Qubit Quality

At the recent Q2B conference it was emphasized that qubit quality is a very important factor in creating a viable quantum computer.  So kudos to IBM and Rigetti for documenting the specifications of their respective 20 qubit and 19 qubit chips. However, they are using different formats making it difficult to compare so we decided to put them into a common format.  These parameters are shown below in the three tables below.   Table 1 shows the qubit count, qubit connectivity, T1 and T2 or T2* times.   Table 2 shows the single qubit gate fidelity, the two qubit gate fidelity and the readout fidelities.   And Table 3 shows the sources of the data for the first two tables.  Note that all of this data is for physical qubits without any error correction.

Although we don’t have complete data for Google’s implementations, they recently provided some information at the Adiabatic Quantum Computing Conference (AQC 2018) where they compared metrics from their 5 and 9 qubit test chips with IBM’s 20 qubit implementation.  In the presentation Google indicated that their T1 times are roughly 2-4X worse than IBM’s, but that their single and two qubit gate fidelities are 2-10X better, and their measurement fidelities are roughly 10X better.  They also gave their opinion that gate fidelities are a more realistic measure than coherence times and that they expect their forthcoming 49 qubit chip will show similar results as their 5 and 9 qubit chips.   You can view Google’s complete presentation at AQC 2018 here.

We encourage anyone else who has data on other implementations to contact us at info@quantumcomputingreport.com so we can include your data into these tables.  Please see the notes at the end of this page that provides additional details of how these tables were generated.

Table 1

    Qubit Connectivity T1 (µsec) T2/T2* (µsec)
Computer Qubit Count Min Max Ave Min Max Ave Min Max Ave
IBM QX2 5 2 4 2.4 44.9 63.1 53.2 27.7 61.4 44.5
IBM QX4 5 2 4 2.4 36.2 54.8 48.1 14.9 55.7 31.1
IBM QX5 16 2 3 2.75 28.3 69.9 42.8 14.5 127.3 59.0
IBM QS1_1 20 2 6 3.9 47.5 173.5 80.1 15.6 94.2 41.3
Rigetti 19Q 19 1 3 2.21 8.2 31.0 20.3 4.9 26.8 10.9

Table 2

  1-Qubit Gate Fidelity 2-Qubit Gate Fidelity Read Out Fidelity
Computer Min Max Ave Min Max Ave Min Max Ave
IBM QX2 99.71% 99.88% 99.79% 94.22% 97.12% 95.33% 92.20% 98.20% 96.24%
IBM QX4 99.83% 99.96% 99.88% 95.11% 98.39% 97.11% 94.80% 97.10% 95.60%
IBM QX5 99.59% 99.87% 99.77% 91.98% 97.29% 95.70% 88.53% 96.66% 93.32%
IBM QS1_1 96.93% 99.92% 99.48% 82.28% 98.87% 95.68% 69.05% 93.55% 83.95%
Rigetti 19Q 94.96% 99.42% 98.63% 79.00% 93.60% 87.50% 84.00% 97.00% 93.30%

Table 3

Computer Reference Date
IBM QX2 https://quantumexperience.ng.bluemix.net/qx/devices 12/25/2017
IBM QX4 https://quantumexperience.ng.bluemix.net/qx/devices 12/25/2017
IBM QX5 https://quantumexperience.ng.bluemix.net/qx/devices 12/13/2017
IBM QS1_1 https://quantumexperience.ng.bluemix.net/qx/devices 12/13/2017
Rigetti 19Q http://pyquil.readthedocs.io/en/latest/qpu.html 12/18/2017


  1. For most of the parameters we show the Min, Max, and Average values.   Since both IBM and Rigetti publicize the individual values for every qubit, the Min shows the value for the worst of the qubits, the Max shows the value for the best of the qubits, and the Average shows the mean calculations for all of the qubits.
  2. The connectivity shows the number of connections from a qubit to a other qubits in the array for use in creating a CNOT gate.  The higher the connectivity, the easier it would be to fit a quantum calculation into the structure.  At this time, we do not differentiate on the flexibility of a connection.  For example, if qubit 1 is connected to qubit 2, many implementation require one of the qubits to be the CONTROL and the other qubit to be the TARGET.  Some implementations may be flexible enough so that either qubit can serve as the CONTROL and either qubit can serve as the TARGET.  That implementation may have some configuration advantages, but for the purposes of the table we are still only counting it as one connection and not as two.
  3. The T1 measure is called the relaxation time and the T2 or T2* measure is called the decoherence time.  For details of these definitions we refer you to this paper.  Note that IBM only publishes the T2 times while Rigetti only publishes the T2* time.  The measures are similar, but not exactly the same.
  4. The IBM reference link in Table 3 may require you to register for the IBM Q Experience.  If you click on this link it may ask you for a logon and password to see in more detail the referenced data.
  5. Questions, suggestions, and any additions you may have to the data are welcomed.   You can send them to info@quantumcomputingreport.com.
Benno Broer (Qu & Co)
@ 3:49 am

Besides this overview of the quality of IBM and Rigetti qubits, I believe adding overviews comparing different technology types (e.g. arXiv:1610.02208 or, a little older, arXiv:quant-ph/0607065 chapter 4.2.1) can be helpful to get a general understanding the pros-and-cons of different qubit types

@ 11:51 pm


I have seen various parameters but I am a little confused by the relations of them (coherence time/T1/T2, fidelity, connectivity, error rate)? Is error rate resulted by the other three parameters?)

Can I generally say:
Power of a quantum computer=Quality of qubits (connectivity x error rate) x Number of qubits = Number of logical qubits?

Thank you!


    @ 10:36 am

    There is no simple agreed upon formula that fully describes the power of a quantum computer. All that can be said is that the higher the qubit count, quality level, and connectivity, the better. The number of logical qubits is a function of the specific error correction algorithm that is used with the physical qubits. It is not directly related to the qubit quality. However, the lower the qubit quality, the more error correction you may want to put in. Describing the details of the different error types can get a little complicated and I would refer you to a quantum computing textbook to get a more complete description.

    Doug Finke
    Managing Editor


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