \(\approx 100 \, \textrm{kHz}\). The coupling strength \(\lambda\) between the two qubits can be calculated in the following way:

\[\lambda^{(1,2)} = \sqrt{\left|\frac{\chi^{(1,2)}_\textrm{qq} \left[\,f^\textrm{(1)}_{01}-f^\textrm{(2)}_{12}\right]\left[\,f^\textrm{(1)}_{12}-f^\textrm{(2)}_{01}\right]}{2(\eta_1+\eta_2)}\right|}\]

Figure 2b shows the coupling strength for our device. This quantity is crucial to predict the gate time of our parametric entangling gates.

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\(\textbf{Figure 2 $|$ Coupling strength. a,}\) Quantum circuit implemented to measure the qubit-qubit effective frequency shift. \(\textbf{b,}\) Capacitive coupling between neighboring qubits expressed in MHz.

2-Qubit Gate Performance

Table 2 shows the two-qubit gate performance of Rigetti 19Q. These parameters refer to parametric CZ gates performed on one pair at a time. We analyze these CZ gates through quantum process tomography (QPT). This procedure starts by applying local rotations to the two qubits taken from the set \(\{I,R_x(\pi/2),R_y(\pi/2),R_x(\pi)\}\), followed by a CZ gate and post-rotations that bring the qubit states back to the computational basis. QPT involves the analysis of \(16\times16 =256\) different experiments, each of which we repeat \(500\) times. The reported process tomography fidelity \(\mathsf{F}^\textrm{cptp}_\textrm{PT}\) is the fidelity of the measured process compared with the ideal process, computed imposing complete positivity (cp) and trace preservation (tp) constraints.

\(\textbf{Table 2 | Rigetti 19Q two-qubit gate performance}\)
  \(A_0\) \(f_\textrm{m}\) \(t_\textrm{CZ}\) \(\mathsf{F}^\textrm{cptp}_{\textrm{PT}}\)
  \(\Phi/\Phi_0\) \(\textrm{MHz}\) ns  
0 - 5 0.27 94.5 168 0.936
0 - 6 0.36 123.9 197 0.889
1 - 6 0.37 137.1 173 0.888
1 - 7 0.59 137.9 179 0.919
2 - 7 0.62 87.4 160 0.817
2 - 8 0.23 55.6 189 0.906
4 - 9 0.43 183.6 122 0.854
5 - 10 0.60 152.9 145 0.870
6 - 11 0.38 142.4 180 0.838
7 - 12 0.60 241.9 214 0.87
8 - 13 0.40 152.0 185 0.881
9 - 14 0.62 130.8 139 0.872
10 - 15 0.53 142.1 154 0.854
10 - 16 0.43 170.3 180 0.838
11 - 16 0.38 160.6 155 0.891
11 - 17 0.29 85.7 207 0.844
12 - 17 0.36 177.1 184 0.876
12 - 18 0.28 113.9 203 0.886
13 - 18 0.24 66.2 152 0.936
13 - 19 0.62 109.6 181 0.921
14 - 19 0.59 188.1 142 0.797

Using the QPU

To maintain above performance levels, Rigetti Forest periodically takes the QPU offline to retune single-qubit and two-qubit gates. To access Acorn for running quantum algorithms, see Using the QPU-based stack for a tutorial.