Frequently asked questions

General questions about quantum information science

What does “quantum” mean?

Quantum theory is a revolutionary advancement in physics and chemistry that emerged in the early twentieth century. It is an elegant mathematical theory able to explain the counterintuitive behavior of subatomic particles, most notably the phenomenon of entanglement. In the late twentieth century it was discovered that quantum theory applies not only to atoms and molecules, but to bits and logic operations in a computer. This realization has been bringing about a revolution in the science and technology of information processing, making possible kinds of computing and communication hitherto unknown in the Information Age.

What is a quantum computer?

A quantum computer is a device able to manipulate delicate quantum states in a controlled fashion, not dissimilar to the way an ordinary computer manipulates its bits.

What is a qubit?

A qubit (pronounced “cue-bit” and short for quantum bit) is the physical carrier of quantum information. It is the quantum version of a bit, and its quantum state can take values of |0\rangle , |1\rangle , or the linear combination of both, which is a phenomenon known as superposition.

What is a superposition?

A superposition is a weighted sum or difference of two or more states; for example, the state of the air when two or more musical tones sound at once. Ordinary, or “classical,” superpositions commonly occur in macroscopic phenomena involving waves.

How are quantum superpositions different?

Quantum theory predicts that a computer with n qubits can exist in a superposition of all 2^n of its distinct logical states |000...0\rangle , through |111...1\rangle . This is exponentially more than a classical superposition. Playing n musical tones at once can only produce a superposition of n states.

How is superposition different from probability?

A set of n coins, each of which might be heads or tails, can be described as a probabilistic mixture of 2^n states, but it actually is in only one of them — we just don’t know which. For this reason, quantum superposition is more powerful than classical probabilism. Quantum computers capable of holding their data in superposition can solve some problems exponentially faster than any known deterministic or probabilistic classical algorithm. A more technical difference is that while probabilities must be positive (or zero), the weights in a superposition can be positive, negative, or even complex numbers.

How is a quantum superposition different from exponential parallelism?

Just as a superposition is stronger than a probabilistic mixture, so it is weaker than actually having an exponentially large army of real computers (which is an unrealistic proposition in any case, since the observable universe isn’t big enough to hold 2^{100} of anything). The power of quantum computers remains to be explored, but it is considered to be strictly weaker than exponential parallelism, and strictly stronger than probabilism.

What is entanglement?

Entanglement is a property of most quantum superpositions and does not occur in classical superpositions. In an entangled state, the whole system is in a definite state, even though the parts are not. Observing one of two entangled particles causes it to behave randomly, but tells the observer how the other particle would act if a similar observation were made on it. Because entanglement involves a correlation between individually random behaviors of the two particles, it cannot be used to send a message. Therefore, the term “instantaneous action at a distance,” sometimes used to describe entanglement, is a misnomer. There is no action (in the sense of something that can be used to exert a controllable influence or send a message) – only correlation, which, though uncannily perfect, can only be detected afterward when the two observers compare notes. The ability of quantum computers to exist in entangled states is responsible for much of their extra computing power, as well as many other feats of quantum information processing that cannot be performed, or even described, classically.

What is the uncertainty principle?

In quantum physics, we cannot simultaneously know two non-commuting variables (like the position and momentum of a particle). This implies that a quantum system in a perfectly definite state can be certain under one measurement and completely random under another.  Moreover, if a quantum system starts out in an arbitrary unknown state, no measurement can reveal complete information about that state; the more information the measurement reveals, the more the state is disturbed.  This is a underlying principle of quantum cryptography.

What is a quantum gate?

A quantum gate is an operation applied to a qubit to change its state. To generate entanglement you must have at least a two-qubit gate equivalent to the CNOT.

Questions about quantum computers

Can a quantum computer solve NP-complete problems?

We do not believe that quantum computers are likely to provide any more than a quadratic speedup to NP-complete problems. This is the consensus view of the quantum information research community, and in fact the idea that quantum computers can solve NP-complete problems efficiently has been described as “the central crock about quantum computing”.

What is a universal quantum computer?

A universal quantum computer is a machine that can simulate an arbitrary quantum state from an arbitrary input quantum state.

What is a universal fault-tolerant quantum computer?

A universal fault-tolerant quantum computer is the grand challenge of quantum computing. It is a device that can properly perform universal quantum operations using unreliable components.

When will I have a quantum computer?

You have access to one now with IBM Quantum Experience.

What is a quantum circuit?

A quantum circuit is the set of instructions, or algorithm, to a quantum computer. It is a series of gates versus time played on different qubits.

What does a quantum computer look like?

A quantum computer looks like nothing you have on your desk, or in your office, or in your pocket. It is housed in a large unit known as a dilution refrigerator and is supported by multiple racks of electronic pulse-generating equipment. However, you can access our quantum computer with very familiar personal computing devices, such as laptops, tablets, and smartphones.

General questions about IBM Quantum Experience

What is IBM Quantum Experience?

IBM Quantum Experience is a cloud-based platform where you can learn, research, and interact with a real quantum computer housed in an IBM Research lab.

What is the Circuit Composer?

The Circuit Composer is a graphical interface tool where you can drag and drop different operations to control qubits. The Circuit Composer permits you to develop your own quantum algorithms.

What is a Quantum Circuit?

A quantum circuit is the set of instructions, or algorithm, to a quantum computer. It is a series of gates versus time played on different qubits, much like a musical score.

Questions about our qubits and experiments

What is the qubit that you are physically using?

The qubit we use is a fixed-frequency superconducting transmon qubit. It is a Josephson-junction-based qubit that is insensitive to charge noise. For more information on this type of qubit please see here (Koch et al. 2007). We use fixed-frequency qubits, as opposed to tunable qubits, to minimize our sensitivity to external magnetic field fluctuations that could corrupt the quantum information.

How do you make the qubits?

The superconducting qubits are fabricated at IBM. The devices are made on silicon wafers with superconducting metals such as niobium and aluminum. Details about the fabrication processes are given in these references (Chow et al. 2014, Córcoles et al. 2015).

What are the properties of these qubits?

The properties of the qubits can be seen in the Devices tab. Properties such as relaxation time ( T_1 ), coherence time ( T_2 ), readout errors, and gate errors are given, posted from the last calibration experiment run on the actual quantum processor device.

Where do the qubits live?

The quantum processor itself is housed inside of a printed circuit board package. This package is mounted inside of a light-tight, magnetic-field shielding can, which sits at the coldest stage at the bottom of a dilution refrigerator, housed in one of IBM’s Quantum Computing labs.

What’s a dilution refrigerator?

A dilution refrigerator is the machine we use to cool down our quantum processor device. The refrigerator cools the device down to around 15 miliKelvin. It works by circulating a mixture of two helium isotopes, 3He and 4He, in a closed cycle within a complex system of pipes and chambers.

What happens when I hit the “Run” button?

The graphical quantum circuit is first interpreted, then compiled to run efficiently on a particular qubit configuration. The compiled version is translated into a sequence of operations performed by equipment in our lab to control the qubits. The output is then passed back to the user, and a note is sent to your email address to alert you that the quantum hardware has run your experiment.

How are quantum gates performed in the system?

Quantum gates are performed by sending electromagnetic impulses at microwave frequencies to the qubits through coaxial cables. These electromagnetic pulses have a particular duration, frequency, and phase that determine the angle of rotation of the qubit state around a particular axis of the Bloch sphere. For single-qubit operations, only one pulse type needs to be calibrated, namely X_{\pi/2} . From that pulse, we can create all the gates you find in the Circuit Composer library. For example, we implement the S gate with the help of a phase transform performed purely in software, which affects the physical implementation of subsequent gates; however, this does not mean that the gate is necessarily error-free, as doing a software phase transform still requires very good phase stability from our instruments! Another example is the Hadamard gate performed by the sequence SX_{\pi/2}S .

What about two-qubit gates?

Two-qubit gates typically require tuning to calibrate the interaction between the two qubits during the gate duration, and minimizing the interaction at any other time. Since our qubits of choice are fixed-frequency transmons, we cannot tune the interaction by bringing them closer in frequency during the two-qubit gate. Instead, we exploit the cross-resonance effect (Chow et al., 2011), by driving one of the qubits (called control) with a microwave pulse tuned at the frequency of the second qubit (called target). By doing this, we can actively increase the strength of the coupling between them. The nature of the cross-resonance effect also allows us to perform rotations in the target qubit conditioned on the state of the control qubit, a key characteristic of the CNOT operation required for a universal quantum gate set. From our cross-resonance microwave pulse, we only need to perform an additional frame change S on the control qubit and a X_{\pi/2} on the target to implement a CNOT.

How are measurements performed in the system?

We must perform the qubit measurements in a way that does not destroy the qubit quantum state. One method is to weakly couple each qubit to a microwave resonator whose resonance characteristics depend on the state of the qubit. Once the qubit operations are completed in your score, you can measure the qubits by sending a microwave tone to their resonators and analyzing the signal it reflects back. The phase and amplitude of this reflected signal will be different depending on the qubit state. These signals in the resonator are boosted via a chain of amplifiers inside of our dilution refrigerator, including a quantum-limited amplifier at 15 mK, and a high-electron mobility transistor amplifier at 4 K.

Do not forget to include measurements in your score! Because the measurement of a qubit in a superposition state seems random – the outcome is sometimes 0 and sometimes 1 – you must repeat the measurement multiple times to determine the likelihood of a qubit being in a particular state. When performing the experiment, you will be asked how many “shots” or experiments to run in order to determine the qubit state probabilities.

How often is the system tuned up?

We perform a full round of single- and two-qubit calibrations, as well as measurements of relaxation time, coherence time, and gate errors two times a day at 8AM and 8PM EST. Each full calibration round takes about one hour.  During calibration, you will notice that the device will be “Down for Maintenance.”

Questions about running the simulator versus the experiment

What does the simulator do?

The simulator computes the quantum state we expect a circuit to produce.